Electron transport in coupled double quantum wells and wires
Harff, N.E.; Simmons, J.A.; Lyo, S.K.
1997-04-01
Due to inter-quantum well tunneling, coupled double quantum wells (DQWs) contain an extra degree of electronic freedom in the growth direction, giving rise to new transport phenomena not found in single electron layers. This report describes work done on coupled DQWs subject to inplane magnetic fields B{sub {parallel}}, and is based on the lead author`s doctoral thesis, successfully defended at Oregon State University on March 4, 1997. First, the conductance of closely coupled DQWs in B{sub {parallel}} is studied. B{sub {parallel}}-induced distortions in the dispersion, the density of states, and the Fermi surface are described both theoretically and experimentally, with particular attention paid to the dispersion anticrossing and resulting partial energy gap. Measurements of giant distortions in the effective mass are found to agree with theoretical calculations. Second, the Landau level spectra of coupled DQWs in tilted magnetic fields is studied. The magnetoresistance oscillations show complex beating as Landau levels from the two Fermi surface components cross the Fermi level. A third set of oscillations resulting from magnetic breakdown is observed. A semiclassical calculation of the Landau level spectra is then performed, and shown to agree exceptionally well with the data. Finally, quantum wires and quantum point contacts formed in DQW structures are investigated. Anticrossings of the one-dimensional DQW dispersion curves are predicted to have interesting transport effects in these devices. Difficulties in sample fabrication have to date prevented experimental verification. However, recently developed techniques to overcome these difficulties are described.
Coupled quantum wires as a detector of many-body states below the last conductance plateau.
Sasaki, T.; Lilly, Michael Patrick; Bird, J. P.; Shailos, A.; Reno, John Louis; Ochiai, Y.; Aoki, N.; Iwase, Y.; Morimoto, T.; Simmons, Jerry Alvon
2004-03-01
We demonstrate the presence of a resonant interaction between a pair of coupled quantum wires, which are realized in the ultra-high mobility two-dimensional electron gas of a GaAs/AlGaAs quantum well. Measuring the conductance of one wire, as the width of the other is varied, we observe a resonant peak in its conductance that is correlated with the point at which the swept wire pinches off. We discuss this behavior in terms of recent theoretical predictions concerning local spin-moment formation in quantum wires.
NASA Astrophysics Data System (ADS)
Fu, X.; Gao, H. X.
2016-02-01
We theoretically investigate the spin-dependent conductance, the total conductance and the spin polarization of a quantum wire with the coexistence of Rashba and Dresselhaus spin-orbit coupling in the quantum wire and two leads, respectively. First, we find that the Rashba or Dresselhaus spin-orbit coupling in the quantum wire induces the split of spin-dependent conductance and forms the out-of-plane spin polarization. Moreover, when Rashba strength or Dresselhaus strength in the quantum wire increases, the split of conductance is enlarged and the intensity of spin polarization is enhanced accordingly. Furthermore, when two spin-orbit couplings coexist in the quantum wire and two leads, the addition of spin-orbit coupling in two leads expands the oscillation ranges of conductance plateaus and spin polarization, respectively, and further strengthens the magnitude of spin polarization. In particular, when the Rashba coupling in two leads exists and the quantum wire is wide, the total conductance jumps to upper conductance plateaus and the direction of spin polarization changes, while for the Dresselhaus coupling no such results exist. Our calculations indicate that one can realize the modulation of the strength and direction of spin polarization by altering the width of wire, the Rashba or Dresselhaus strength in quantum wire, the Rashba or Dresselhaus strength in two leads, respectively, which may be used to design the spin filter.
Quantum coherence of mesoscopic stadia and wires coupled to the environment
NASA Astrophysics Data System (ADS)
Xie, Yuantao; Heremans, J. J.; Priol, C. Le; Vijeyaragunathan, S.; Mishima, T. D.; Santos, M. B.
Quantum phase coherence was measured in quasi-1D wires and in mesoscopic stadia connected to wide sample regions by wire-like necks, to investigate the effects of environmental and interdevice coupling on decoherence as contrasted to intrinsic materials properties. Measurements of quantum phase coherence lengths used weak-antilocalization on nanolithographic InGaAs/InAlAs structures at 390 mK. For quantum wires, experiments show that longer wire lengths result in longer phase coherence lengths. The result is understood from the observation that longer wires average out decoherence introduced at the end sections by environmental coupling. For stadia with quantum-wire-like necks, stadium-wire coupling dominates decoherence in the stadia, rather than environmental coupling. Stadia with wider and shorter necks show longer phase coherence lengths. The result is understood from the observation that wider and shorter wires are geometrically similar to stadia, implying a stronger wave function hybridization between stadia and connecting necks and thus weaker decoherence from stadium-wire coupling. The work shows that geometry has to be taken into account in measured mesoscopic coherence. Supported by DOE DE-FG02-08ER46532 and NSF DMR-1207537.
Externally controlled local magnetic field in a conducting mesoscopic ring coupled to a quantum wire
Maiti, Santanu K.
2015-01-14
In the present work, the possibility of regulating local magnetic field in a quantum ring is investigated theoretically. The ring is coupled to a quantum wire and subjected to an in-plane electric field. Under a finite bias voltage across the wire a net circulating current is established in the ring which produces a strong magnetic field at its centre. This magnetic field can be tuned externally in a wide range by regulating the in-plane electric field, and thus, our present system can be utilized to control magnetic field at a specific region. The feasibility of this quantum system in designing spin-based quantum devices is also analyzed.
Direction Controlled Coulomb Drag in Coupled One-Dimensional Quantum Wires
NASA Astrophysics Data System (ADS)
Yamamoto, Michihisa
2007-03-01
In a one-dimensional electron gas (1DEG) with sufficiently low density at low temperature, Coulomb interaction becomes so dominant that Wigner crystallization can occur. Wigner crystal (WC) is generally characterized by collective motion of electrons and strong incompressibility. Therefore, in the presence of an external electrostatic potential, electrons forming a WC do not contribute to microscopic screening and only respond rigidly, whereas those of a Fermi liquid (FL) freely move to screen the external potential and produce a correlation hole. In this work we show that the difference between WC and FL allows us to control the direction of Coulomb drag in coupled pairs of 1DEG wires, each having two 2DEG leads. We prepare parallel coupled pairs of quantum wires in a 2DEG defined by Schottky gates to study the current drag between the two wires. The distance between the two wires and the electron density in each wire are all tunable with gate voltages. We inject a constant current into one of the wires (drive wire) and measure the induced drag current (or voltage drop for Idrag = 0) in the other wire (drag wire). Electrons in the drive wire usually drag electrons in the drag wire in the same direction because momentum is conserved in Coulombic scattering between the wires. However, when the electron density in the drive wire is sufficiently low that the drive wire has charge inhomogeneity and the electrons in the drag wire are strongly correlated, i.e. at low density, high perpendicular magnetic field and low temperature, the direction of the drag current can be reversed. The sign reversal occurs only when the drive wire is adjacent to the boundary between the drag wire and its lead, and can be controlled by changing the geometry of the coupled wires. These behaviors can be modeled by electron pump from WC in the drag wire to its 2DEG lead, driven by particle-like electrons in the drive wire. The drive wire electrons induce a positive screening charge only in the
NASA Astrophysics Data System (ADS)
Franco, R.; Figueira, M. S.; Anda, E. V.
2003-04-01
The transport through a quantum wire with a side-coupled quantum dot is studied. We use the X-boson treatment for the Anderson single impurity model in the limit of U=∞. The conductance presents a minimum for values of T=0 in the crossover from mixed valence to Kondo regime due to a destructive interference between the ballistic channel associated with the quantum wire and the quantum dot channel. We obtain the experimentally studied Fano behavior of the resonance. The conductance as a function of temperature exhibits a logarithmic and universal behavior, that agrees with recent experimental results.
Magnetoresistance of One-Dimensional Subbands in Tunnel-Coupled Double Quantum Wires
Moon, J.S.; Blount, M.A.; Simmons, J.A.; Wendt, J.R.; Lyo, S.K.; Reno, J.L.
1999-08-04
The authors study the low-temperature in-plane magnetoresistance of tunnel-coupled quasi-one-dimensional quantum wires. The wires are defined by two pairs of mutually aligned split gates on opposite sides of a {le} 1 micron thick AlGaAs/GaAs double quantum well heterostructure, allowing independent control of the width of each quantum well. In the ballistic regime, when both wires are defined and the field is perpendicular to the current, a large resistance peak at {approximately}6 Tesla is observed with a strong gate voltage dependence. The data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.
Magnetoresistance of One-Dimensional Subbands in Tunnel-Coupled Double Quantum Wires
Blount, M.A.; Lyo, S.K.; Moon, J.S.; Reno, J.L.; Simmons, J.A.; Wendt, J.R.
1999-04-27
We study the low-temperature in-plane magnetoresistance of tunnel-coupled quasi-one-dimensional quantum wires. The wires are defined by two pairs of mutually aligned split gates on opposite sides of a < 1 micron thick AlGaAs/GaAs double quantum well heterostructure, allowing independent control of their widths. In the ballistic regime, when both wires are defined and the field is perpendicular to the current, a large resistance peak at ~6 Tesla is observed with a strong gate voltage dependence. The data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.
The geometry effect on energy transfer rate in a coupled-quantum-wires structure
NASA Astrophysics Data System (ADS)
Rafee, Vahdat
2016-03-01
The geometry effect on energy transfer rate in a coupled cylindrical quantum wires system is investigated. The corrected random phase approximation by the zero-temperature static Hubbard correction is employed to calculate dielectric function of the system. The geometry effect on energy transfer rate is studied for statically and dynamically screened electron-electron interaction. Both the linear and nonlinear regimes correspond respectively to weak and strong external field are considered. The calculations show that increasing wire radius increases energy transfer rate in both the static and dynamic screening approximations for electron-electron interactions. Moreover, the same trend is predicted by the calculations for both the linear and nonlinear regimes.
Charging of a one-dimensional metallic segment side-coupled to a quantum wire.
NASA Astrophysics Data System (ADS)
Kakashvili, Paata; Johannesson, Henrik
2003-03-01
We study a one-dimensional metallic segment (modeled by a finite Luttinger liquid) side-coupled to a quantum wire via a tunnel junction. Coulomb blockade effects are pronounced because of the finite size of the segment. We map this problem onto a spin-1/2 Kondo model, where an impurity susceptibility acts as a capacitance. Using conformal field theory we study how interactions in the wire influence the charging of the segment. The role of forward and backward Kondo scatterings and anisotropy of the Kondo interaction is also discussed.
Coupled force-balance and scattering equations for nonlinear transport in quantum wires
NASA Astrophysics Data System (ADS)
Huang, Danhong; Gumbs, Godfrey
2009-07-01
The coupled force-balance and scattering equations have been derived and applied to study nonlinear transport of electrons subjected to a strong dc electric field in an elastic-scattering-limited quantum wire. Numerical results have demonstrated both field-induced heating-up and cooling-down behaviors in the nonequilibrium part of the total electron-distribution function by varying the impurity density or the width of the quantum wire. The obtained asymmetric distribution function in momentum space invalidates the application of the energy-balance equation to our quantum-wire system in the center-of-mass frame. The experimentally observed suppression of mobility by a driving field for the center-of-mass motion in the quantum-wire system has been reproduced [see K. Tsubaki , Electr. Lett. 24, 1267 (1988); M. Hauser , Sci. Technol. 9, 951 (1994)]. In addition, the thermal enhancement of mobility in the elastic-scattering-limited system has been demonstrated, in accordance with a similar prediction made for graphene nanoribbons [see T. Fang , Phys. Rev. B 78, 205403 (2008)]. This thermal enhancement has been found to play a more and more significant role with higher lattice temperature and becomes stronger for a low-driving field.
Bracher, G; Schraml, K; Ossiander, M; Frédérick, S; Finley, J J; Kaniber, M
2014-02-21
We present an optical investigation of surface plasmon polaritons propagating along nanoscale Au-wires, lithographically defined on GaAs substrates. A two-axis confocal microscope was used to perform spatially and polarization resolved measurements in order to confirm the guiding of surface plasmon polaritons over lengths ranging from 5 to 20 μm along nanowires with a lateral dimension of only ≈ 100 nm. Finite difference time domain simulations are used to corroborate our experimental observations, and highlight the potential to couple proximal quantum emitters to propagating plasmon modes in such extreme subwavelength devices. Our findings are of strong relevance for the development of semiconductor based integrated plasmonic and active quantum plasmonic nanosystems that merge quantum emitters with nanoscale plasmonic elements.
Spin-dependent coupling between quantum dots and topological quantum wires
NASA Astrophysics Data System (ADS)
Hoffman, Silas; Chevallier, Denis; Loss, Daniel; Klinovaja, Jelena
2017-07-01
Considering Rashba quantum wires with a proximity-induced superconducting gap as physical realizations of Majorana bound states and quantum dots, we calculate the overlap of the Majorana wave functions with the local wave functions on the dot. We determine the spin-dependent tunneling amplitudes between these two localized states and show that we can tune into a fully spin polarized tunneling regime by changing the distance between dot and Majorana bound state. Upon directly applying this to the tunneling model Hamiltonian, we calculate the effective magnetic field on the quantum dot flanked by two Majorana bound states. The direction of the induced magnetic field on the dot depends on the occupation of the nonlocal fermion formed from the two Majorana end states which can be used as a readout for such a Majorana qubit.
Negative tunneling magneto-resistance in quantum wires with strong spin-orbit coupling
NASA Astrophysics Data System (ADS)
Han, Seungju; Serra, Llorenç; Choi, Mahn-Soo
2015-06-01
We consider a two-dimensional magnetic tunnel junction of the FM/I/QW(FM+SO)/I/N structure, where FM, I and QW(FM+SO) stand for a ferromagnet, an insulator and a quantum wire with both magnetic ordering and Rashba spin-orbit (SOC), respectively. The tunneling magneto-resistance (TMR) exhibits strong anisotropy and switches sign as the polarization direction varies relative to the quantum-wire axis, due to interplay among the one-dimensionality, the magnetic ordering, and the strong SOC of the quantum wire.
Nonlinear optical rectification in laterally-coupled quantum well wires with applied electric field
NASA Astrophysics Data System (ADS)
Liu, Guanghui; Guo, Kangxian; Zhang, Zhongmin; Hassanbadi, Hassan; Lu, Liangliang
2017-03-01
Nonlinear optical rectification coefficient χ0(2) in laterally-coupled AlxGa1-xAs/GaAs quantum well wires with an applied electric field is theoretically investigated using the effective mass approximation as well as the numerical energy levels and wavefunctions of electrons. We find that χ0(2) is greatly influenced by the electric field as well as both the distance and the radius of the coupled system. A blue shift of χ0(2) with increasing electric field is exhibited while a red shift followed by a blue shift with increasing distance or radius is exhibited. A nonmonotonic behavior can be found in the resonant peak values of χ0(2) along with the increase of the electric field, the distance or the radius. One or two of the following physical mechanisms: the increased localization of the ground and first-excited states, the reduced coupling and the reduced quantum confinement effect are applied to elucidate the results above. Our results play a potential role in infrared photodetectors based on the coupled system.
NASA Astrophysics Data System (ADS)
Vernek, Edson; Penteado, Poliana; Seridonio, Antonio; Egues, José C.
2014-03-01
The search for Majorana bound state (MBS) is topological superconductor nanowires is currently a topic of great interest. Despite the various theoretical proposals and the experimental results, the question of whether the possible signatures of MBS can be distinguished from those arising from other phenomena such as the Kondo effect is still under debate. A recent proposal for detecting MBS using a quantum dot coupled to normal two leads and to a topological quantum wire has proven to be very appropriate structure to investigate this problem. In this system, the presence of MBS in the wire is marked as a e2 / 2 h conductance through the dot. In this work we find, that the e2 / 2 h conductance peak is not per se an distinct signature of a MBS in the wire. We show instead that it results from a leaking of the Majorana state into the dot. Moreover, by gating the dot level (ɛd) far away below and above the Fermi level of the leads (ɛF), the conductance remains at e2 / 2 h . The surviving of the conductance plateau for ɛd >ɛF contrasts with Kondo effect plateau known to emerge only for ɛd <ɛF . This work is supported by FAPESP, CNPq, CAPES and FAPEMIG.
Kondo effect in a quantum wire with spin-orbit coupling
NASA Astrophysics Data System (ADS)
de Sousa, G. R.; Silva, Joelson F.; Vernek, E.
2016-09-01
The influence of spin-orbit interactions on the Kondo effect has been under debate recently. Studies conducted recently on a system composed of an Anderson impurity on a two-dimensional electron gas with a Rashba spin orbit have shown that it can enhance or suppress the Kondo temperature (TK), depending on the relative energy level position of the impurity with respect to the particle-hole symmetric point. Here, we investigate a system composed of a single Anderson impurity, side coupled to a quantum wire with spin-orbit coupling (SOC). We derive an effective Hamiltonian in which the Kondo coupling is modified by the SOC. In addition, the Hamiltonian contains two other scattering terms, the so-called Dzyaloshinskii-Moriya interaction, known to appear in these systems, and another one describing processes similar to the Elliott-Yafet scattering mechanisms. By performing a renormalization group analysis on the effective Hamiltonian, we find that the correction on the Kondo coupling due to the SOC favors the enhancement of the Kondo temperature even in the particle-hole symmetric point of the Anderson model, agreeing with the numerical renormalization group results. Moreover, away from the particle-hole symmetric point, TK always increases with the SOC, accordingly with a previous renormalization group analysis.
Weakly-coupled quasi-1D helical modes in disordered 3D topological insulator quantum wires
NASA Astrophysics Data System (ADS)
Dufouleur, J.; Veyrat, L.; Dassonneville, B.; Xypakis, E.; Bardarson, J. H.; Nowka, C.; Hampel, S.; Schumann, J.; Eichler, B.; Schmidt, O. G.; Büchner, B.; Giraud, R.
2017-04-01
Disorder remains a key limitation in the search for robust signatures of topological superconductivity in condensed matter. Whereas clean semiconducting quantum wires gave promising results discussed in terms of Majorana bound states, disorder makes the interpretation more complex. Quantum wires of 3D topological insulators offer a serious alternative due to their perfectly-transmitted mode. An important aspect to consider is the mixing of quasi-1D surface modes due to the strong degree of disorder typical for such materials. Here, we reveal that the energy broadening γ of such modes is much smaller than their energy spacing Δ, an unusual result for highly-disordered mesoscopic nanostructures. This is evidenced by non-universal conductance fluctuations in highly-doped and disordered Bi2Se3 and Bi2Te3 nanowires. Theory shows that such a unique behavior is specific to spin-helical Dirac fermions with strong quantum confinement, which retain ballistic properties over an unusually large energy scale due to their spin texture. Our result confirms their potential to investigate topological superconductivity without ambiguity despite strong disorder.
Weakly-coupled quasi-1D helical modes in disordered 3D topological insulator quantum wires
Dufouleur, J.; Veyrat, L.; Dassonneville, B.; Xypakis, E.; Bardarson, J. H.; Nowka, C.; Hampel, S.; Schumann, J.; Eichler, B.; Schmidt, O. G.; Büchner, B.; Giraud, R.
2017-01-01
Disorder remains a key limitation in the search for robust signatures of topological superconductivity in condensed matter. Whereas clean semiconducting quantum wires gave promising results discussed in terms of Majorana bound states, disorder makes the interpretation more complex. Quantum wires of 3D topological insulators offer a serious alternative due to their perfectly-transmitted mode. An important aspect to consider is the mixing of quasi-1D surface modes due to the strong degree of disorder typical for such materials. Here, we reveal that the energy broadening γ of such modes is much smaller than their energy spacing Δ, an unusual result for highly-disordered mesoscopic nanostructures. This is evidenced by non-universal conductance fluctuations in highly-doped and disordered Bi2Se3 and Bi2Te3 nanowires. Theory shows that such a unique behavior is specific to spin-helical Dirac fermions with strong quantum confinement, which retain ballistic properties over an unusually large energy scale due to their spin texture. Our result confirms their potential to investigate topological superconductivity without ambiguity despite strong disorder. PMID:28374744
Magnetoconductance of quantum wires
NASA Astrophysics Data System (ADS)
Ferreira, Gerson J.; Sammarco, Filipe; Egues, Carlos
2010-03-01
At low temperatures the conductance of a quantum wires exhibit characteristic plate-aus due to the quantization of the transverse modes [1]. In the presence of high in-plane magnetic fields these spin-split transverse modes cross. Recently, these crossings were observed experimentally [2] via measurements of the differential conductance as a function of the gate voltage and the in-plane magnetic-field. These show structures described as either anti-crossings or magnetic phase transitions. Motivated by our previous works on magnetotransport in 2DEGs via the Spin Density Functional Theory (SDFT) [3], here we propose a similar model to investigate the magnetoconductance of quantum wires. We use (i) the SDFT via the Kohn-Sham self-consistent scheme within the local spin density approximation to obtain the electronic structure and (ii) the Landauer-Buettiker formalism to calculate the conductance of a quantum wire. Our results show qualitative agreement with the data of Ref. [2]. [1] B. J. van Wees et al., Phys. Rev. Lett. 60, 848 (1988). [2] A. C. Graham et al., Phys. Rev. Lett. 100, 226804 (2008). [3] H. J. P. Freire, and J. C. Egues, Phys. Rev. Lett. 99, 026801 (2007); G. J. Ferreira, and J. Carlos Egues, J. Supercond. Nov. Mag., in press; G. J. Ferreira, H. J. P. Freire, J. Carlos Egues, submitted.
Non-Abelian S U (N -1 ) -singlet fractional quantum Hall states from coupled wires
NASA Astrophysics Data System (ADS)
Fuji, Y.; Lecheminant, P.
2017-03-01
The construction of fractional quantum Hall (FQH) states from the two-dimensional array of quantum wires provides a useful way to control strong interactions in microscopic models and has been successfully applied to the Laughlin, Moore-Read, and Read-Rezayi states. We extend this construction to the Abelian and non-Abelian S U (N -1 ) -singlet FQH states at filling fraction ν =k (N -1 )/[N +k (N -1 )m ] labeled by integers k and m , which are potentially realized in multicomponent quantum Hall systems or S U (N ) spin systems. Utilizing the bosonization approach and conformal field theory (CFT), we show that their bulk quasiparticles and gapless edge excitations are both described by an (N -1 ) -component free-boson CFT and the S U (N) k/[U(1 ) ] N -1 CFT known as the Gepner parafermion. Their generalization to different filling fractions is also proposed. In addition, we argue possible applications of these results to two kinds of lattice systems: bosons interacting via occupation-dependent correlated hoppings and an S U (N ) Heisenberg model.
Growth and fabrication of proximity-coupled topological quantum wire circuits from thin InAs films
NASA Astrophysics Data System (ADS)
Kan, Carolyn; Xue, Chi; Bai, Yang; Eckstein, James
The realization of topological states in strongly spin orbit coupled semiconductors proximity-coupled to conventional superconductors requires delicate materials engineering. Key areas for improvement include the crystalline quality of the semiconductor itself, but a high-quality interface between the semiconductor and superconductor is essential. Recent results have demonstrated the necessity of forming an in situ interface to eliminate the ``soft gap'' observed in earlier experiments. While much work has focused on vertically grown nanowires, we take a lithographic approach to fabricating quantum wires out of MBE-grown thin films, which allow for increased flexibility and scalability of device structures. Notably, our films are grown entirely in situ in linked MBE systems, vastly improving interface transmission and cleanliness. Aspects of growth architecture aimed toward increasing the InAs mobility, such as substrate choice and layer structure, are also discussed.
Quantum wire as a charge-qubit detector
NASA Astrophysics Data System (ADS)
Kwapiński, Tomasz; Taranko, Ryszard
2012-11-01
We present a proposal for a qubit charge meter (detector) based on a linear wire of quantum dots placed between two electron reservoirs. A qubit formed by an excess electron in a double quantum dot is coupled electrostatically with a single wire site and the dynamics of this system is studied using the equation of motion for appropriate correlation functions and the evolution operator method. For the qubit-wire system, depending on the qubit position on the wire the readout current oscillates over a long time or the current oscillations decrease very rapidly. This effect cannot be explained in terms of different charges of the wire sites (the same occupancies of all sites are considered). We have found that the qubit's decoherence strongly depends on the structure of the local density of states of the site which is coupled with the qubit. Additionally, the period of the readout current oscillations changes with the wire-qubit electrostatic coupling.
Negative excess noise in gated quantum wires
Dolcini, F.; Trauzettel, B.; Safi, I.; Grabert, H.
2009-04-23
The electrical current noise of a quantum wire is expected to increase with increasing applied voltage. We show that this intuition can be wrong. Specifically, we consider a single channel quantum wire with impurities and with a capacitive coupling to a metallic gate, and find that its excess noise, defined as the change in the noise caused by the finite voltage, can be negative at zero temperature. This feature is present both for large (c>>c{sub q}) and small (c<
Charging effects in a quantum wire with leads
NASA Astrophysics Data System (ADS)
Sablikov, V. A.; Polyakov, S. V.; Büttiker, M.
2000-05-01
We investigate the distribution of the electron density and the potential in a quantum wire coupled to reservoirs, treating this structure as a unified quantum system and taking into account the Coulomb interaction of electrons. The chemical potential difference that exists between a decoupled, isolated quantum wire and the reservoirs gives rise to charge transfer in the coupled system. We show that the quantum wire can be charged positively or negatively or remain neutral as a whole, depending on such factors as the wire radius and the background charge density in the wire. The magnitude of the charge and its sign are to a large extent determined by the exchange interaction of the electrons in the wire. Using a Hartree-Fock approach, we develop a model of a quantum wire which includes the reservoirs. This model allows us to find the self-consistent distribution of the electron density and the potential in the wire both at equilibrium and in the presence of transport. The linear conductance is investigated as a function of the chemical potential. The nonadiabatic transition from the reservoirs to the wire leads to conductance oscillations caused by multiple scattering of electron waves. The period of the oscillations depends on the charge acquired by the wire and the exchange energy. We find that the exchange interaction strongly enhances the Friedel oscillations near the contacts. However, they do not noticeably suppress the conductance because the wire has a finite length and is charged. Under far from equilibrium conditions, which appear when the applied voltage exceeds the Fermi energy in the wire, the system becomes unstable with respect to fluctuations of the electric potential and the electron density. The instability results in the appearance of multistable electron states.
Superconductivity in quantum wires: A symmetry analysis
NASA Astrophysics Data System (ADS)
Samokhin, K. V.
2017-10-01
We study properties of quantum wires with spin-orbit coupling and time reversal symmetry breaking, in normal and superconducting states. Electronic band structures are classified according to quasi-one-dimensional magnetic point groups, or magnetic classes. The latter belong to one of three distinct types, depending on the way the time reversal operation appears in the group elements. The superconducting gap functions are constructed using antiunitary operations and have different symmetry properties depending on the type of the magnetic point group. We obtain the spectrum of the Andreev boundary modes near the end of the wire in a model-independent way, using the semiclassical approach with the boundary conditions described by a phenomenological scattering matrix. Explicit expressions for the bulk topological invariants controlling the number of the boundary zero modes are presented in the general multiband case for two types of the magnetic point groups with real order parameters, corresponding to DIII and BDI symmetry classes.
Wire recycling for quantum circuit optimization
NASA Astrophysics Data System (ADS)
Paler, Alexandru; Wille, Robert; Devitt, Simon J.
2016-10-01
Quantum information processing is expressed using quantum bits (qubits) and quantum gates which are arranged in terms of quantum circuits. Here, each qubit is associated with a quantum circuit wire which is used to conduct the desired operations. Most of the existing quantum circuits allocate a single quantum circuit wire for each qubit and hence introduce significant overhead. In fact, qubits are usually not needed during the entire computation, only between their initialization and measurement. Before and after that, corresponding wires may be used by other qubits. In this work, we propose a solution which exploits this fact in order to optimize the design of quantum circuits with respect to the required wires. To this end, we introduce a representation of the lifetimes of all qubits which is used to analyze the respective need for wires. Based on this analysis, a method is proposed which "recycles" the available wires and, as a result, reduces the size of the resulting circuit. Numerical tests based on established reversible and fault-tolerant quantum circuits confirm that the proposed solution reduces the number of wires by more than 90% compared to unoptimized quantum circuits.
Braiding errors in interacting Majorana quantum wires
NASA Astrophysics Data System (ADS)
Sekania, Michael; Plugge, Stephan; Greiter, Martin; Thomale, Ronny; Schmitteckert, Peter
2017-09-01
Avenues of Majorana bound states (MBSs) have become one of the primary directions towards a possible realization of topological quantum computation. For a Y junction of Kitaev quantum wires, we numerically investigate the braiding of MBSs while considering the full quasiparticle background. The two central sources of braiding errors are found to be the fidelity loss due to the incomplete adiabaticity of the braiding operation as well as the finite hybridization of the MBSs. The explicit extraction of the braiding phase from the full many-particle states allows us to analyze the breakdown of the independent-particle picture of Majorana braiding. Furthermore, we find nearest-neighbor interactions to significantly affect the braiding performance for better or worse, depending on the sign and magnitude of the coupling.
Superradiance in a two-channel quantum wire
Tayebi, A.; Zelevinsky, V.
2014-10-15
A one-dimensional, two-channel quantum wire is studied in the effective non-Hermitian Hamiltonian framework. Analytical expressions are derived for the band structure of the isolated wire. Quantum states and transport properties of the wire coupled to two ideal leads at the edges are studied in detail. The width distribution of the quasistationary states varies as a function of the coupling strength to the environment. At weak coupling, all the eigenenergies uniformly acquire small widths. The picture changes entirely at strong coupling, a certain number of states (“super-radiant”) are greatly broadened, while the rest remain long-lived states, a pure quantum mechanical effect as a consequence of quantum interference. The transition between the two regimes greatly influences the transport properties of the system. The maximum transmission through the wire occurs at the super-radiance transition. We consider also a realistic situation with energy-dependent coupling to the continuum due to the existence of decay threshold where super-radiance still plays a significant role in transport properties of the system.
Anisotropic intrinsic spin Hall effect in quantum wires.
Cummings, A W; Akis, R; Ferry, D K
2011-11-23
We use numerical simulations to investigate the spin Hall effect in quantum wires in the presence of both Rashba and Dresselhaus spin-orbit coupling. We find that the intrinsic spin Hall effect is highly anisotropic with respect to the orientation of the wire, and that the nature of this anisotropy depends strongly on the electron density and the relative strengths of the Rashba and Dresselhaus spin-orbit couplings. In particular, at low densities, when only one subband of the quantum wire is occupied, the spin Hall effect is strongest for electron momentum along the [N110] axis, which is the opposite of what is expected for the purely 2D case. In addition, when more than one subband is occupied, the strength and anisotropy of the spin Hall effect can vary greatly over relatively small changes in electron density, which makes it difficult to predict which wire orientation will maximize the strength of the spin Hall effect. These results help to illuminate the role of quantum confinement in spin-orbit-coupled systems, and can serve as a guide for future experimental work on the use of quantum wires for spin-Hall-based spintronic applications.
Charge- and spin-density modulations in semiconductor quantum wires
NASA Astrophysics Data System (ADS)
Lee, Minchul; Bruder, Christoph
2005-07-01
We investigate static charge- and spin-density modulation patterns along a ferromagnet-semiconductor single-junction quantum wire in the presence of spin-orbit coupling. Coherent scattering theory is used to calculate the charge and spin densities in the ballistic regime. The observed oscillatory behavior is explained in terms of the symmetry of the charge and spin distributions of eigenstates in the semiconductor quantum wire. Also, we discuss the condition that these charge- and spin-density oscillations can be observed experimentally.
LDRD final report on quantum computing using interacting semiconductor quantum wires.
Lyo, Sungkwun Kenneth; Dunn, Roberto G.; Lilly, Michael Patrick; Tibbetts, Denise R. ); Stephenson, Larry L.; Seamons, John Andrew; Reno, John Louis; Bielejec, Edward Salvador; Simmons, Jerry Alvon
2006-01-01
For several years now quantum computing has been viewed as a new paradigm for certain computing applications. Of particular importance to this burgeoning field is the development of an algorithm for factoring large numbers which obviously has deep implications for cryptography and national security. Implementation of these theoretical ideas faces extraordinary challenges in preparing and manipulating quantum states. The quantum transport group at Sandia has demonstrated world-leading, unique double quantum wires devices where we have unprecedented control over the coupling strength, number of 1 D channels, overlap and interaction strength in this nanoelectronic system. In this project, we study 1D-1D tunneling with the ultimate aim of preparing and detecting quantum states of the coupled wires. In a region of strong tunneling, electrons can coherently oscillate from one wire to the other. By controlling the velocity of the electrons, length of the coupling region and tunneling strength we will attempt to observe tunneling oscillations. This first step is critical for further development double quantum wires into the basic building block for a quantum computer, and indeed for other coupled nanoelectronic devices that will rely on coherent transport. If successful, this project will have important implications for nanoelectronics, quantum computing and information technology.
Coupled wire construction of chiral spin liquids
NASA Astrophysics Data System (ADS)
Thomale, Ronny; Meng, Tobias; Neupert, Titus; Greiter, Martin
We develop a coupled wire construction of chiral spin liquids. The starting point are individual wires of electrons in the Mott regime that are subject to a Zeeman field and Rashba spin-orbit coupling. Suitable spin-flip couplings between the wires yield an Abelian chiral spin liquid state which supports spinon excitations above a bulk gap, and chiral edge states. The approach generalizes to non-Abelian chiral spin liquids at level k with parafermionic edge states. RT is supported by the European Research Council through ERC-StG-336012-TOPOLECTRICS. MG and RT are supported by DFG-SFB 1170.
Nonequilibrium functional bosonization of quantum wire networks
Ngo Dinh, Stephane; Bagrets, Dmitry A.; Mirlin, Alexander D.
2012-11-15
We develop a general approach to nonequilibrium nanostructures formed by one-dimensional channels coupled by tunnel junctions and/or by impurity scattering. The formalism is based on nonequilibrium version of functional bosonization. A central role in this approach is played by the Keldysh action that has a form reminiscent of the theory of full counting statistics. To proceed with evaluation of physical observables, we assume the weak-tunneling regime and develop a real-time instanton method. A detailed exposition of the formalism is supplemented by two important applications: (i) tunneling into a biased Luttinger liquid with an impurity, and (ii) quantum Hall Fabry-Perot interferometry. - Highlights: Black-Right-Pointing-Pointer A nonequilibrium functional bosonization framework for quantum wire networks is developed Black-Right-Pointing-Pointer For the study of observables in the weak tunneling regime a real-time instanton method is elaborated. Black-Right-Pointing-Pointer We consider tunneling into a biased Luttinger liquid with an impurity. Black-Right-Pointing-Pointer We analyze electronic Fabry-Perot interferometers in the integer quantum Hall regime.
Conductance oscillations of a metallic quantum wire
NASA Astrophysics Data System (ADS)
Kwapinski, T.
2005-09-01
The electron transport through a monatomic metallic wire connected to leads is investigated using the tight-binding Hamiltonian and the Green function technique. Analytical formulae for the transmittance are derived and M-atom oscillations of the conductance versus the length of the wire are found. Maxima of the transmittance function versus the energy, for a wire consisting of N atoms, determine the (N+1) period of the conductance. The periods of conductance oscillations are discussed and the local and average quantum wire charges are presented. The average charge of the wire is linked with the period of the conductance oscillations and for M-atom periodicity there are possible (M-1) average occupations of the wire states.
Directional coupling for quantum computing and communication.
Nikolopoulos, Georgios M
2008-11-14
We introduce the concept of directional coupling, i.e., the selective transfer of a state between adjacent quantum wires, in the context of quantum computing and communication. Our analysis rests upon a mathematical analogy between a dual-channel directional coupler and a composite spin system.
Antenna coupled photonic wire lasers
Kao, Tsung-Kao; Cai, Xiaowei; Lee, Alan W. M.; ...
2015-06-22
Slope efficiency (SE) is an important performance metric for lasers. In conventional semiconductor lasers, SE can be optimized by careful designs of the facet (or the modulation for DFB lasers) dimension and surface. However, photonic wire lasers intrinsically suffer low SE due to their deep sub-wavelength emitting facets. Inspired by microwave engineering techniques, we show a novel method to extract power from wire lasers using monolithically integrated antennas. These integrated antennas significantly increase the effective radiation area, and consequently enhance the power extraction efficiency. When applied to wire lasers at THz frequency, we achieved the highest single-side slope efficiency (~450more » mW/A) in pulsed mode for DFB lasers at 4 THz and a ~4x increase in output power at 3 THz compared with a similar structure without antennas. This work demonstrates the versatility of incorporating microwave engineering techniques into laser designs, enabling significant performance enhancements.« less
Antenna coupled photonic wire lasers
Kao, Tsung-Kao; Cai, Xiaowei; Lee, Alan W. M.; Reno, John L.; Hu, Qing
2015-06-22
Slope efficiency (SE) is an important performance metric for lasers. In conventional semiconductor lasers, SE can be optimized by careful designs of the facet (or the modulation for DFB lasers) dimension and surface. However, photonic wire lasers intrinsically suffer low SE due to their deep sub-wavelength emitting facets. Inspired by microwave engineering techniques, we show a novel method to extract power from wire lasers using monolithically integrated antennas. These integrated antennas significantly increase the effective radiation area, and consequently enhance the power extraction efficiency. When applied to wire lasers at THz frequency, we achieved the highest single-side slope efficiency (~450 mW/A) in pulsed mode for DFB lasers at 4 THz and a ~4x increase in output power at 3 THz compared with a similar structure without antennas. This work demonstrates the versatility of incorporating microwave engineering techniques into laser designs, enabling significant performance enhancements.
Antenna coupled photonic wire lasers
Kao, Tsung-Kao; Cai, Xiaowei; Lee, Alan W. M.; ...
2015-06-22
Slope efficiency (SE) is an important performance metric for lasers. In conventional semiconductor lasers, SE can be optimized by careful designs of the facet (or the modulation for DFB lasers) dimension and surface. However, photonic wire lasers intrinsically suffer low SE due to their deep sub-wavelength emitting facets. Inspired by microwave engineering techniques, we show a novel method to extract power from wire lasers using monolithically integrated antennas. These integrated antennas significantly increase the effective radiation area, and consequently enhance the power extraction efficiency. When applied to wire lasers at THz frequency, we achieved the highest single-side slope efficiency (~450more » mW/A) in pulsed mode for DFB lasers at 4 THz and a ~4x increase in output power at 3 THz compared with a similar structure without antennas. This work demonstrates the versatility of incorporating microwave engineering techniques into laser designs, enabling significant performance enhancements.« less
Quantum interference and conductance in silicon quantum wires
NASA Astrophysics Data System (ADS)
Bagraev, Nikolai T.; Gehlhoff, Wolfgang; Ivanov, Vadim K.; Klyachkin, Leonid E.; Malyarenko, Anna M.; Naeser, Alexander; Rykov, Serguei A.; Shelykh, Ivan A.
1999-05-01
We present the findings of quantized conductance (QC), Coulomb staircase (CS) and local tunneling spectroscopy (LTS) techniques which reveal the single-hole confinement and charging phenomena in the smooth and modulated quantum wires created electrostatically inside self-assembly longitudinal (SLQW) and lateral (SLaQW) silicon quantum wells. The current- voltage (CV) characteristics obtained are in a good agreement with the data of the theoretical calculations taking account of quantum interference effects in the field-dependent value of the transmission coefficient through the quantum wires that exhibit the different degree of a modulation.
Coupled atomic wires in a synthetic magnetic field
NASA Astrophysics Data System (ADS)
Budich, J. C.; Elben, A.; ŁÄ cki, M.; Sterdyniak, A.; Baranov, M. A.; Zoller, P.
2017-04-01
We propose and study systems of coupled atomic wires in a perpendicular synthetic magnetic field as a platform to realize exotic phases of quantum matter. This includes (fractional) quantum Hall states in arrays of many wires inspired by the pioneering work [C. L. Kane et al., Phys. Rev. Lett. 88, 036401 (2002), 10.1103/PhysRevLett.88.036401], as well as Meissner phases and vortex phases in double wires. With one continuous and one discrete spatial dimension, the proposed setup naturally complements recently realized discrete counterparts, i.e., the Harper-Hofstadter model and the two-leg flux ladder, respectively. We present both an in-depth theoretical study and a detailed experimental proposal to make the unique properties of the semicontinuous Harper-Hofstadter model accessible with cold-atom experiments. For the minimal setup of a double wire, we explore how a subwavelength spacing of the wires can be implemented. This construction increases the relevant energy scales by at least an order of magnitude compared to ordinary optical lattices, thus rendering subtle many-body phenomena such as Lifshitz transitions in Fermi gases observable in an experimentally realistic parameter regime. For arrays of many wires, we discuss the emergence of Chern bands with readily tunable flatness of the dispersion and show how fractional quantum Hall states can be stabilized in such systems. Using for the creation of optical potentials Laguerre-Gauss beams that carry orbital angular momentum, we detail how the coupled atomic wire setups can be realized in nonplanar geometries such as cylinders, disks, and tori.
Charge transport through weakly open one-dimensional quantum wires
NASA Astrophysics Data System (ADS)
Kopnin, N. B.; Galperin, Y. M.; Vinokur, V. M.
2009-01-01
We consider resonant transmission through a gated finite-length quantum wire connected to leads via finite-transparency junctions, such that the escape time is much smaller than the energy relaxation time in the wire. The coherent electron transport is strongly modified by the Coulomb interaction. The low-temperature current-voltage (IV) curves show steplike dependence on the bias voltage determined by the distance between the quantum levels inside the conductor, the pattern being dependent on the ratio between the charging energy and level spacing. If the system is tuned close to the resonance condition by the gate voltage, the low-voltage IV curve is ohmic. At large Coulomb energy and low temperatures, the conductance is temperature independent for any relationship between temperature, level spacing, and coupling between the wire and the leads.
Electrochemical Fabrication of Metallic Quantum Wires
ERIC Educational Resources Information Center
Tao, Nongjian
2005-01-01
The fabrication of metallic quantum wires using simple electrochemical techniques is described. The conductance of the system can be readily measured that allows one to constantly monitor the conductance during fabrication and use conductance quantization as a signature to guide the fabrication.
Electrochemical Fabrication of Metallic Quantum Wires
ERIC Educational Resources Information Center
Tao, Nongjian
2005-01-01
The fabrication of metallic quantum wires using simple electrochemical techniques is described. The conductance of the system can be readily measured that allows one to constantly monitor the conductance during fabrication and use conductance quantization as a signature to guide the fabrication.
Coupled spatial multimode solitons in microcavity wires
NASA Astrophysics Data System (ADS)
Slavcheva, G.; Gorbach, A. V.; Pimenov, A.
2016-12-01
A modal expansion approach is developed and employed to investigate and elucidate the nonlinear mechanism behind the multistability and formation of coupled multimode polariton solitons in microcavity wires. With pump switched on and realistic dissipation parameters, truncating the expansion up to the second-order wire mode, our model predicts two distinct coupled soliton branches: stable and unstable. Modulational stability of the stationary homogeneous solution and soliton branches stability are studied. Our simplified 1D model is in remarkably good agreement with the full 2D mean-field Gross-Pitaevskii model, reproducing correctly the soliton existence domain upon variation of pump amplitude and the onset of multistability.
Theoretical study of excitons in semiconductor quantum wires and related systems
NASA Astrophysics Data System (ADS)
Sidor, Yosyp
The main goal of this thesis is a theoretical study of the excitonic properties in semiconductor quantum wires. Excitons dominate the optical properties of these one-dimensional structures, producing broad or sharp absorption and photoluminescence lines. The confinement of the electron and the hole is responsible for the properties of the exciton in a quantum wire. Confinement of the particles can be controlled through the size and shape of the quantum wire as well as through the selection of structure and barrier materials to produce various band offsets. The application of a magnetic field can give important information about the exciton confinement. Therefore, theoretical investigations of excitons in quantum wires is a strong theoretical tool to provide valuable information about quantum wire characteristics, as size uniformity, dimensions and photoluminescence spectrum. In the present thesis self-assembled InAs/InP and GaAs/AlGaAs V-shaped quantum wires are considered. The calculated photoluminescence transition energies in these structures are compared with available experimental data in order to deduce the dimensions of the wires. Both wires are investigated theoretically in the presence of an external magnetic field applied along different directions of the quantum wires. The computed exciton diamagnetic shift for both V-shaped and self-assembled quantum wires are reported and a detailed comparison is obtained with available magneto-photoluminescence experimental data. Since strain is important for the formation of the self-assembled quantum wires, results on the influence of strain on the electron and hole confinement will also be presented. Further, exciton coupling in self-assembled InAs/InP coupled quantum wires is considered. The charge confinement in InAs/InP based quantum wells and self-assembled quantum wires is examined, where for the narrow quantum well a local circular width fluctuation is included in order to describe the localization of the
Spectroscopic properties of colloidal indium phosphide quantum wires
Wang, Lin-Wang; Wang, Fudong; Yu, Heng; Li, Jingbo; Hang, Qingling; Zemlyanov, Dmitry; Gibbons, Patrick C.; Wang, Lin-Wang; Janes, David B.; Buhro, William E.
2008-07-11
Colloidal InP quantum wires are grown by the solution-liquid-solid (SLS) method, and passivated with the traditional quantum dots surfactants 1-hexadecylamine and tri-n-octylphosphine oxide. The size dependence of the band gaps in the wires are determined from the absorption spectra, and compared to other experimental results for InP quantum dots and wires, and to the predictions of theory. The photoluminescence behavior of the wires is also investigated. Efforts to enhance photoluminescence efficiencies through photochemical etching in the presence of HF result only in photochemical thinning or photo-oxidation, without a significant influence on quantum-wire photoluminescence. However, photo-oxidation produces residual dot and rod domains within the wires, which are luminescent. The results establish that the quantum-wire band gaps are weakly influenced by the nature of the surface passivation, and that colloidal quantum wires have intrinsically low photoluminescence efficiencies.
Spin Polarized Transport and Spin Relaxation in Quantum Wires
NASA Astrophysics Data System (ADS)
Wenk, Paul; Yamamoto, Masayuki; Ohe, Jun-Ichiro; Ohtsuki, Tomi; Kramer, Bernhard; Kettemann, Stefan
We give an introduction to spin dynamics in quantum wires. After a review of spin-orbit coupling (SOC) mechanisms in semiconductors, the spin diffusion equation with SOC is introduced. We discuss the particular conditions in which solutions of the spin diffusion equation with vanishing spin relaxation rates exist, where the spin density forms persistent spin helices. We give an overview of spin relaxation mechanisms, with particular emphasis on the motional narrowing mechanism in disordered conductors, the D'yakonov-Perel' spin relaxation. The solution of the spin diffusion equation in quantum wires shows that the spin relaxation becomes diminished when reducing the wire width below the spin precession length L SO. This corresponds to an effective alignment of the spin-orbit field in quantum wires and the formation of persistent spin helices whose form as well as amplitude is a measure of the particular SOCs, the linear Rashba and the linear Dresselhaus coupling. Cubic Dresselhaus coupling is found to yield in diffusive wires an undiminished contribution to the spin relaxation rate, however. We discuss recent experimental results which confirm the reduction of the spin relaxation rate. We next review theoretical proposals for creating spin-polarized currents in a T-shape structure with Rashba-SOC. For relatively small SOC, high spin polarization can be obtained. However, the corresponding conductance has been found to be small. Due to the self-duality of the scattering matrix for a system with spin-orbit interaction, no spin polarization of the current can be obtained for single-channel transport in two-terminal devices. Therefore, one has to consider at least a conductor with three terminals. We review results showing that the amplitude of the spin polarization becomes large if the SOC is sufficiently strong. We argue that the predicted effect should be experimentally accessible in InAs. For a possible experimental realization of InAs spin filters, see [1].
Efficient numerical simulation of electron states in quantum wires
NASA Technical Reports Server (NTRS)
Kerkhoven, Thomas; Galick, Albert T.; Ravaioli, Umberto; Arends, John H.; Saad, Youcef
1990-01-01
A new algorithm is presented for the numerical simulation of electrons in a quantum wire as described by a two-dimensional eigenvalue problem for Schroedinger's equation coupled with Poisson's equation. Initially, the algorithm employs an underrelaxed fixed point iteration to generate an approximation which is reasonably close to the solution. Subsequently, this approximate solution is employed as an initial guess for a Jacobian-free implementation of an approximate Newton method. In this manner the nonlinearity in the model is dealt with effectively. The effectiveness of this approach is demonstrated in a set of numerical experiments which study the electron states on the cross section of a quantum wire structure based on III-V semiconductors at 4.2 and 77 K.
Universal quantum computation in a semiconductor quantum wire network
NASA Astrophysics Data System (ADS)
Sau, Jay D.; Tewari, Sumanta; Das Sarma, S.
2010-11-01
Universal quantum computation (UQC) using Majorana fermions on a two-dimensional topological superconducting (TS) medium remains an outstanding open problem. This is because the quantum gate set that can be generated by braiding of the Majorana fermions does not include any two-qubit gate and also no single-qubit π/8 phase gate. In principle, it is possible to create these crucial extra gates using quantum interference of Majorana fermion currents. However, it is not clear if the motion of the various order parameter defects (vortices, domain walls, etc.), to which the Majorana fermions are bound in a TS medium, can be quantum coherent. We show that these obstacles can be overcome using a semiconductor quantum wire network in the vicinity of an s-wave superconductor, by constructing topologically protected two-qubit gates and any arbitrary single-qubit phase gate in a topologically unprotected manner, which can be error corrected using magic-state distillation. Thus our strategy, using a judicious combination of topologically protected and unprotected gate operations, realizes UQC on a quantum wire network with a remarkably high error threshold of 0.14 as compared to 10-3 to 10-4 in ordinary unprotected quantum computation.
Quasiclassical theory of disordered multi-channel Majorana quantum wires
NASA Astrophysics Data System (ADS)
Neven, Patrick; Bagrets, Dmitry; Altland, Alexander
2013-05-01
Multi-channel spin-orbit quantum wires, when subjected to a magnetic field and proximity coupled to an s-wave superconductor, may support Majorana states. We study what happens to these systems in the presence of disorder. Inspired by the widely established theoretical methods of mesoscopic superconductivity, we develop á la Eilenberger a quasiclassical approach to topological nanowires valid in the limit of strong spin-orbit coupling. We find that the ‘Majorana number’ {\\cal M} , distinguishing between the state with Majorana fermions (symmetry class B) and no Majorana fermions (class D), is given by the product of two Pfaffians of gapped quasiclassical Green's functions fixed by the right and left terminals connected to the wire. A numerical solution of the Eilenberger equations reveals that the class D disordered quantum wires are prone to the formation of the zero-energy anomaly (class D impurity spectral peak) in the local density of states that shares the key features of the Majorana peak. In this way, we confirm the robustness of our previous conclusions (Bagrets and Altland 2012 Phys. Rev. Lett. 109 227005) on a more restrictive system setup. Generally speaking, we find that the quasiclassical approach provides a highly efficient means to address disordered class D superconductors both in the presence and in the absence of topological structures.
Topological phases and transport properties of screened interacting quantum wires
NASA Astrophysics Data System (ADS)
Xu, Hengyi; Xiong, Ye; Wang, Jun
2016-10-01
We study theoretically the effects of long-range and on-site Coulomb interactions on the topological phases and transport properties of spin-orbit-coupled quasi-one-dimensional quantum wires imposed on a s-wave superconductor. The distributions of the electrostatic potential and charge density are calculated self-consistently within the Hartree approximation. Due to the finite width of the wires and charge repulsion, the potential and density distribute inhomogeneously in the transverse direction and tend to accumulate along the lateral edges where the hard-wall confinement is assumed. This result has profound effects on the topological phases and the differential conductance of the interacting quantum wires and their hybrid junctions with superconductors. Coulomb interactions renormalize the gate voltage and alter the topological phases strongly by enhancing the topological regimes and producing jagged boundaries. Moreover, the multicritical points connecting different topological phases are modified remarkably in striking contrast to the predictions of the two-band model. We further suggest the possible non-magnetic topological phase transitions manipulated externally with the aid of long-range interactions. Finally, the transport properties of normal-superconductor junctions are further examined, in particular, the impacts of Coulomb interactions on the zero-bias peaks related to the Majorana fermions and near zero-energy peaks.
Phase-dependent electron transport through a quantum wire on a surface.
Kwapiński, T
2012-02-08
Electron transport through a quantum wire in the presence of external periodic energy-level modulations with different on-site phases is studied within the time evolution operator method for a tight-binding Hamiltonian. It is found that in the presence of spatial symmetry of the system and no source-drain and static gate voltages the pumping current can be generated. Moreover, for a wire which is tunnel-coupled to the underlying substrate, the current flowing through an unbiased wire does not fade away but increases with the wire-surface coupling. For randomly chosen phases at every wire site two regimes of the phase-averaged current are found which are related to small and high wire density of states.
Quantum stability and magic lengths of metal atom wires
NASA Astrophysics Data System (ADS)
Cui, Ping; Choi, Jin-Ho; Lan, Haiping; Cho, Jun-Hyung; Niu, Qian; Yang, Jinlong; Zhang, Zhenyu
2016-06-01
Metal atom wires represent an important class of nanomaterials in the development of future electronic devices and other functional applications. Using first-principles calculations within density functional theory, we carry out a systematic study of the quantum stability of freestanding atom wires consisting of prototypical metal elements with s -, s p -, and s d -valence electrons. We explore how the quantum mechanically confined motion and local bonding of the valence electrons in these different wire systems can dictate their overall structural stability and find that the formation energy of essentially all the wires oscillates with respect to their length measured by the number n of atoms contained in the wires, establishing the existence of highly preferred (or magic) lengths. Furthermore, different wire classes exhibit distinctively different oscillatory characteristics and quantum stabilities. Alkali metal wires possessing an unpaired s valence electron per atom exhibit simple damped even-odd oscillations. In contrast, Al and Ga wires containing three s2p1 valence electrons per atom generally display much larger and undamped even-odd energy oscillations due to stronger local bonding of the p orbitals. Among the noble metals, the s -dominant Ag wires behave similarly to the linear alkali metal wires, while Au and Pt wires distinctly prefer to be structurally zigzagged due to strong relativistic effects. These findings are discussed in connection with existing experiments and should also be instrumental in future experimental realization of different metal atom wires in freestanding or supported environments with desirable functionalities.
Research on quantum efficiency of GaN wire photocathode
NASA Astrophysics Data System (ADS)
Xia, Sihao; Liu, Lei; Diao, Yu; Kong, Yike
2017-02-01
On the basis of three-dimensional continuity equation in semiconductors and finite difference method, the carrier concentration and the quantum efficiency of GaN wire photocathode as a function of incident photon energy are achieved. Results show that the quantum efficiency of the wire photocathode is largely enhanced compared with the conventional planar photocathode. The superiority of the wire photocathode is reflected in its structure with surrounding surfaces. The quantum efficiency of the wire photocathode largely depends on the wire width, surface reflectivity, surface escape probability and incident angle of light. The back interface recombination rate, however, has little influences on the quantum efficiency of the wire photocathode. The simulation results suggest that the optimal width for photoemission is 150-200 nm. Besides, the quantum efficiency increases and decreases linearly with increasing surface escape probability and surface reflectivity, respectively. With increasing ratio of wire spacing to wire height, the optimal incident angle of light is reduced. These simulations are expected to guide the preparation of a better performing GaN wire photocathode.
Heat Current Fluctuations in Quantum Wires
NASA Astrophysics Data System (ADS)
Bogachek, E. N.; Krive, I. V.; Scherbakov, A. G.; Landman, Uzi
2001-03-01
The noise in the heat current for fermionic and bosonic systems is investigated in the Landauer-Buttiker approach. We show that the thermal (Jonhson-Nyquist) noise in perfect quantum wires does not depend on the statistics of the heat carriers. The nonequilibrium noise produced by the temperature difference between the heat reservoirs (hot and cold leads) is different for bosons and fermions. The contribution of ''fermionic'' nonequilibrium noise to the thermal noise is positive, while that due to ''bosonic'' noise is negative. At all temperatures the nonequilibrium noise P_Δ T is much smaller then the Johnson-Nyquist noise P_JN. Even in the most favorable situations | P_Δ T | /P_JN<= 0.3 for bosons and P_Δ T/P_JN<= 0.1 for fermions. The expressions obtained for bosons are applied for a description of the thermal transport through a Luttinger liquid constriction.
Peltier effect in strongly driven quantum wires
NASA Astrophysics Data System (ADS)
Mierzejewski, M.; Crivelli, D.; Prelovšek, P.
2014-08-01
We study a microscopic model of a thermocouple device with two connected correlated quantum wires driven by a constant electric field. In such a closed system we follow the time and position dependence of the entropy density using the concept of the reduced density matrix. At weak driving, the initial changes of the entropy at the junctions can be described by the linear Peltier response. At longer times the quasiequilibrium situation is reached with well defined local temperatures which increase due to an overall Joule heating. On the other hand, a strong electric field induces a nontrivial nonlinear thermoelectric response, e.g., the Bloch oscillations of the energy current. Moreover, we show for the doped Mott insulators that strong driving can reverse the Peltier effect.
Geometrical dependence of quantum decoherence in circular arenas with side-wires
NASA Astrophysics Data System (ADS)
Xie, Yuantao; Le Priol, Clément; Heremans, Jean J.
2016-12-01
Low-temperature quantum phase coherence lengths were experimentally measured in mesoscopic circular arenas fabricated on InGaAs quantum wells. The arenas are connected to wide sample regions by short side-wires, to investigate the effects of geometry in comparison to intrinsic materials properties on quantum decoherence. Universal conductance fluctuations were used to quantify the phase coherence lengths as a function of temperature and geometry. The experimental data show a dependence of phase coherence lengths on side-wire length and width-to-length ratio, which is accounted for by the competing effects of decoherence by coupling to the classical environment and Nyquist decoherence in ergodic wires. The observed decay of phase coherence lengths with the increasing temperature is consistent with expectations. The work demonstrates that geometrical effects influence the measured mesoscopic quantum decoherence.
Keselman, Anna; Fu, Liang; Stern, Ady; Berg, Erez
2013-09-13
We propose a setup to realize time-reversal-invariant topological superconductors in quantum wires, proximity coupled to conventional superconductors. We consider a model of quantum wire with strong spin-orbit coupling and proximity coupling to two s-wave superconductors. When the relative phase between the two superconductors is ϕ=π a Kramers pair of Majorana zero modes appears at each edge of the wire. We study the robustness of the phase in the presence of both time-reversal-invariant and time-reversal-breaking perturbations. In addition, we show that the system forms a natural realization of a fermion parity pump, switching the local fermion parity of both edges when the relative phase between the superconductors is changed adiabatically by 2π.
Theory of Spin Seebeck Effects in a Quantum Wire
NASA Astrophysics Data System (ADS)
Ogata, Masao; Fukuyama, Hidetoshi
2017-09-01
Spin Seebeck coefficient in a quantum wire is microscopically derived using the Kubo formula and thermal Green’s functions, taking account of the effects of disorder in a self-consistent t-matrix approximation. It is found that the induced spin current to be detected through the inverse spin Hall effect will be in the range of experimental detectability when the chemical potential for electrons in the quantum wire is close to the band edge.
Si, Ge, and SiGe quantum wires and quantum dots
NASA Astrophysics Data System (ADS)
Pearsall, T. P.
This document is part of subvolume C3 'Optical Properties' of volume 34 'Semiconductor quantum structures' of Landolt-Börnstein, Group III, Condensed Matter, on the optical properties of quantum structures based on group IV semiconductors. It discusses Si, Ge, and SiGe quantum wire and quantum dot structures, the synthesis of quantum wires and quantum dots, and applications of SiGe quantum-dot structures as photodetectors, light-emitting diodes, for optical amplification and as Si quantum-dot memories.
Spectrum of localized states in graphene quantum dots and wires
NASA Astrophysics Data System (ADS)
Zalipaev, V. V.; Maksimov, D. N.; Linton, C. M.; Kusmartsev, F. V.
2013-01-01
We developed semiclassical method and show that any smooth potential in graphene describing elongated a quantum dot or wire may behave as a barrier or as a trapping well or as a double barrier potential, Fabry-Perot structure, for 1D Schrödinger equation. The energy spectrum of quantum wires has been found and compared with numerical simulations. We found that there are two types of localized states, stable and metastable, having finite life time. These life times are calculated, as is the form of the localized wave functions which are exponentially decaying away from the wire in the perpendicular direction.
NASA Astrophysics Data System (ADS)
Thomas, George; Johal, Ramandeep S.
2011-03-01
We study the one-dimensional isotropic Heisenberg model of two spin-1/2 systems as a quantum heat engine. The engine undergoes a four-step Otto cycle where the two adiabatic branches involve changing the external magnetic field at a fixed value of the coupling constant. We find conditions for the engine efficiency to be higher than in the uncoupled model; in particular, we find an upper bound which is tighter than the Carnot bound. A domain of parameter values is pointed out which was not feasible in the interaction-free model. Locally, each spin seems to cause a flow of heat in a direction opposite to the global temperature gradient. This feature is explained by an analysis of the local effective temperature of the spins.
Thomas, George; Johal, Ramandeep S
2011-03-01
We study the one-dimensional isotropic Heisenberg model of two spin-1/2 systems as a quantum heat engine. The engine undergoes a four-step Otto cycle where the two adiabatic branches involve changing the external magnetic field at a fixed value of the coupling constant. We find conditions for the engine efficiency to be higher than in the uncoupled model; in particular, we find an upper bound which is tighter than the Carnot bound. A domain of parameter values is pointed out which was not feasible in the interaction-free model. Locally, each spin seems to cause a flow of heat in a direction opposite to the global temperature gradient. This feature is explained by an analysis of the local effective temperature of the spins.
Colloidal GaAs quantum wires: solution-liquid-solid synthesis and quantum-confinement studies.
Dong, Angang; Yu, Heng; Wang, Fudong; Buhro, William E
2008-05-07
Colloidal GaAs quantum wires with diameters of 5-11 nm and narrow diameter distributions (standard deviation = 12-21% of the mean diameter) are grown by two methods based on the solution-liquid-solid (SLS) mechanism. Resolved excitonic absorption features arising from GaAs quantum wires are detected, allowing extraction of the size-dependent effective band gaps of the wires. The results allow the first systematic comparison of the size dependences of the effective band gaps in corresponding sets of semiconductor quantum wires and quantum wells. The GaAs quantum wire and well band gaps scale according to the prediction of a simple effective-mass-approximation, particle-in-a-box (EMA-PIB) model, which estimates the kinetic confinement energies of electron-hole pairs in quantum nanostructures of different shapes and confinement dimensionalities.
Growth and Characterization of III-V Nitride Quantum Dots and Quantum Wires
2010-03-26
REPORT Growth and Characterization of III - V Nitride Quantum Dots and Quantum Wires 14. ABSTRACT 16. SECURITY CLASSIFICATION OF: Our research program...ANSI Std. Z39.18 - 14-Sep-2009 Final report Growth and Characterization of III - V Nitride Quantum Dots and Quantum Wires Statement of the...has two interrelated components: the growth of GaN nanowires and the fabrication of electronic devices, including gas sensors, on these nanowires . A
Observation of charged excitons in V-groove quantum wires
NASA Astrophysics Data System (ADS)
Otterburg, T.; Oberli, D. Y.; Dupertuis, M.-A.; Dwir, B.; Pelucchi, E.; Kapon, E.
2004-02-01
We report on the observation of negatively and positively charged excitons in the photoluminescence spectra of V-groove quantum wires. The charged exciton binding energy increases with the strength of the quantum confinement. We demonstrate that the charged excitons are localized by the fluctuations of the confinement potential and estimate a minimal value of the localization length.
Optical properties of GaN wurtzite quantum wires
NASA Astrophysics Data System (ADS)
Zhang, X. W.; Xia, J. B.
2006-03-01
The electronic structure and optical properties of freestanding GaN wurtzite quantum wires are studied in the framework of six-band effective-mass envelope function theory. It is found that the electron states are either twofold or fourfold degenerate. There is a dark exciton effect when the radius R of GaN wurtzite quantum wires is in the range of [0.7, 10.9] nm. The linear polarization factors are calculated in three cases, the quantum confinement effect (finite long wire), the dielectric effect and both effects (infinitely long wire). It is found that the linear polarization factor of a finite long wire whose length is much less than the electromagnetic wavelength decreases as R increases, is very close to unity (0.979) at R = 1 nm, and changes from a positive value to a negative value around R = 4.1 nm. The linear polarization factor of the dielectric effect is 0.934, independent of radius, as long as the radius remains much less than the electromagnetic wavelength. The result for the two effects shows that the quantum confinement effect gives a correction to the dielectric effect result. It is found that the linear polarization factor of very long (treated approximately as infinitely long) quantum wires is in the range of [0.8, 1]. The linear polarization factors of the quantum confinement effect of CdSe wurtzite quantum wires are calculated for comparison. In the CdSe case, the linear polarization factor of R = 1 nm is 0.857, in agreement with the experimental results (Hu et al 2001 Science 292 2060). This value is much smaller than unity, unlike 0.979 in the GaN case, mainly due to the big spin-orbit splitting energy Δso of CdSe material with wurtzite structure.
Inversion of hysteresis in quantum dot controlled quantum-wire transistor
NASA Astrophysics Data System (ADS)
Müller, C. R.; Worschech, L.; Forchel, A.
2009-05-01
In a quantum-wire transistor, pronounced floating-gate function of quantum dots is demonstrated with large threshold hysteresis exceeding 1.5 V. The charge state of the quantum dots is electrically controlled and, by applying a critical bias voltage along the quantum wire, the charging mechanism of the quantum dots is deactivated or, for bias voltages above this critical bias point, even inverted. It is shown that the charging as well as discharging of the quantum dots can be selectively switched off; i.e., the floating-gate function of the quantum dots is suppressed. The inversion of the hysteresis is explained within the framework of a capacitor model and the control of the charging mechanism is attributed to a dynamic gate efficiency of the quantum wire, which can be either larger or smaller than the quantum dot gate efficiency.
The Quantum Socket: Wiring for Superconducting Qubits - Part 2
NASA Astrophysics Data System (ADS)
Bejanin, J. H.; McConkey, T. G.; Rinehart, J. R.; Bateman, J. D.; Earnest, C. T.; McRae, C. H.; Rohanizadegan, Y.; Shiri, D.; Mariantoni, M.; Penava, B.; Breul, P.; Royak, S.; Zapatka, M.; Fowler, A. G.
Quantum computing research has reached a level of maturity where quantum error correction (QEC) codes can be executed on linear arrays of superconducting quantum bits (qubits). A truly scalable quantum computing architecture, however, based on practical QEC algorithms, requires nearest neighbor interaction between qubits on a two-dimensional array. Such an arrangement is not possible with techniques that rely on wire bonding. To address this issue, we have developed the quantum socket, a device based on three-dimensional wires that enables the control of superconducting qubits on a two-dimensional grid. In this talk, we present experimental results characterizing this type of wiring. We will show that the quantum socket performs exceptionally well for the transmission and reflection of microwave signals up to 10 GHz, while minimizing crosstalk between adjacent wires. Under realistic conditions, we measured an S21 of -5 dB at 6 GHz and an average crosstalk of -60 dB. We also describe time domain reflectometry results and arbitrary pulse transmission tests, showing that the quantum socket can be used to control superconducting qubits.
IR photodetector based on rectangular quantum wire in magnetic field
Jha, Nandan
2014-04-24
In this paper we study rectangular quantum wire based IR detector with magnetic field applied along the wires. The energy spectrum of a particle in rectangular box shows level repulsions and crossings when external magnetic field is applied. Due to this complex level dynamics, we can tune the spacing between any two levels by varying the magnetic field. This method allows user to change the detector parameters according to his/her requirements. In this paper, we numerically calculate the energy sub-band levels of the square quantum wire in constant magnetic field along the wire and quantify the possible operating wavelength range that can be obtained by varying the magnetic field. We also calculate the photon absorption probability at different magnetic fields and give the efficiency for different wavelengths if the transition is assumed between two lowest levels.
Interaction-induced backscattering in short quantum wires
Rieder, M. -T.; Micklitz, T.; Levchenko, A.; Matveev, K. A.
2014-10-06
We study interaction-induced backscattering in clean quantum wires with adiabatic contacts exposed to a voltage bias. Particle backscattering relaxes such systems to a fully equilibrated steady state only on length scales exponentially large in the ratio of bandwidth of excitations and temperature. Here in this paper we focus on shorter wires in which full equilibration is not accomplished. Signatures of relaxation then are due to backscattering of hole excitations close to the band bottom which perform a diffusive motion in momentum space while scattering from excitations at the Fermi level. This is reminiscent to the first passage problem of a Brownian particle and, regardless of the interaction strength, can be described by an inhomogeneous Fokker-Planck equation. From general solutions of the latter we calculate the hole backscattering rate for different wire lengths and discuss the resulting length dependence of interaction-induced correction to the conductance of a clean single channel quantum wire.
Papp, E.; Micu, C.; Racolta, D.
2013-11-13
In this paper one deals with the theoretical derivation of energy bands and of related wavefunctions characterizing quasi 1D semiconductor heterostructures, such as InAs quantum wire models. Such models get characterized this time by equal coupling strength superpositions of Rashba and Dresselhaus spin-orbit interactions of dimensionless magnitude a under the influence of in-plane magnetic fields of magnitude B. We found that the orientations of the field can be selected by virtue of symmetry requirements. For this purpose one resorts to spin conservations, but alternative conditions providing sensible simplifications of the energy-band formula can be reasonably accounted for. Besides the wavenumber k relying on the 1D electron, one deals with the spin-like s=±1 factors in the front of the square root term of the energy. Having obtained the spinorial wavefunction, opens the way to the derivation of spin precession effects. For this purpose one resorts to the projections of the wavenumber operator on complementary spin states. Such projections are responsible for related displacements proceeding along the Ox-axis. This results in a 2D rotation matrix providing both the precession angle as well as the precession axis.
Time-Domain Simulation of Three Dimensional Quantum Wires
Mossman, Sean; Kuzyk, Mark G.
2016-01-01
A method is presented to calculate the eigenenergies and eigenfunctions of quantum wires. This is a true three-dimensional method based on a direct implementation of the time-dependent Schrödinger equation. It makes no approximations to the Schrödinger equation other than the finite-difference approximation of the space and time derivatives. The accuracy of our method is tested by comparing it to analytical results in a cylindrical wire. PMID:27124603
A Novel InAs Quantum Wire System
2003-01-01
length. In summary, we have developed a novel technique for the fabrication of one-dimensional quantum wires. The new fabrication method utilize the...Available online at www.sciencedirect.com Physica E 17 (2003) 161–163 www.elsevier.com/locate/physe A novel InAs quantum wire system C.H. Yanga...reserved. doi:10.1016/S1386-9477(02)00731-2 Report Documentation Page Form ApprovedOMB No. 0704-0188 Public reporting burden for the collection of
Conductance of a quantum wire in the Wigner-crystal regime.
Matveev, K A
2004-03-12
We study the effect of Coulomb interactions on the conductance of a single-mode quantum wire connecting two bulk leads. When the density of electrons in the wire is very low, they arrange in a finite-length Wigner crystal. In this regime the electron spins form an antiferromagnetic Heisenberg chain with an exponentially small coupling J. An electric current in the wire perturbs the spin chain and gives rise to a temperature-dependent contribution of the spin subsystem to the resistance. At low temperature T
Strongly Coupled Quantum Heat Machines
Gelbwaser-Klimovsky, David; Aspuru-Guzik, Alán
2015-09-03
Energy conversion of heat into work at the quantum level is modeled by quantum heat machines (QHMs) generally assumed to operate at weak coupling to the baths. This supposition is grounded in the separability principle between systems and allows the derivation of the evolution equation. In the weak coupling regime, the machine’s output is limited by the coupling strength, restricting their application. Seeking to overcome this limitation, we analyze QHMs in the virtually unexplored strong coupling regime here, where separability, as well as other standard thermodynamic assumptions, may no longer hold. We show that strongly coupled QHMs may be as efficient as their weakly coupled counterparts. In addition, we find a novel turnover behavior where their output saturates and disappears in the limit of ultrastrong coupling.
The Quantum Socket: Wiring for Superconducting Qubits - Part 1
NASA Astrophysics Data System (ADS)
McConkey, T. G.; Bejanin, J. H.; Rinehart, J. R.; Bateman, J. D.; Earnest, C. T.; McRae, C. H.; Rohanizadegan, Y.; Shiri, D.; Mariantoni, M.; Penava, B.; Breul, P.; Royak, S.; Zapatka, M.; Fowler, A. G.
Quantum systems with ten superconducting quantum bits (qubits) have been realized, making it possible to show basic quantum error correction (QEC) algorithms. However, a truly scalable architecture has not been developed yet. QEC requires a two-dimensional array of qubits, restricting any interconnection to external classical systems to the third axis. In this talk, we introduce an interconnect solution for solid-state qubits: The quantum socket. The quantum socket employs three-dimensional wires and makes it possible to connect classical electronics with quantum circuits more densely and accurately than methods based on wire bonding. The three-dimensional wires are based on spring-loaded pins engineered to insure compatibility with quantum computing applications. Extensive design work and machining was required, with focus on material quality to prevent magnetic impurities. Microwave simulations were undertaken to optimize the design, focusing on the interface between the micro-connector and an on-chip coplanar waveguide pad. Simulations revealed good performance from DC to 10 GHz and were later confirmed against experimental measurements.
Quantum optics in coupled quantum dots
NASA Astrophysics Data System (ADS)
Garrido, Mauricio
Coupled quantum dots present an active field of study, both at the fundamental and applied level, due to their atomic and molecular-like energy structure and the ability to design and tune their parameters. Being single-photon emitters, they are systems that behave fully according to the laws of quantum mechanics. The work presented here involved the experimental study of the electro-optical properties of Indium Arsenide, coupled quantum dots. Initial experiments involved the use of spectroscopic methods such as photoluminescence and photoluminescence excitation (PLE). Through such techniques, the top dot's hole energy level structure was mapped and different types of resonant absorption were identified. The characterization of these excited states and the knowledge of how to resonantly excite into them is an integral part of the development of certain controlled spin gates in quantum computation. Additionally, a shift of the spectra in the electric field was observed with varying excitation wavelength through and above the wetting layer, which allowed for direct measurement of the optically-created electric field within the device. This extends the quantum dots' capabilities to using them as electric-field nano-probes and opens up the possibility of an all-optical, fast switching mechanism. In the course of these studies, a novel data visualization method for PLE in this type of system was developed. Finally, to study correlated photon effects, a Hanbury Brown - Twiss experiment was built which revealed bunching and antibunching signals typical of quantum statistics in biexciton cascade emissions. This is an important step towards the experimental investigation of entangled states in coupled quantum dots.
Lattice thermal conductance of quantum wires with disorder
NASA Astrophysics Data System (ADS)
Vyhmeister, Erik; Hershfield, Selman
We model the lattice thermal conductance in long quantum wires connected to two large heat baths at different temperatures in the harmonic approximation. The thermal conductance is computed with the Landauer formula for phonons, where it is related to the sum over all transmission probabilities for phonons through the wire. The net transmission probability is computed using a recursive Green function technique, which allows one to study long wires efficiently. We consider several different kinds of disorder to reduce the lattice thermal conductivity: periodic rectangular holes of varying sizes and shapes, periodic triangular holes, and narrow bands, averaged over randomness to account for variance in manufacturing. Depending on the model, the thermal conductance was reduced by 80 percent or more from the perfectly ordered wire case. Funded by NSF grant DMR-1461019.
Quantum Dot with Attached Wires: Resonant States Completeness
NASA Astrophysics Data System (ADS)
Popov, I. Y.; Popov, A. I.
2017-08-01
3D quantum dot with two 1D wires attached is considered. Scattering matrix is constructed in the framework of Lax-Phillips approach. The completeness of resonant states is proved using the factorization criterion for the characteristic function of Sz.-Nagy functional model.
Interlevel cascade transition in electrically confined quantum wire arrays.
Wu, Wei; Hassani, Iman; Mohseni, Hooman
2011-09-27
Vertical stacks of electrically confined quantum wires were demonstrated in devices with large areas. Multiple current plateaus and strong differential conductance oscillations were observed at above liquid nitrogen temperatures because of interlevel cascade transition of carriers. Our simulation results for charge transport, as well as interlevel infrared photoresponse red-shift, due to lateral electric field confinement show good agreement with experimental data.
Coupled Quantum Fluctuations and Quantum Annealing
NASA Astrophysics Data System (ADS)
Hormozi, Layla; Kerman, Jamie
We study the relative effectiveness of coupled quantum fluctuations, compared to single spin fluctuations, in the performance of quantum annealing. We focus on problem Hamiltonians resembling the the Sherrington-Kirkpatrick model of Ising spin glass and compare the effectiveness of different types of fluctuations by numerically calculating the relative success probabilities and residual energies in fully-connected spin systems. We find that for a small class of instances coupled fluctuations can provide improvement over single spin fluctuations and analyze the properties of the corresponding class. Disclaimer: This research was funded by ODNI, IARPA via MIT Lincoln Laboratory under Air Force Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the US Government.
Interface effect in coupled quantum wells
Hao, Ya-Fei
2014-06-28
This paper intends to theoretically investigate the effect of the interfaces on the Rashba spin splitting of two coupled quantum wells. The results show that the interface related Rashba spin splitting of the two coupled quantum wells is both smaller than that of a step quantum well which has the same structure with the step quantum well in the coupled quantum wells. And the influence of the cubic Dresselhaus spin-orbit interaction of the coupled quantum wells is larger than that of a step quantum well. It demonstrates that the spin relaxation time of the two coupled quantum wells will be shorter than that of a step quantum well. As for the application in the spintronic devices, a step quantum well may be better than the coupled quantum wells, which is mentioned in this paper.
Lens Coupled Quantum Cascade Laser
NASA Technical Reports Server (NTRS)
Hu, Qing (Inventor); Lee, Alan Wei Min (Inventor)
2013-01-01
Terahertz quantum cascade (QC) devices are disclosed that can operate, e.g., in a range of about 1 THz to about 10 THz. In some embodiments, QC lasers are disclosed in which an optical element (e.g., a lens) is coupled to an output facet of the laser's active region to enhance coupling of the lasing radiation from the active region to an external environment. In other embodiments, terahertz amplifier and tunable terahertz QC lasers are disclosed.
Gate-Defined Wires in HgTe Quantum Wells: From Majorana Fermions to Spintronics
NASA Astrophysics Data System (ADS)
Reuther, Johannes; Alicea, Jason; Yacoby, Amir
2013-07-01
We introduce a promising new platform for Majorana zero modes and various spintronics applications based on gate-defined wires in HgTe quantum wells. Because of the Dirac-like band structure for HgTe, the physics of such systems differs markedly from that of conventional quantum wires. Most strikingly, we show that the subband parameters for gate-defined HgTe wires exhibit exquisite tunability: Modest gate voltage variation allows one to modulate the Rashba spin-orbit energies from zero up to about 30 K, and the effective g factors from zero up to giant values exceeding 600. The large achievable spin-orbit coupling and g factors together allow one to access Majorana modes in this setting at exceptionally low magnetic fields while maintaining robustness against disorder. As an additional benefit, gate-defined wires (in HgTe or other settings) should greatly facilitate the fabrication of networks for refined transport experiments used to detect Majoranas, as well as the realization of non-Abelian statistics and quantum information devices.
Hetero-junction of two quantum wires: Critical line and duality
NASA Astrophysics Data System (ADS)
Lee, Taejin
2016-08-01
Applying the Fermi-Bose equivalence and the boundary state formulation, we study the hetero-junction of two quantum wires. Two quantum wires are described by using Tomonaga-Luttinger (TL) liquids with different TL parameters, and electron transport between the two wires is depicted by using a simple hopping interaction. We calculate the radiative corrections to the hopping interaction and obtain the renormalization (RG) exponent, making use of perturbation theory based on the boundary state formulation. The model exhibits a phase transition at zero temperature. We discuss the critical line that defines the phase boundary on the two-dimensional parameter space. The model also exhibits the particle-kink duality, which maps the strong coupling region of the model onto the weak coupling region of the dual model. The strong coupling region of the model is found to match exactly the weak coupling region of the dual model. This model is also important to study the critical behaviors of two-dimensional dissipative systems with anisotropic friction coefficients.
Signatures of subband quantization in the Coulomb blockade regime of a disordered quantum wire
NASA Astrophysics Data System (ADS)
Liu, Wei; He, Jianhong; Guo, Huazhong; Gao, Jie
2017-01-01
We report experiments on the two-terminal conductance of a long disordered quantum wire in a perpendicular magnetic field. Pronouncedly enhanced magnetoconductance in magnetic fields of intermediate strength is observed in the Coulomb blockade regime, which is well explained using the boundary roughness scattering and the subband quantization of the quantum wire, by modeling the disordered quantum wire as that of a quantum dot defined in a quantum wire. Assuming a parabolic constriction in the disordered quantum wire, we further obtained the magnetic field dependence of high energy levels in the quantum dot and the gate voltage dependence of the effective width of the quantum wire. Our results may provide useful information for further studies on integrated structures in on-chip laboratories.
Development and Characterization of Intermediate-Band Quantum Wire Solar Cells
NASA Astrophysics Data System (ADS)
Furrow, Colin S.
The effects of a quantum wire intermediate band, grown by molecular beam epitaxy, on the optical and electrical properties of solar cells are reported. To investigate the behavior of the intermediate band, the quantum wires were remotely doped at three different doping concentrations, the number of quantum wire layers was varied from three to twenty, and the solar cell structure was optimized. For all the structures, current-voltage and external quantum efficiency measurements were performed to examine the effect of absorption and power conversion of the intermediate band solar cell (IBSC). Time-resolved photoluminescence measurements showed that ?-doping can increase the lifetime of the excited electrons in the quantum wires. The quantum efficiency measurements revealed that the quantum wires extend the absorption spectrum in the infrared and produce a photocurrent by absorption of photons with energies below the GaAs band gap energy. In addition, the quantum wire intermediate band solar cell increased the solar conversion efficiency by 13.3% over the reference cell. An increase in the quantum efficiency was observed by increasing the number of quantum wire layers in the intermediate band. Furthermore, by optimizing the solar cell structure, the quantum efficiency and solar power conversion efficiency were substantially improved. Finally, temperature dependent current-voltage measurements reveal that the quantum wire intermediate band does not degrade the temperature sensitivity of the device. This research shows the potential for a quantum wire intermediate band as a viable option for creating higher efficiency solar cell devices.
Gate-defined wires in HgTe quantum wells as a robust Majorana platform
NASA Astrophysics Data System (ADS)
Reuther, Johannes; Alicea, Jason; Yacoby, Amir
2013-03-01
We propose a new quasi-1D platform for Majorana zero-modes based on gate-defined wires in HgTe. Due to the Dirac-like band structure for HgTe such wires exhibit several remarkable properties. Most strikingly, modest gate-tuning allows one to modulate the Rashba spin-orbit energy from zero up to ~ 30 K , and the effective g-factor from zero up to giant values of ~ 600 . The large achievable spin-orbit coupling and g-factor together allow one to access Majorana modes in this setting at exceptionally low magnetic fields while maintaining robustness against disorder. Moreover, gate-defined wires may facilitate the fabrication of networks required for realizing non-Abelian statistics and quantum information devices. The exquisite tunablity of parameters further suggests applications in spintronics. Research supported by the Deutsche Akademie der Naturforscher Leopoldina through Grant No. LPDS 2011-14.
Quantum wires as sensors of the electric field: a model into quantum plasmonics
NASA Astrophysics Data System (ADS)
Alves, R. A.; Costa, J. C.; Gomes, M.; Silva, Nuno A.; Guerreiro, A.
2017-04-01
This paper presents a study for a fibre optic sensor based on quantum wires to detect and measure the amplitude and direction of a static electric field. This study is supported by the analogy of the fluid equations describing the free electrons in the quantum wires and the Madelung formalism of Quantum Mechanics. In this context, it is possible to construct a diatomic plasmonic molecule whose energy levels can be Stark shifted by an external electric field and readout using a light beam tuned to the Rabi oscillations of these levels. Choosing the adequate design parameters it is possible to estimate a sensitivity of 100nm/NC-1.
Dissipation in a Quantum Wire: Fact and Fantasy
NASA Astrophysics Data System (ADS)
Das, Mukunda P.; Green, Frederick
2008-10-01
Where, and how, does energy dissipation of electrical energy take place in a ballistic wire? Fully two decades after the advent of the transmissive phenomenology of electrical conductance, this deceptively simple query remains unanswered. We revisit the quantum kinetic basis of dissipation and show its power to give a definitive answer to our query. Dissipation leaves a clear, quantitative trace in the non-equilibrium current noise of a quantum point contact; this signature has already been observed in the laboratory. We then highlight the current state of accepted understandings in the light of well-known yet seemingly contradictory measurements. The physics of mesoscopic transport rests not in coherent carrier transmission through a perfect and dissipationless metallic channel, but explicitly in their dissipative inelastic scattering at the wire's interfaces and adjacent macroscopic leads.
Wigner crystallization in quantum wires within the Yukawa approximation
NASA Astrophysics Data System (ADS)
Méndez-Camacho, Reyna; Cruz-Hernández, Esteban; Castañeda-Priego, Ramón
2017-02-01
One crucial and important aspect to account for the nature of the quantum wires is the understanding of the effects associated to many-body interactions between confined electrons. The inclusion of such many-body forces in any theoretical framework is a difficult and computationally demanding task. Then one has to make use of coarse-grained descriptions that allow one to incorporate the contribution of all the electrons. In a simple physical picture, the interaction between two electrons can be considered screened due to the presence of the other ones. If the latter are homogeneously distributed inside the wire, the interaction between the former can then be assumed of the Yukawa form. In this contribution, we report on the lower energy states of n -doped GaAs circular-quantum wires with two electrons in the conduction band interacting through a repulsive Yukawa potential. By varying the length and the electronic density of the wire, quite different trends in the electronic distribution are observed. By changing the material parameters to InSb and InAs nanowires, we found that our results are consistent with available experimental data that have reported the formation of Wigner crystals.
Interaction-induced backscattering in short quantum wires
Rieder, M. -T.; Micklitz, T.; Levchenko, A.; ...
2014-10-06
We study interaction-induced backscattering in clean quantum wires with adiabatic contacts exposed to a voltage bias. Particle backscattering relaxes such systems to a fully equilibrated steady state only on length scales exponentially large in the ratio of bandwidth of excitations and temperature. Here in this paper we focus on shorter wires in which full equilibration is not accomplished. Signatures of relaxation then are due to backscattering of hole excitations close to the band bottom which perform a diffusive motion in momentum space while scattering from excitations at the Fermi level. This is reminiscent to the first passage problem of amore » Brownian particle and, regardless of the interaction strength, can be described by an inhomogeneous Fokker-Planck equation. From general solutions of the latter we calculate the hole backscattering rate for different wire lengths and discuss the resulting length dependence of interaction-induced correction to the conductance of a clean single channel quantum wire.« less
Charged excitons in modulation-doped quantum wires
NASA Astrophysics Data System (ADS)
Otterburg, T.; Oberli, D. Y.; Dupertuis, M.-A.; Moret, N.; Malko, A.; Pelucchi, E.; Dwir, B.; Kapon, E.
2005-06-01
We report on the observation of negatively- and positively-charged excitons in the photoluminescence spectra of V-groove quantum wires. The charged exciton binding energy increases with the strength of the quantum confinement. We demonstrate that fluctuations of the confinement potential cause the localization of the exciton and of the charged exitons on the same location. We discover that a large fraction of the enhancement of the charged exciton "binding energies" has a kinetic origin associated with the recoil energy transferred to the remaining carrier during the emission process.
NASA Astrophysics Data System (ADS)
Saha, Arijit; Rainis, Diego; Tiwari, Rakesh P.; Loss, Daniel
2014-07-01
We study the phenomenon of adiabatic quantum charge pumping in systems supporting fractionally charged fermionic bound states in two different setups. The first quantum pump setup consists of a charge density modulated quantum wire, and the second one is based on a semiconducting nanowire with Rashba spin-orbit interaction, in the presence of a spatially oscillating magnetic field. In both these quantum pumps transport is investigated in an N-X-N geometry, with the system of interest (X) connected to two normal-metal leads (N), and the two pumping parameters are the strengths of the effective wire-lead barriers. Pumped charge is calculated within the scattering matrix formalism. We show that quantum pumping in both setups provides a unique signature of the presence of the fractional-fermion bound states, in terms of the asymptotically quantized pumped charge. Furthermore, we investigate shot noise arising due to quantum pumping, verifying that the quantized pumped charge corresponds to minimal shot noise.
Cadmium selenide quantum wires and the transition from 3D to 2D confinement.
Yu, Heng; Li, Jingbo; Loomis, Richard A; Gibbons, Patrick C; Wang, Lin-Wang; Buhro, William E
2003-12-31
Soluble CdSe quantum wires are prepared by the solution-liquid-solid mechanism, using monodisperse bismith nanoparticles to catalyze wire growth. The quantum wires have micrometer lengths, diameters in the range of 5-20 nm, and diameter distributions of +/-10-20%. Spectroscopically determined wire band gaps compare closely to those calculated by the semiemipirical pseudopotential method, confirming 2D quantum confinement. The diameter dependence of the quantum wire band gaps is compared to that of CdSe quantum dots and rods. Quantum rod band gaps are shown to be delimited by the band gaps of dots and wires of like diameter, for short and long rods, respectively. The experimental data suggest that a length of ca. 30 nm is required for the third dimension of quantum confinement to fully vanish in CdSe rods. That length is about six times the bulk CdSe exciton Bohr radius.
Optophononics with Coupled Quantum Dots
2014-02-18
the molecular polaron can be used as an efficient and tunable coherent coupler for quantum states in spatially separated low-dimensional structures...cold finger of a closed cycle microscopy cryostat and kept at a temperature of 20K. A tunable diode laser with a tuning range from about 900 to 1,000...et al. Tunable exciton relaxation in vertically coupled semiconductor InAs quantum dots. Phys. Rev. B 84, 081404(R) (2011). 10 100 1,000 0 5 A m pl ifi
Quantum speed meter based on dissipative coupling
NASA Astrophysics Data System (ADS)
Vyatchanin, Sergey P.; Matsko, Andrey B.
2017-01-01
We consider dissipative coupling Fabry-Perot cavity, i.e. its input mirror transmittance depends on position of probe mass. We show that dissipative coupling provide possibility to realize quantum speed meter by natural way, without additional setup for subtraction of position x(t) and delayed position x(t-τ). Quantum speed meter is a quantum non demolition (QND) meter which allow to overcome Standatd Quantum Limit — we show it for speed meter based on dissipative coupling.
NASA Astrophysics Data System (ADS)
Morath, D.; Sedlmayr, N.; Sirker, J.; Eggert, S.
2016-09-01
We study electron and spin transport in interacting quantum wires contacted by noninteracting leads. We theoretically model the wire and junctions as an inhomogeneous chain where the parameters at the junction change on the scale of the lattice spacing. We study such systems analytically in the appropriate limits based on Luttinger liquid theory and compare the results to quantum Monte Carlo calculations of the conductances and local densities near the junction. We first consider an inhomogeneous spinless fermion model with a nearest-neighbor interaction and then generalize our results to a spinful model with an on-site Hubbard interaction.
Three-Dimensional Wiring for Extensible Quantum Computing: The Quantum Socket
NASA Astrophysics Data System (ADS)
Béjanin, J. H.; McConkey, T. G.; Rinehart, J. R.; Earnest, C. T.; McRae, C. R. H.; Shiri, D.; Bateman, J. D.; Rohanizadegan, Y.; Penava, B.; Breul, P.; Royak, S.; Zapatka, M.; Fowler, A. G.; Mariantoni, M.
2016-10-01
Quantum computing architectures are on the verge of scalability, a key requirement for the implementation of a universal quantum computer. The next stage in this quest is the realization of quantum error-correction codes, which will mitigate the impact of faulty quantum information on a quantum computer. Architectures with ten or more quantum bits (qubits) have been realized using trapped ions and superconducting circuits. While these implementations are potentially scalable, true scalability will require systems engineering to combine quantum and classical hardware. One technology demanding imminent efforts is the realization of a suitable wiring method for the control and the measurement of a large number of qubits. In this work, we introduce an interconnect solution for solid-state qubits: the quantum socket. The quantum socket fully exploits the third dimension to connect classical electronics to qubits with higher density and better performance than two-dimensional methods based on wire bonding. The quantum socket is based on spring-mounted microwires—the three-dimensional wires—that push directly on a microfabricated chip, making electrical contact. A small wire cross section (approximately 1 mm), nearly nonmagnetic components, and functionality at low temperatures make the quantum socket ideal for operating solid-state qubits. The wires have a coaxial geometry and operate over a frequency range from dc to 8 GHz, with a contact resistance of approximately 150 m Ω , an impedance mismatch of approximately 10 Ω , and minimal cross talk. As a proof of principle, we fabricate and use a quantum socket to measure high-quality superconducting resonators at a temperature of approximately 10 mK. Quantum error-correction codes such as the surface code will largely benefit from the quantum socket, which will make it possible to address qubits located on a two-dimensional lattice. The present implementation of the socket could be readily extended to accommodate a
Optophononics with coupled quantum dots.
Kerfoot, Mark L; Govorov, Alexander O; Czarnocki, Cyprian; Lu, Davis; Gad, Youstina N; Bracker, Allan S; Gammon, Daniel; Scheibner, Michael
2014-01-01
Modern technology is founded on the intimate understanding of how to utilize and control electrons. Next to electrons, nature uses phonons, quantized vibrations of an elastic structure, to carry energy, momentum and even information through solids. Phonons permeate the crystalline components of modern technology, yet in terms of technological utilization phonons are far from being on par with electrons. Here we demonstrate how phonons can be employed to render a single quantum dot pair optically transparent. This phonon-induced transparency is realized via the formation of a molecular polaron, the result of a Fano-type quantum interference, which proves that we have accomplished making typically incoherent and dissipative phonons behave in a coherent and non-dissipative manner. We find the transparency to be widely tunable by electronic and optical means. Thereby we show amplification of weakest coupling channels. We further outline the molecular polaron's potential as a control element in phononic circuitry architecture.
van Hove singularities in disordered multichannel quantum wires and nanotubes
NASA Astrophysics Data System (ADS)
Hügle, S.; Egger, R.
2002-11-01
We present a theory for the van Hove singularity (VHS) in the tunneling density of states (TDOS) of disordered multichannel quantum wires, in particular multiwall carbon nanotubes. We assume close-by gates that screen off electron-electron interactions. Diagrammatic perturbation theory within the noncrossing approximation yields analytical expressions governing the disorder-induced broadening and shift of VHS's as new subbands are opened. This problem is nontrivial because the (lowest-order) Born approximation breaks down close to the VHS. Interestingly, compared to the bulk case, the boundary TDOS shows drastically altered VHS's, even in the clean limit.
Polariton dispersion of a quantum wire superlattice system
NASA Astrophysics Data System (ADS)
Wilson, K. S. Joseph; Amalanathan, M.; Revathy, V.; Lenin, S. Maria
2015-06-01
Superlattices have drawn considerable attention in the recent years. In this work, the behaviour of polaritons in a quantum wire superlattice is analysed both at the brillouin zone edge and at centre of the brillouin zone using LiNbO3/ LiTaO3 as an example. The significance of the polariton modes in both the cases are analysed. New modes on the polaritonic gap, where the propagation of electromagnetic wave is forbidden, is obtained in the system as suggested by some recent literature. The effect on nonlinear interactions of phonon polaritons in LiNbO3/ LiTaO3 superlattices is also discussed.
Polariton dispersion of a quantum wire superlattice system
Wilson, K. S. Joseph; Revathy, V.; Amalanathan, M.; Lenin, S. Maria
2015-06-24
Superlattices have drawn considerable attention in the recent years. In this work, the behaviour of polaritons in a quantum wire superlattice is analysed both at the brillouin zone edge and at centre of the brillouin zone using LiNbO3/ LiTaO3 as an example. The significance of the polariton modes in both the cases are analysed. New modes on the polaritonic gap, where the propagation of electromagnetic wave is forbidden, is obtained in the system as suggested by some recent literature. The effect on nonlinear interactions of phonon polaritons in LiNbO3/ LiTaO3 superlattices is also discussed.
The Quantum Socket: Wiring for Superconducting Qubits - Part 3
NASA Astrophysics Data System (ADS)
Mariantoni, M.; Bejianin, J. H.; McConkey, T. G.; Rinehart, J. R.; Bateman, J. D.; Earnest, C. T.; McRae, C. H.; Rohanizadegan, Y.; Shiri, D.; Penava, B.; Breul, P.; Royak, S.; Zapatka, M.; Fowler, A. G.
The implementation of a quantum computer requires quantum error correction codes, which allow to correct errors occurring on physical quantum bits (qubits). Ensemble of physical qubits will be grouped to form a logical qubit with a lower error rate. Reaching low error rates will necessitate a large number of physical qubits. Thus, a scalable qubit architecture must be developed. Superconducting qubits have been used to realize error correction. However, a truly scalable qubit architecture has yet to be demonstrated. A critical step towards scalability is the realization of a wiring method that allows to address qubits densely and accurately. A quantum socket that serves this purpose has been designed and tested at microwave frequencies. In this talk, we show results where the socket is used at millikelvin temperatures to measure an on-chip superconducting resonator. The control electronics is another fundamental element for scalability. We will present a proposal based on the quantum socket to interconnect a classical control hardware to a superconducting qubit hardware, where both are operated at millikelvin temperatures.
Quantum Monte Carlo Studies of Interaction-Induced Localization in Quantum Dots and Wires
NASA Astrophysics Data System (ADS)
Devrim Güçlü, A.
2009-03-01
We investigate interaction-induced localization of electrons in both quantum dots and inhomogeneous quantum wires using variational and diffusion quantum Monte Carlo methods. Quantum dots and wires are highly tunable systems that enable the study of the physics of strongly correlated electrons. With decreasing electronic density, interactions become stronger and electrons are expected to localize at their classical positions, as in Wigner crystallization in an infinite 2D system. (1) Dots: We show that the addition energy shows a clear progression from features associated with shell structure to those caused by commensurability of a Wigner crystal. This cross-over is, then, a signature of localization; it occurs near rs˜20. For higher values of rs, the configuration symmetry of the quantum dot becomes fully consistent with the classical ground state. (2) Wires: We study an inhomogeneous quasi-one-dimensional system -- a wire with two regions, one at low density and the other high. We find that strong localization occurs in the low density quantum point contact region as the gate potential is increased. The nature of the transition from high to low density depends on the density gradient -- if it is steep, a barrier develops between the two regions, causing Coulomb blockade effects. We find no evidence for ferromagnetic spin polarization for the range of parameters studied. The picture emerging here is in good agreement with the experimental measurements of tunneling between two wires. Collaborators: C. J. Umrigar (Cornell), Hong Jiang (Fritz Haber Institut), Amit Ghosal (IISER Calcutta), and H. U. Baranger (Duke).
Disorder-induced bound states within an adatom-quantum wire system
NASA Astrophysics Data System (ADS)
Magnetta, Bradley; Ordonez, Gonzalo
2014-03-01
Bound states induced by disorder are theoretically observed within a quantum wire and adatom system. The quantum wire is modeled as an array of quantum wells with random energies and exhibits Anderson Localization. By varying the energy of our adatom and adjusting the tunneling strength between the adatom and the quantum wire we observe disorder-induced bound states between the the adatom and its attached point. The characteristics of these disorder-induced bound states are greatly influenced by the site of interest on the quantum wire. Utilizing random quantum wires and disordered superlattices to produce bound states may offer flexibility in fabrication as well as provide grounds for energy transmission in photovoltaics.
Piled up charge effects in a ballistic transport in quantum wires
NASA Astrophysics Data System (ADS)
Sablikov, V. A.; Shchamkhalova, B. S.
2003-04-01
We have found that a high-field domain appears in a quantum wire with ballistic transport under far from equilibrium condition. The domain is located near the injecting electrode. The applied voltage drops mainly across the domain while the rest part of the wire remains nearly equipotential. The potential hump in the domain limits the current through the wire.
Density-functional theory of interacting electrons in inhomogeneous quantum wires
NASA Astrophysics Data System (ADS)
Abedinpour, Saeed H.; Polini, Marco; Xianlong, Gao; Tosi, Mario P.
2007-03-01
Motivated by the experimental evidence of electron localization in cleaved edge overgrowth quantum wires and by the recent interest in the development of density-functional schemes for inhomogeneous Luttinger and Luther-Emery liquids, we present a novel density-functional study of a few interacting electrons confined by power-law external potentials into a short portion of a thin quantum wire. The theory employs the quasi-one-dimensional (Q1D) homogeneous electron liquid as the reference system and transfers the appropriate Q1D ground-state correlations to the confined inhomogeneous system through a suitable local-density approximation (LDA) to the exchange and correlation energy functional. The LDA describes accurately ``liquid-like'' phases at weak coupling but fails in describing the emergence of ``Wigner molecules'' at strong coupling. A local spin-density approximation allowing for the formation of antiferromagnetic quasi-order with increasing coupling strength is proposed as a first step to overcome this problem.
Quantum conductance of silicon-doped carbon wire nanojunctions
2012-01-01
Unknown quantum electronic conductance across nanojunctions made of silicon-doped carbon wires between carbon leads is investigated. This is done by an appropriate generalization of the phase field matching theory for the multi-scattering processes of electronic excitations at the nanojunction and the use of the tight-binding method. Our calculations of the electronic band structures for carbon, silicon, and diatomic silicon carbide are matched with the available corresponding density functional theory results to optimize the required tight-binding parameters. Silicon and carbon atoms are treated on the same footing by characterizing each with their corresponding orbitals. Several types of nanojunctions are analyzed to sample their behavior under different atomic configurations. We calculate for each nanojunction the individual contributions to the quantum conductance for the propagating σ, Π, and σ∗electron incidents from the carbon leads. The calculated results show a number of remarkable features, which include the influence of the ordered periodic configurations of silicon-carbon pairs and the suppression of quantum conductance due to minimum substitutional disorder and artificially organized symmetry on these nanojunctions. Our results also demonstrate that the phase field matching theory is an efficient tool to treat the quantum conductance of complex molecular nanojunctions. PMID:23130998
Quantum Phase Transition and Entanglement in Topological Quantum Wires.
Cho, Jaeyoon; Kim, Kun Woo
2017-06-05
We investigate the quantum phase transition of the Su-Schrieffer-Heeger (SSH) model by inspecting the two-site entanglements in the ground state. It is shown that the topological phase transition of the SSH model is signified by a nonanalyticity of local entanglement, which becomes discontinuous for finite even system sizes, and that this nonanalyticity has a topological origin. Such a peculiar singularity has a universal nature in one-dimensional topological phase transitions of noninteracting fermions. We make this clearer by pointing out that an analogous quantity in the Kitaev chain exhibiting the identical nonanalyticity is the local electron density. As a byproduct, we show that there exists a different type of phase transition, whereby the pattern of the two-site entanglements undergoes a sudden change. This transition is characterised solely by quantum information theory and does not accompany the closure of the spectral gap. We analyse the scaling behaviours of the entanglement in the vicinities of the transition points.
Synthesizing Majorana zero-energy modes in a periodically gated quantum wire
NASA Astrophysics Data System (ADS)
Malard, Mariana; Japaridze, George I.; Johannesson, Henrik
2016-09-01
We explore a scheme for engineering a one-dimensional spinless p -wave superconductor hosting unpaired Majorana zero-energy modes, using an all-electric setup with a spin-orbit-coupled quantum wire in proximity to an s -wave superconductor. The required crossing of the Fermi level by a single spin-split energy band is ensured by employing a periodically modulated Rashba interaction, which, assisted by electron-electron interactions and a uniform Dresselhaus interaction, opens a gap at two of the spin-orbit shifted Fermi points. While an implementation in a hybrid superconductor-semiconductor device requires improvements upon present-day capabilities, a variant of our scheme where spin-orbit-coupled cold fermions are effectively proximity coupled to a BEC reservoir of Feshbach molecules may provide a ready-to-use platform.
Micro-Photoluminescence Confocal Mapping of Single V-Grooved GaAs Quantum Wire
NASA Astrophysics Data System (ADS)
Huang, Shao-Hua; Chen, Zhang-Hai; Bai, Li-Hui; Shen, Xue-Chu; Tan H., H.; L., Fu; Fraser, M.; Jagadish, C.
2006-12-01
We perform the micro-photoluminescence measurement at low temperatures and a scanning optical mapping with high spatial resolution of a single V-grooved GaAs quantum wire modified by the selective ion-implantation and rapid thermally annealing. While the mapping shows the luminescences respectively from the quantum wires and from quantum well areas between quantum wires in general, the micro-photoluminescence at liquid He temperatures reveals a plenty of spectral structures of the PL band for a single quantum wire. The spectral structures are attributed to the inhomogeneity and non-uniformity of both the space structure and compositions of real wires as well as the defects nearby the interface between quantum wire and surrounding quantum well structures. All these make the excitons farther localized in quasi-zero-dimensional quantum potential boxes related to these non-uniformity and/or defects. The results also demonstrate the ability of micro-photoluminescence measurement and mapping for the characterization of both opto-electronic and structural properties of real quantum wires.
Two-layer synchronized ternary quantum-dot cellular automata wire crossings.
Bajec, Iztok Lebar; Pečar, Primož
2012-04-16
: Quantum-dot cellular automata are an interesting nanoscale computing paradigm. The introduction of the ternary quantum-dot cell enabled ternary computing, and with the recent development of a ternary functionally complete set of elementary logic primitives and the ternary memorizing cell design of complex processing structures is becoming feasible. The specific nature of the ternary quantum-dot cell makes wire crossings one of the most problematic areas of ternary quantum-dot cellular automata circuit design. We hereby present a two-layer wire crossing that uses a specific clocking scheme, which ensures the crossed wires have the same effective delay.
Micklitz, T; Levchenko, A; Rosch, A
2012-07-20
We calculate the linear and nonlinear conductance of spinless fermions in clean, long quantum wires, where short-ranged interactions lead locally to equilibration. Close to the quantum phase transition, where the conductance jumps from zero to one conductance quantum, the conductance obtains a universal form governed by the ratios of temperature, bias voltage, and gate voltage. Asymptotic analytic results are compared to solutions of a Boltzmann equation which includes the effects of three-particle scattering. Surprisingly, we find that for long wires the voltage predominantly drops close to one end of the quantum wire due to a thermoelectric effect.
Spin resonance and spin fluctuations in a quantum wire
NASA Astrophysics Data System (ADS)
Pokrovsky, V. L.
2017-02-01
This is a review of theoretical works on spin resonance in a quantum wire associated with the spin-orbit interaction. We demonstrate that the spin-orbit induced internal "magnetic field" leads to a narrow spin-flip resonance at low temperatures in the absence of an applied magnetic field. An applied dc magnetic field perpendicular to and small compared with the spin-orbit field enhances the resonance absorption by several orders of magnitude. The component of applied field parallel to the spin-orbit field separates the resonance frequencies of right and left movers and enables a linearly polarized ac electric field to produce a dynamic magnetization as well as electric and spin currents. We start with a simple model of noninteracting electrons and then consider the interaction that is not weak in 1d electron system. We show that electron spin resonance in the spin-orbit field persists in the Luttinger liquid. The interaction produces an additional singularity (cusp) in the spin-flip channel associated with the plasma oscillation. As it was shown earlier by Starykh and his coworkers, the interacting 1d electron system in the external field with sufficiently large parallel component becomes unstable with respect to the appearance of a spin-density wave. This instability suppresses the spin resonance. The observation of the electron spin resonance in a thin wires requires low temperature and high intensity of electromagnetic field in the terahertz diapason. The experiment satisfying these two requirements is possible but rather difficult. An alternative approach that does not require strong ac field is to study two-time correlations of the total spin of the wire with an optical method developed by Crooker and coworkers. We developed theory of such correlations. We prove that the correlation of the total spin component parallel to the internal magnetic field is dominant in systems with the developed spin-density waves but it vanishes in Luttinger liquid. Thus, the
Optical properties of quantum-wire arrays in (Al,Ga)As serpentine-superlattice structures
NASA Astrophysics Data System (ADS)
Weman, H.; Miller, M. S.; Pryor, C. E.; Li, Y. J.; Bergman, P.; Petroff, P. M.; Merz, J. L.
1993-09-01
Serpentine superlattices with a built-in quantum-wire array have been grown on vicinal (100) GaAs substrates by molecular-beam epitaxy. The quantum wires have parabolic cross sections with confinement dimensions on the order of 100 Å. The goal of this paper is to make a comprehensive optical study of two different serpentine-superlattice samples. The serpentine structures have been characterized by low-temperature cathodoluminescence, photoluminescence, and photoluminescence-excitation measurements. In photoluminescence, a rather sharp peak (typically 7 meV full width at half maximum) is attributed to excitonic recombination in the built-in quantum-wire array. The linear polarization dependence of the serpentine-superlattice emission has been measured with a photoelastic modulation technique, showing a pronounced polarization anisotropy in both photoluminescence and photoluminescence excitation. The serpentine-superlattice photoluminescence emission normal to the vicinal surface shows a linear polarization along the wires of up to about 30% due to the lateral confinement. The carrier confinement has been further characterized by measuring the linear polarization dependence of the photoluminescence normal to the cleaved edges. The measured polarization anisotropy has been compared with the calculated polarization dependence as a function of intermixing between the lateral barriers and wells. It is found that there is a substantial intermixing between the barriers and wells, with at least 30% of the Al intended for the barrier ending up in the well. This results in hole states confined to one dimension, while the electron states are two dimensional due to coupling through the lateral barriers. Linearly polarized photoluminescence excitation has been used to reveal the laterally induced heavy-hole-light-hole splitting. Photoluminescence decay-time measurements of the serpentine-superlattice emission (390 ps) show a longer radiative decay time at low temperature than
The superconductor-metal quantum phase transition in ultra-narrow wires
NASA Astrophysics Data System (ADS)
Del Maestro, Adrian Giuseppe
We present a complete description of a zero temperature phase transition between superconducting and diffusive metallic states in very thin wires due to a Cooper pair breaking mechanism originating from a number of possible sources. These include impurities localized to the surface of the wire, a magnetic field orientated parallel to the wire or, disorder in an unconventional superconductor. The order parameter describing pairing is strongly overdamped by its coupling to an effectively infinite bath of unpaired electrons imagined to reside in the transverse conduction channels of the wire. The dissipative critical theory thus contains current reducing fluctuations in the guise of both quantum and thermally activated phase slips. A full cross-over phase diagram is computed via an expansion in the inverse number of complex components of the superconducting order parameter (equal to one in the physical case). The fluctuation corrections to the electrical and thermal conductivities are determined, and we find that the zero frequency electrical transport has a non-monotonic temperature dependence when moving from the quantum critical to low temperature metallic phase, which may be consistent with recent experimental results on ultra-narrow MoGe wires. Near criticality, the ratio of the thermal to electrical conductivity displays a linear temperature dependence and thus the Wiedemann-Franz law is obeyed. We compute the constant of proportionality in a systematic expansion and find a universal and experimentally verifiable fluctuation correction to the Lorenz number. In the presence of quenched disorder, a novel algorithm is developed to solve the self-consistency condition arising when the number of complex order parameter components is taken to be large. In this limit, we find striking evidence for the flow to infinite randomness, and observe dynamically activated scaling consistent with predictions from the strong disorder renormalization group. Moreover, the infinite
Comparison studies of infrared photodetectors with a quantum-dot and a quantum-wire base
NASA Astrophysics Data System (ADS)
El Tokhy, M. S.; Mahmoud, I. I.; Konber, H. A.
2011-12-01
This paper mainly presents a theoretical analysis for the characteristics of quantum dot infrared photodetectors (QDIPs) and quantum wire infrared photodetectors (QRIPs). The paper introduces a unique mathematical model of solving Poisson's equations with the usage of Lambert W functions for infrared detectors' structures based on quantum effects. Even though QRIPs and QDIPs have been the subject of extensive researches and development during the past decade, it is still essential to implement theoretical models allowing to estimate the ultimate performance of those detectors such as photocurrent and its figure-of-merit detectivity vs. various parameter conditions such as applied voltage, number of quantum wire layers, quantum dot layers, lateral characteristic size, doping density, operation temperature, and structural parameters of the quantum dots (QDs), and quantum wires (QRs). A comparison is made between the computed results of the implemented models and fine agreements are observed. It is concluded from the obtained results that the total detectivity of QDIPs can be significantly lower than that in the QRIPs and main features of the QRIPs such as large gap between the induced photocurrent and dark current of QRIP which allows for overcoming the problems in the QDIPs. This confirms what is evaluated before in the literature. It is evident that by increasing the QD/QR absorption volume in QDIPs/QRIPs as well as by separating the dark current and photocurrents, the specific detectivity can be improved and consequently the devices can operate at higher temperatures. It is an interesting result and it may be benefit to the development of QDIP and QRIP for infrared sensing applications.
Quantum Transport in Gated Dangling-Bond Atomic Wires.
Bohloul, S; Shi, Q; Wolkow, Robert A; Guo, Hong
2017-01-11
A single line of dangling bonds (DBs) on Si(100)-2 × 1:H surface forms a perfect metallic atomic-wire. In this work, we investigate quantum transport properties of such dangling bond wires (DBWs) by a state-of-the-art first-principles technique. It is found that the conductance of the DBW can be gated by electrostatic potential and orbital overlap due to only a single DB center (DBC) within a distance of ∼16 Å from the DBW. The gating effect is more pronounced for two DBCs and especially, when these two DB "gates" are within ∼3.9 Å from each other. These effective length scales are in excellent agreement with those measured in scanning tunnelling microscope experiments. By analyzing transmission spectrum and density of states of DBC-DBW systems, with or without subsurface doping, for different length of the DBW, distance between DBCs and the DBW, and distance between DB gates, we conclude that charge transport in a DBW can be regulated to have both an on-state and an off-state using only one or two DBs.
Rashba quantum wire: exact solution and ballistic transport.
Perroni, C A; Bercioux, D; Ramaglia, V Marigliano; Cataudella, V
2007-05-08
The effect of Rashba spin-orbit interaction in quantum wires with hard-wall boundaries is discussed. The exact wavefunction and eigenvalue equation are worked out, pointing out the mixing between the spin and spatial parts. The spectral properties are also studied within perturbation theory with respect to the strength of the spin-orbit interaction and diagonalization procedure. A comparison is made with the results of a simple model, the two-band model, that takes account only of the first two sub-bands of the wire. Finally, the transport properties within the ballistic regime are analytically calculated for the two-band model and through a tight-binding Green function for the entire system. Single and double interfaces separating regions with different strengths of spin-orbit interaction are analysed by injecting carriers into the first and the second sub-band. It is shown that in the case of a single interface the spin polarization in the Rashba region is different from zero, and in the case of two interfaces the spin polarization shows oscillations due to spin-selective bound states.
Non-Abelian topological spin liquids from arrays of quantum wires or spin chains
NASA Astrophysics Data System (ADS)
Huang, Po-Hao; Chen, Jyong-Hao; Gomes, Pedro R. S.; Neupert, Titus; Chamon, Claudio; Mudry, Christopher
2016-05-01
We construct two-dimensional non-Abelian topologically ordered states by strongly coupling arrays of one-dimensional quantum wires via interactions. In our scheme, all charge degrees of freedom are gapped, so the construction can use either quantum wires or quantum spin chains as building blocks, with the same end result. The construction gaps the degrees of freedom in the bulk, while leaving decoupled states at the edges that are described by conformal field theories (CFT) in (1 +1 ) -dimensional space and time. We consider both the cases where time-reversal symmetry (TRS) is present or absent. When TRS is absent, the edge states are chiral and stable. We prescribe, in particular, how to arrive at all the edge states described by the unitary CFT minimal models with central charges c <1 . These non-Abelian spin liquid states have vanishing quantum Hall conductivities, but nonzero thermal ones. When TRS is present, we describe scenarios where the bulk state can be a non-Abelian, nonchiral, and gapped quantum spin liquid, or a gapless one. In the former case, we find that the edge states are also gapped. The paper provides a brief review of non-Abelian bosonization and affine current algebras, with the purpose of being self-contained. To illustrate the methods in a warm-up exercise, we recover the tenfold way classification of two-dimensional noninteracting topological insulators using the Majorana representation that naturally arises within non-Abelian bosonization. Within this scheme, the classification reduces to counting the number of null singular values of a mass matrix, with gapless edge modes present when left and right null eigenvectors exist.
Driving a first order quantum phase transition by coupling a quantum dot to a 1D charge density wave
NASA Astrophysics Data System (ADS)
Weiss, Y.; Goldstein, M.; Berkovits, R.
2007-02-01
The ground state properties of a one-dimensional system with particle-hole symmetry, consisting of a gate controlled dot coupled to an interacting reservoir, are explored using the numerical DMRG method. It has previously been shown that the system's thermodynamic properties as a function of the gate voltage in the Luttinger liquid phase are qualitatively similar to the behaviour of a non-interacting wire with an effective (renormalized) dot-lead coupling. Here we examine the thermodynamic properties of the wire in the charge density wave phase, and show that these properties behave quite differently. The number of electrons in the system remains constant as a function of the gate voltage, while the total energy becomes linear. Moreover, by tuning the gate voltage on the dot in the charge density wave phase it is possible to drive the wire through a first order quantum phase transition in which the population of each site in the wire is inverted.
First-principles calculations: half-metallic Au-V(Cr) quantum wires as spin filters.
Min, Y; Yao, K L; Liu, Z L; Gao, G Y; Cheng, H G; Zhu, S C
2009-03-04
The half-metallic behavior of Au-V(Cr) quantum wires adsorbed on an armchair (5, 5) boron nitride nanotube is obtained by performing spin-polarized density functional calculations. The density of states shows a metallic property at the Fermi level for the majority spin channel and a semiconductor gap in the minority spin channel. The half-metallic behavior of the quantum wire/nanotube complex originates from the half-metallic behavior of the free-standing Au-V(Cr) quantum wires. The calculations of spin-polarized transport indicate that such a one-dimensional half-metallic magnet can be used as a spin filter.
Quantum Ising model coupled with conducting electrons
NASA Astrophysics Data System (ADS)
Yamashita, Yasufumi; Yonemitsu, Kenji
2005-01-01
The effect of photo-doping on the quantum paraelectric SrTiO3 is studied by using the one-dimensional quantum Ising model, where the Ising spin describes the effective lattice polarization of an optical phonon. Two types of electron-phonon couplings are introduced through the modulation of transfer integral via lattice deformations. After the exact diagonalization and the perturbation studies, we find that photo-induced low-density carriers can drastically alter quantum fluctuations when the system locates near the quantum critical point between the quantum para- and ferro-electric phases.
Electron motion induced by magnetic pulse in a bilayer quantum wire
NASA Astrophysics Data System (ADS)
Chwiej, T.
2016-06-01
We consider theoretical stimulation of electron motion in a quantum wire by means of ultrashort magnetic pulses of time duration between several and a few tens of picoseconds. In our considerations, an electron is confined in a nanowire which consists of two vertically stacked tunnel-coupled layers. If a magnetic pulse pierces this nanowire and its direction is parallel to the plane established by the layers, and additionally, it is perpendicular to the wire's axis, then the eigenstates of a single electron energy operator for vertical direction are hybridized by the off-diagonal terms of the full Hamiltonian. These terms depend linearly on the momentum operator, which means that such magnetically forced hybridization may induce electron motion in a nanowire. The classical counterpart of this quantum-mechanical picture is a situation in which the rotational electric field generated by a time-varying magnetic field pushes the charge densities localized in the upper and lower layers in opposite directions. We have found, however, that for an asymmetric vertical confinement in a bilayer nanowire, the major part of the single electron density starts to move in the direction of the local electric field in its layer forcing the minority part to move in this direction as well. It results in coherent motion of both densities in a particular direction. We analyze the dynamics of such motion in dependence on the time characteristics of a magnetic pulse and discuss potential applications of this effect in the construction of a magnetic valve.
Quantum Optical Signature of Plasmonically Coupled Nanocrystal Quantum Dots.
Wang, Feng; Karan, Niladri S; Nguyen, Hue Minh; Mangum, Benjamin D; Ghosh, Yagnaseni; Sheehan, Chris J; Hollingsworth, Jennifer A; Htoon, Han
2015-10-01
Small clusters of two to three silica-coated nanocrystals coupled to plasmonic gap-bar antennas can exhibit photon antibunching, a characteristic of single quantum emitters. Through a detailed analysis of their photoluminescence emissions characteristics, it is shown that the observed photon antibunching is the evidence of coupled quantum dot formation resulting from the plasmonic enhancement of dipole-dipole interaction. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
NASA Astrophysics Data System (ADS)
Das, Tanmoy
2016-07-01
We study directional dependent band gap evolutions and metal-insulator transitions (MITs) in model quantum wire systems within the spin-orbit density wave (SODW) model. The evolution of MIT is studied as a function of varying anisotropy between the intra-wire hopping ({{t}\\parallel} ) and inter-wire hopping ({{t}\\bot} ) with Rashba spin-orbit coupling. We find that as long as the anisotropy ratio (β ={{t}\\bot}/{{t}\\parallel} ) remains below 0.5, and the Fermi surface nesting is tuned to {{\\mathbf{Q}}1}=≤ft(π,0\\right) , an exotic SODW induced MIT easily develops, with its critical interaction strength increasing with increasing anisotropy. As β \\to 1 (2D system), the nesting vector switches to {{\\mathbf{Q}}2}=≤ft(π,π \\right) , making this state again suitable for an isotropic MIT. Finally, we discuss various physical consequences and possible applications of the directional dependent MIT.
Observation of correlated spin-orbit order in a strongly anisotropic quantum wire system.
Brand, C; Pfnür, H; Landolt, G; Muff, S; Dil, J H; Das, Tanmoy; Tegenkamp, Christoph
2015-09-10
Quantum wires with spin-orbit coupling provide a unique opportunity to simultaneously control the coupling strength and the screened Coulomb interactions where new exotic phases of matter can be explored. Here we report on the observation of an exotic spin-orbit density wave in Pb-atomic wires on Si(557) surfaces by mapping out the evolution of the modulated spin-texture at various conditions with spin- and angle-resolved photoelectron spectroscopy. The results are independently quantified by surface transport measurements. The spin polarization, coherence length, spin dephasing rate and the associated quasiparticle gap decrease simultaneously as the screened Coulomb interaction decreases with increasing excess coverage, providing a new mechanism for generating and manipulating a spin-orbit entanglement effect via electronic interaction. Despite clear evidence of spontaneous spin-rotation symmetry breaking and modulation of spin-momentum structure as a function of excess coverage, the average spin polarization over the Brillouin zone vanishes, indicating that time-reversal symmetry is intact as theoretically predicted.
Polar Interface-Optical Vibrational Spectra in a Wurtzite GaN/AlN Rectangular Quantum Wire
NASA Astrophysics Data System (ADS)
Zhang, L.
Under the dielectric continuum model and Loudon's uniaxial crystal model, the interface optical (IO) phonon modes in a quasi-one-dimensional (Q1D) wurtzite rectangular quantum wire are deduced and analyzed. Numerical calculation on a wurtzite GaN/AlN rectangular wurtzite quantum wire was performed. Results reveal that the dispersion frequencies of IO modes sensitively depend on the geometric structures of the Q1D wurtzite rectangular quantum wires. The degenerating behavior of the IO phonon modes in the Q1D wurtzite rectangular quantum wire has been clearly observed for small free wave number kz in z-direction. The limited frequency behaviors of IO modes have been analyzed deeply, and detailed comparisons with those in wurtzite planar quantum wells and cylindrical quantum wires are also done. Moreover, once the anisotropy of the wurtzite material has been ignored, the present theories can be naturally reduced to the situation of Q1D cubic rectangular quantum wire systems.
Quantum emitters dynamically coupled to a quantum field
NASA Astrophysics Data System (ADS)
Acevedo, O. L.; Quiroga, L.; Rodríguez, F. J.; Johnson, N. F.
2013-12-01
We study theoretically the dynamical response of a set of solid-state quantum emitters arbitrarily coupled to a single-mode microcavity system. Ramping the matter-field coupling strength in round trips, we quantify the hysteresis or irreversible quantum dynamics. The matter-field system is modeled as a finite-size Dicke model which has previously been used to describe equilibrium (including quantum phase transition) properties of systems such as quantum dots in a microcavity. Here we extend this model to address non-equilibrium situations. Analyzing the system's quantum fidelity, we find that the near-adiabatic regime exhibits the richest phenomena, with a strong asymmetry in the internal collective dynamics depending on which phase is chosen as the starting point. We also explore signatures of the crossing of the critical points on the radiation subsystem by monitoring its Wigner function; then, the subsystem can exhibit the emergence of non-classicality and complexity.
Quantum emitters dynamically coupled to a quantum field
Acevedo, O. L.; Quiroga, L.; Rodríguez, F. J.; Johnson, N. F.
2013-12-04
We study theoretically the dynamical response of a set of solid-state quantum emitters arbitrarily coupled to a single-mode microcavity system. Ramping the matter-field coupling strength in round trips, we quantify the hysteresis or irreversible quantum dynamics. The matter-field system is modeled as a finite-size Dicke model which has previously been used to describe equilibrium (including quantum phase transition) properties of systems such as quantum dots in a microcavity. Here we extend this model to address non-equilibrium situations. Analyzing the system’s quantum fidelity, we find that the near-adiabatic regime exhibits the richest phenomena, with a strong asymmetry in the internal collective dynamics depending on which phase is chosen as the starting point. We also explore signatures of the crossing of the critical points on the radiation subsystem by monitoring its Wigner function; then, the subsystem can exhibit the emergence of non-classicality and complexity.
Coherent tunneling through a double quantum dot coupled to Majorana bound states
NASA Astrophysics Data System (ADS)
Ivanov, T. I.
2017-07-01
We consider a double quantum dot coupled to a one-dimensional superconducting quantum wire with Majorana bound states at the ends of the wire. We compute the conductance of the double dot in the coherent tunneling regime. When only one of the dots is coupled to one Majorana bound state the conductance is enhanced/diminished in the vicinity of zero voltage if it has minimum/maximum at this voltage with no Majorana bound state and has two local maximums/minimums at voltage equal plus or minus the Majorana bound states overlapping energy. When each dot is coupled to one Majorana bound state with zero overlapping energy it is possible by tuning the magnetic flux through the system to change the zero-voltage conductance from minimum to local maximum. We show that when both electron levels in the double quantum dot are below the right chemical potential the Fano resonance occurs only for the lower energy level.
Multiple Majorana zero modes in atomic Fermi double wires with spin-orbit coupling
NASA Astrophysics Data System (ADS)
Wang, Liang-Liang; Gong, Ming; Liu, W.-M.
2017-08-01
Majorana zero modes, quasiparticles with non-Abelian statistics, have gained increasing interest for their fundamental role as building blocks in topological quantum computation. Previous studies have mainly focused on two well-separated Majorana zero modes, which could form two degenerate states serving as one nonlocal qubit for fault-tolerant quantum memory. However, creating and manipulating multiple Majorana zero modes, which could encode more qubits, remain an ongoing research topic. Here we report that multiple Majorana zero modes can exist in atomic Fermi double wires with spin-orbit coupling and perpendicular Zeeman field. This system belongs to the topological BDI class, thus all the topological superfluids are classified by integer numbers. Especially, diverse topological superfluids can be formed in a trap, where the zero energy modes can be found at the interfaces between different topological superfluids. The structure of these zero energy modes in the trap can be engineered by the trapping potential as well as other system parameters. This system would be a significant step towards utilization of Majorana zero modes in quantum computation.
Energy and matter-efficient size-selective growth of thin quantum wires in a plasma
Ostrikov, K.; Mehdipour, H.
2011-01-17
It is shown that plasmas can minimize the adverse Gibbs-Thompson effect in thin quantum wire growth. The model of Si nanowire nucleation includes the unprecedented combination of the plasma sheath, ion- and radical-induced species creation and heating effects on the surface and within an Au catalyst nanoparticle. Compared to neutral gas thermal processes, much thinner, size-selective wires can nucleate at the same temperature and pressure while much lower energy and matter budget is needed to grow same-size wires. This explains the experimental observations and may lead to energy- and matter-efficient synthesis of a broader range of one-dimensional quantum structures.
Quantum light in coupled interferometers for quantum gravity tests.
Ruo Berchera, I; Degiovanni, I P; Olivares, S; Genovese, M
2013-05-24
In recent years quantum correlations have received a lot of attention as a key ingredient in advanced quantum metrology protocols. In this Letter we show that they provide even larger advantages when considering multiple-interferometer setups. In particular, we demonstrate that the use of quantum correlated light beams in coupled interferometers leads to substantial advantages with respect to classical light, up to a noise-free scenario for the ideal lossless case. On the one hand, our results prompt the possibility of testing quantum gravity in experimental configurations affordable in current quantum optics laboratories and strongly improve the precision in "larger size experiments" such as the Fermilab holometer; on the other hand, they pave the way for future applications to high precision measurements and quantum metrology.
Temperature dependence of coulomb drag between finite-length quantum wires.
Peguiron, J; Bruder, C; Trauzettel, B
2007-08-24
We evaluate the Coulomb drag current in two finite-length Tomonaga-Luttinger-liquid wires coupled by an electrostatic backscattering interaction. The drag current in one wire shows oscillations as a function of the bias voltage applied to the other wire, reflecting interferences of the plasmon standing waves in the interacting wires. In agreement with this picture, the amplitude of the current oscillations is reduced with increasing temperature. This is a clear signature of non-Fermi-liquid physics because for coupled Fermi liquids the drag resistance is always expected to increase as the temperature is raised.
Spin Seebeck effect in quantum dot side-coupled to topological superconductor.
Weymann, Ireneusz
2017-03-08
The spin-resolved thermoelectric transport properties of a quantum dot coupled to ferromagnetic leads and side-coupled to a topological superconductor wire hosting Majorana zero-energy modes are studied theoretically. The calculations are performed in the linear response regime by using the numerical renormalization group method. It is shown that transport characteristics are determined by the interplay of Kondo correlations, exchange field due to the presence of ferromagnets and the strength of coupling to Majorana wire. These different energy scales are revealed in the behavior of the Seebeck and spin Seebeck coefficients, which exhibit an enhancement for temperatures of the order of the coupling strength to topological wire. Moreover, it is demonstrated that additional sign changes of the thermopower can occur due to the presence of Majorana zero-energy modes. These findings may provide additional fingerprints of the presence of Majorana fermions.
Spin Seebeck effect in quantum dot side-coupled to topological superconductor
NASA Astrophysics Data System (ADS)
Weymann, Ireneusz
2017-03-01
The spin-resolved thermoelectric transport properties of a quantum dot coupled to ferromagnetic leads and side-coupled to a topological superconductor wire hosting Majorana zero-energy modes are studied theoretically. The calculations are performed in the linear response regime by using the numerical renormalization group method. It is shown that transport characteristics are determined by the interplay of Kondo correlations, exchange field due to the presence of ferromagnets and the strength of coupling to Majorana wire. These different energy scales are revealed in the behavior of the Seebeck and spin Seebeck coefficients, which exhibit an enhancement for temperatures of the order of the coupling strength to topological wire. Moreover, it is demonstrated that additional sign changes of the thermopower can occur due to the presence of Majorana zero-energy modes. These findings may provide additional fingerprints of the presence of Majorana fermions.
Nanofabrication Of Quantum Coupled Devices
NASA Astrophysics Data System (ADS)
Randall, J. N.; Reed, M. A.; Matyi, R. J.; Moore, T. M.; Aggarwal, R. J.; Wetsel, A. E.
1988-08-01
A new generation of integrated circuits is foreseen in which quantum dots with discrete electronic states are used to construct logic and memory elements. Successful implementation of this technology will require significant improvements in semiconductor processing technology.
Coupling single emitters to quantum plasmonic circuits
NASA Astrophysics Data System (ADS)
Huck, Alexander; Andersen, Ulrik L.
2016-09-01
In recent years, the controlled coupling of single-photon emitters to propagating surface plasmons has been intensely studied, which is fueled by the prospect of a giant photonic nonlinearity on a nanoscaled platform. In this article, we will review the recent progress on coupling single emitters to nanowires towards the construction of a new platform for strong light-matter interaction. The control over such a platform might open new doors for quantum information processing and quantum sensing at the nanoscale and for the study of fundamental physics in the ultrastrong coupling regime.
Quantum Computation Using Optically Coupled Quantum Dot Arrays
NASA Technical Reports Server (NTRS)
Pradhan, Prabhakar; Anantram, M. P.; Wang, K. L.; Roychowhury, V. P.; Saini, Subhash (Technical Monitor)
1998-01-01
A solid state model for quantum computation has potential advantages in terms of the ease of fabrication, characterization, and integration. The fundamental requirements for a quantum computer involve the realization of basic processing units (qubits), and a scheme for controlled switching and coupling among the qubits, which enables one to perform controlled operations on qubits. We propose a model for quantum computation based on optically coupled quantum dot arrays, which is computationally similar to the atomic model proposed by Cirac and Zoller. In this model, individual qubits are comprised of two coupled quantum dots, and an array of these basic units is placed in an optical cavity. Switching among the states of the individual units is done by controlled laser pulses via near field interaction using the NSOM technology. Controlled rotations involving two or more qubits are performed via common cavity mode photon. We have calculated critical times, including the spontaneous emission and switching times, and show that they are comparable to the best times projected for other proposed models of quantum computation. We have also shown the feasibility of accessing individual quantum dots using the NSOM technology by calculating the photon density at the tip, and estimating the power necessary to perform the basic controlled operations. We are currently in the process of estimating the decoherence times for this system; however, we have formulated initial arguments which seem to indicate that the decoherence times will be comparable, if not longer, than many other proposed models.
Quantum Computation Using Optically Coupled Quantum Dot Arrays
NASA Technical Reports Server (NTRS)
Pradhan, Prabhakar; Anantram, M. P.; Wang, K. L.; Roychowhury, V. P.; Saini, Subhash (Technical Monitor)
1998-01-01
A solid state model for quantum computation has potential advantages in terms of the ease of fabrication, characterization, and integration. The fundamental requirements for a quantum computer involve the realization of basic processing units (qubits), and a scheme for controlled switching and coupling among the qubits, which enables one to perform controlled operations on qubits. We propose a model for quantum computation based on optically coupled quantum dot arrays, which is computationally similar to the atomic model proposed by Cirac and Zoller. In this model, individual qubits are comprised of two coupled quantum dots, and an array of these basic units is placed in an optical cavity. Switching among the states of the individual units is done by controlled laser pulses via near field interaction using the NSOM technology. Controlled rotations involving two or more qubits are performed via common cavity mode photon. We have calculated critical times, including the spontaneous emission and switching times, and show that they are comparable to the best times projected for other proposed models of quantum computation. We have also shown the feasibility of accessing individual quantum dots using the NSOM technology by calculating the photon density at the tip, and estimating the power necessary to perform the basic controlled operations. We are currently in the process of estimating the decoherence times for this system; however, we have formulated initial arguments which seem to indicate that the decoherence times will be comparable, if not longer, than many other proposed models.
Raman scattering from confined phonons in GaAs/AlGaAs quantum wires
NASA Astrophysics Data System (ADS)
Bairamov, B. H.; Aydinli, A.; Tanatar, B.; Güven, K.; Gurevich, S.; Mel'tser, B. Ya.; Ivanov, S. V.; Kop'ev, P. S.; Smirnitskii, V. B.; Timofeev, F. N.
1998-10-01
We report on photoluminescence and Raman scattering performed at low temperature (T = 10 K) on GaAs/Al0.3Ga0.7As quantum-well wires with effective wire widths ofL = 100.0 and 10.9 nm prepared by molecular beam epitaxial growth followed by holographic patterning, reactive ion etching, and anodic thinning. We find evidence for the existence of longitudinal optical phonon modes confined to the GaAs quantum wire. The observed frequency at οL10 = 285.6 cm-1forL = 11.0 nm is in good agreement with that calculated on the basis of the dispersive dielectric continuum theory of Enderleinas applied to the GaAs/Al0.3Ga0.7As system. Our results indicate the high crystalline quality of the quantum-well wires fabricated using these techniques.
Coupling InSb quantum dots to a superconducting microwave resonator
NASA Astrophysics Data System (ADS)
Cassidy, Maja; Kammhuber, Jakob; Car, Diana; Plissard, Sebastien; Bakkers, Erik; Dicarlo, Leo; Kouwenhoven, Leo
2014-03-01
We present measurements of a superconducting half-wave resonator coupled to two InSb nanowire quantum dots. Precise nanowire alignment at the electric field antinodes at opposite ends of the microwave cavity allows for a maximal electric field along the wire axis, without compromising the intrinsic quality factor of the cavity. This architecture may be useful for reaching the strong coupling limit between a single spin and a microwave photon, paving the way to on-chip coupling of single spins for quantum information processing.
The quantum pinch effect in semiconducting quantum wires: A bird’s-eye view
NASA Astrophysics Data System (ADS)
Kushwaha, Manvir S.
2016-01-01
Those who measure success with culmination do not seem to be aware that life is a journey not a destination. This spirit is best reflected in the unceasing failures in efforts for solving the problem of controlled thermonuclear fusion for even the simplest pinches for over decades; and the nature keeps us challenging with examples. However, these efforts have permitted researchers the obtention of a dense plasma with a lifetime that, albeit short, is sufficient to study the physics of the pinch effect, to create methods of plasma diagnostics, and to develop a modern theory of plasma processes. Most importantly, they have impregnated the solid state plasmas, particularly the electron-hole plasmas in semiconductors, which do not suffer from the issues related with the confinement and which have demonstrated their potential not only for the fundamental physics but also for the device physics. Here, we report on a two-component, cylindrical, quasi-one-dimensional quantum plasma subjected to a radial confining harmonic potential and an applied magnetic field in the symmetric gauge. It is demonstrated that such a system, as can be realized in semiconducting quantum wires, offers an excellent medium for observing the quantum pinch effect at low temperatures. An exact analytical solution of the problem allows us to make significant observations: Surprisingly, in contrast to the classical pinch effect, the particle density as well as the current density display a determinable maximum before attaining a minimum at the surface of the quantum wire. The effect will persist as long as the equilibrium pair density is sustained. Therefore, the technological promise that emerges is the route to the precise electronic devices that will control the particle beams at the nanoscale.
Quantum analysis of plasmonic coupling between quantum dots and nanoparticles
NASA Astrophysics Data System (ADS)
Ahmad, SalmanOgli
2016-10-01
In this study, interaction between core-shells nanoparticles and quantum dots is discussed via the full-quantum-theory method. The electromagnetic field of the nanoparticles is derived by the quasistatic approximation method and the results for different regions of the nanoparticles are quantized from the time-harmonic to the wave equation. Utilizing the optical field quantization, the nanoparticles' and quantum dots' deriving amplitudes contributing to the excitation waves are determined. In the current model, two counterpropagating waves with two different frequencies are applied. We derived the Maxwell-Bloch equations from the Heisenberg-Langevin equations; thus the nanoparticles-quantum dots interaction is perused. Moreover, by full quantum analyzing of the analytical expression, the quantum-plasmonic coupling relation and the Purcell factor are achieved. We show that the spontaneous emission of quantum dots can be dramatically manipulated by engineering the plasmon-plasmon interaction in the core-shells nanoparticles. This issue is a very attractive point for designing a wide variety of quantum-plasmonic sensors. Through the investigation of the nanoparticle plasmonic interaction effects on absorbed power, the results show that the nanoparticles' and quantum dots' absorption saturation state can be switched to each other just by manipulation of their deriving amplitudes. In fact, we manage the interference between the two waves' deriving amplitudes just by the plasmonic interactions effect.
Theoretical study of rectification in Tour wires with asymmetric coupling to gold contacts
NASA Astrophysics Data System (ADS)
Taylor, Jeremy; Brandbyge, Mads; Stokbro, Kurt
2002-03-01
We report first-principles studies of electronic transport properties (using the TranSIESTA^1,2 package) of a molecular-scale rectifier consisting of a Tour wire^3 connected to two gold electrodes. The relationship between current rectification and coupling to the electrodes is studied. The device consists of a molecule which is thiol-gold bonded to one electrode and weakly coupled to another electrode through a tunnel barrier. One key feature of the TranSIESTA package is that it uses the same model chemistry for the electrodes and the molecule/device region, thus allowing an ab-initio evaluation of the self-energy due to the electrodes and a quantitative measure of the alignment and broadening of molecular levels. We find that the strong thiol-gold bond leads to a broad transmission resonance which follows below the Fermi level of the strongly coupled electrode, leading to rectification of the current. We find that the width of the resonance must be taken into account in order to understand the rectification properties of such devices. ^1 M. Brandbyge, J. Taylor, K. Stokbro, J.L. Mozos, P. Ordejon,http://xxx.lanl.gov/abs/cond-mat/0108257 and references therein ^2 SIESTA: D. Sanchez-Portal, P. Ordejon, E. Artacho, and J. Soler, Int. J. Quantum Chem. 65, 453 (1997). ^3 J. M. Tour, M. Kozoki, and J.M. Seminario, J. Am. Chem. Soc. 120, 8486 (2001).
Fiber-Coupled Diamond Quantum Nanophotonic Interface
NASA Astrophysics Data System (ADS)
Burek, Michael J.; Meuwly, Charles; Evans, Ruffin E.; Bhaskar, Mihir K.; Sipahigil, Alp; Meesala, Srujan; Machielse, Bartholomeus; Sukachev, Denis D.; Nguyen, Christian T.; Pacheco, Jose L.; Bielejec, Edward; Lukin, Mikhail D.; Lončar, Marko
2017-08-01
Color centers in diamond provide a promising platform for quantum optics in the solid state, with coherent optical transitions and long-lived electron and nuclear spins. Building upon recent demonstrations of nanophotonic waveguides and optical cavities in single-crystal diamond, we now demonstrate on-chip diamond nanophotonics with a high-efficiency fiber-optical interface achieving >90 % power coupling at visible wavelengths. We use this approach to demonstrate a bright source of narrow-band single photons based on a silicon-vacancy color center embedded within a waveguide-coupled diamond photonic crystal cavity. Our fiber-coupled diamond quantum nanophotonic interface results in a high flux (approximately 38 kHz) of coherent single photons (near Fourier limited at <1 -GHz bandwidth) into a single-mode fiber, enabling possibilities for realizing quantum networks that interface multiple emitters, both on chip and separated by long distances.
Coherent Coupled Qubits for Quantum Annealing
NASA Astrophysics Data System (ADS)
Weber, Steven J.; Samach, Gabriel O.; Hover, David; Gustavsson, Simon; Kim, David K.; Melville, Alexander; Rosenberg, Danna; Sears, Adam P.; Yan, Fei; Yoder, Jonilyn L.; Oliver, William D.; Kerman, Andrew J.
2017-07-01
Quantum annealing is an optimization technique which potentially leverages quantum tunneling to enhance computational performance. Existing quantum annealers use superconducting flux qubits with short coherence times limited primarily by the use of large persistent currents Ip. Here, we examine an alternative approach using qubits with smaller Ip and longer coherence times. We demonstrate tunable coupling, a basic building block for quantum annealing, between two flux qubits with small (approximately 50-nA) persistent currents. Furthermore, we characterize qubit coherence as a function of coupler setting and investigate the effect of flux noise in the coupler loop on qubit coherence. Our results provide insight into the available design space for next-generation quantum annealers with improved coherence.
Quantum phase diagram of Polar Molecules in 1D Double Wire Systems
NASA Astrophysics Data System (ADS)
Chang, Chi-Ming; Wang, Daw-Wei
2007-03-01
We study the quantum phase transitions of fermionic polar molecules loaded in a double wire potential. By tuning the magnitude and direction of external electric field we observed many interesting quantum phases in different parameter range, including an easy-plane spin density wave, a triplet superconducting phase, and a truly long range order of easy-axis ferromagnetic phase in strong interacting regime. We also discuss how these exotic quantum phases can be measured in the existing experimental techniques.
Temperature-dependent magnetotransport properties for systems of few quantum wires
NASA Astrophysics Data System (ADS)
Ploner, G.; Smoliner, J.; Strasser, G.; Gornik, E.
1996-09-01
We have investigated temperature-dependent magnetotransport properties of quantum wires fabricated on high mobility GaAsGaAlAs modulation doped heterostructures. Laser holography and optical lithography were used to define multiple quantum wire systems with 40 wires in parallel. These “few wire systems” turn out to have the best signal to noise ratio for systematic magnetic depopulation and magnetophonon resonance measurements. In the examined temperature range between 1.9 and 160 K it was found that the 1D subband energies increase strongly with decreasing 1D electron density and the polaron mass increases with increasing 1D subband spacing. Between 100 and 160 K, magnetophonon resonance data indicate a decline of both the subband spacing and also the polaron mass with increasing temperature. This effect is most probably due to an increase of the electron concentration with increasing temperature.
Spin-dependent electron transport in a Rashba quantum wire with rough edges
NASA Astrophysics Data System (ADS)
Xiao, X. B.; Li, H. L.; Zhou, G. H.; Liu, N. H.
2012-09-01
We investigate theoretically the spin-dependent electron transport in a Rashba quantum wire with rough edges. The charge and spin conductances are calculated as function of the electron energy and wire length by adopting the spin-resolved lattice Green function method. For a single disordered Rashba wire, it is found that the charge conductance quantization is destroyed by the edge disorder. However, a nonzero spin conductance can be generated and its amplitude can be manipulated by varying the wire length, which is attributed to the broken structure symmetries and the spin-dependent quantum interference induced by the rough boundaries. For a large ensemble of disordered Rashba wires, the average charge conductance decreases monotonically, however, the average spin conductance increases to a maximum value and then decreases, with increasing wire length. Further study shows that the influence of the rough edges on the charge and spin conductances can be eliminated by applying a perpendicular magnetic field to the wire. In addition, a very large magnitude of the spin conductance can be achieved when the electron energy lies between the two thresholds of each pair of subbands. These findings may not only benefit to further apprehend the transport properties of the Rashba low-dimensional systems but also provide some theoretical instructions to the application of spintronics devices.
Laser field induced optical gain in a group III-V quantum wire
NASA Astrophysics Data System (ADS)
Saravanan, Subramanian; Peter, Amalorpavam John; Lee, Chang Woo
2016-08-01
Effect of intense high frequency laser field on the electronic and optical properties of heavy hole exciton in an InAsP/InP quantum well wire is investigated taking into consideration of the spatial confinement. Laser field induced exciton binding energies, optical band gap, oscillator strength and the optical gain in the InAs0.8P0.2/InP quantum well wire are studied. The variational formulism is applied to find the respective energies. The laser field induced optical properties are studied. The optical gain as a function of photon energy, in the InAs0.8P0.2/InP quantum wire, is obtained in the presence of intense laser field. The compact density matrix method is employed to obtain the optical gain. The results show that the 1.55 μm wavelength for the fibre optic telecommunication applications is achieved for 45 Å wire radius in the absence of laser field intensity whereas the 1.55 μm wavelength is obtained for 40 Å if the amplitude of the laser field amplitude parameter is 50 Å. The characterizing wavelength for telecommunication network is optimized when the intense laser field is applied for the system. It is hoped that the obtained optical gain in the group III-V narrow quantum wire can be applied for fabricating laser sources for achieving the preferred telecommunication wavelength.
Quantum Strong Coupling with Protein Vibrational Modes.
Vergauwe, Robrecht M A; George, Jino; Chervy, Thibault; Hutchison, James A; Shalabney, Atef; Torbeev, Vladimir Y; Ebbesen, Thomas W
2016-10-07
In quantum electrodynamics, matter can be hybridized to confined optical fields by a process known as light-matter strong coupling. This gives rise to new hybrid light-matter states and energy levels in the coupled material, leading to modified physical and chemical properties. Here, we report for the first time the strong coupling of vibrational modes of proteins with the vacuum field of a Fabry-Perot mid-infrared cavity. For two model systems, poly(l-glutamic acid) and bovine serum albumin, strong coupling is confirmed by the anticrossing in the dispersion curve, the square root dependence on the concentration, and a vacuum Rabi splitting that is larger than the cavity and vibration line widths. These results demonstrate that strong coupling can be applied to the study of proteins with many possible applications including the elucidation of the role of vibrational dynamics in enzyme catalysis and in H/D exchange experiments.
Adiabatic manipulations of Majorana fermions in a three-dimensional network of quantum wires
NASA Astrophysics Data System (ADS)
Halperin, Bertrand I.; Oreg, Yuval; Stern, Ady; Refael, Gil; Alicea, Jason; von Oppen, Felix
2012-04-01
It has been proposed that localized zero-energy Majorana states can be realized in a two-dimensional network of quasi-one-dimensional semiconductor wires that are proximity coupled to a bulk superconductor. The wires should have strong spin-orbit coupling with appropriate symmetry, and their electrons should be partially polarized by a strong Zeeman field. Then, if the Fermi level is in an appropriate range, the wire can be in a topological superconducting phase, with Majorana states that occur at wire ends and at Y junctions, where three topological superconductor segments may be joined. Here we generalize these ideas to consider a three-dimensional network. The positions of Majorana states can be manipulated, and their non-Abelian properties made visible, by using external gates to selectively deplete portions of the network or by physically connecting and redividing wire segments. Majorana states can also be manipulated by reorientations of the Zeeman field on a wire segment, by physically rotating the wire about almost any axis, or by evolution of the phase of the order parameter in the proximity-coupled superconductor. We show how to keep track of sign changes in the zero-energy Hilbert space during adiabatic manipulations by monitoring the evolution of each Majorana state separately, rather than keeping track of the braiding of all possible pairs. This has conceptual advantages in the case of a three-dimensional network, and may be computationally useful even in two dimensions, if large numbers of Majorana sites are involved.
NASA Astrophysics Data System (ADS)
Khan, Mayukh; Teo, Jeffrey; Hughes, Taylor
2015-03-01
Non-abelian anyons exhibit exotic braiding statistics which can be utilized to realize a universal topological quantum computer. In this work we focus on Fibonacci anyons which occur in Z3 Read Rezayi fractional quantum hall states. Traditionally they have been constructed using su(2)3 / u (1) coset theories. We introduce conformal field theories(CFTs) of exceptional and non-simply laced Lie Algebras at level 1, for example G2 ,F4 which host Fibonacci anyons. We realize these CFT's concretely on the 1d gapless edge of an anisotropic 2d system built out of coupled, interacting Luttinger wires. Interactions are introduced within a bundle of wires to fractionalize the original chiral bosons into different sectors. Next, we couple these sectors to get the desired topological phase in the bulk. The 2d bulk of the stack is gapped by backscattering terms between counterpropagating modes on different bundles. The emergence of this topological phase can be interpreted using techniques of anyon condensation . We also explicitly construct the Kac Moody algebra on the edge CFT using original bosonic degrees of freedom.We acknowledge support from NSF CAREER DMR-1351895(TH) and Simons Foundation (JT).
Dielectric confinement influenced screened Coulomb potential for a semiconductor quantum wire
NASA Astrophysics Data System (ADS)
Aharonyan, K. H.; Margaryan, N. B.
2016-01-01
A formalism of the Thomas-Fermi method has been applied for studying the screening effect due to quasi-one-dimensional electron gas in a semiconductor cylindrical quantum wire embedded in the barrier environment. With taking into account of strongly low dielectric properties of the barrier material, an applicability of the quantum wire effective interaction potential of the confined charge carriers has been revealed. Both screened quasi- one-dimensional interaction potential and effective screening length analytical expressions are derived in the first time. It is shown that in the long wavelength moderate limit dielectric confinement effect enhances strength of the screening potential depending on the both radius of the wire and effective screening length, whereas in the long wavelength strong limit the screening potential solely is determined by barrier environment dielectric properties.
Effect of geometry on the screened acceptor binding energy in a quantum wire
Shanthi, R. Vijaya Nithiananthi, P.
2014-04-24
The effect of various Geometries G(x, y) of the GaAs/Al{sub x}Ga{sub 1−x}As Quantum wire like G{sub 1}: (L, L) {sub 2}: (L, L/2) {sub 3}: (L/2, L/4) on the binding energy of an on-center acceptor impurity has been investigated through effective mass approximation using variational technique. The observations were made including the effect of spatial dependent dielectric screening for different concentration of Al, at T=300K. The influence of spatial dielectric screening on different geometries of the wire has been compared and hence the behavior of the acceptor impurity in GaAs/Al{sub x}Ga{sub 1−x}As Quantum wire has been discussed.
Non-Abelian statistics and topological quantum information processing in 1D wire networks
NASA Astrophysics Data System (ADS)
Alicea, Jason; Oreg, Yuval; Refael, Gil; von Oppen, Felix; Fisher, Matthew P. A.
2011-03-01
Topological quantum computation provides an elegant way around decoherence, as one encodes quantum information in a non-local fashion that the environment finds difficult to corrupt. Here we establish that one of the key operations---braiding of non-Abelian anyons---can be implemented in one-dimensional semiconductor wire networks. Previous work [Lutchyn et al., arXiv:1002.4033 and Oreg et al., arXiv:1003.1145] provided a recipe for driving semiconducting wires into a topological phase supporting long-sought particles known as Majorana fermions that can store topologically protected quantum information. Majorana fermions in this setting can be transported, created, and fused by applying locally tunable gates to the wire. More importantly, we show that networks of such wires allow braiding of Majorana fermions and that they exhibit non-Abelian statistics like vortices in a p+ip superconductor. We propose experimental setups that enable the Majorana fusion rules to be probed, along with networks that allow for efficient exchange of arbitrary numbers of Majorana fermions. This work paves a new path forward in topological quantum computation that benefits from physical transparency and experimental realism.
Coulomb interaction effects on the Majorana states in quantum wires.
Manolescu, A; Marinescu, D C; Stanescu, T D
2014-04-30
The stability of the Majorana modes in the presence of a repulsive interaction is studied in the standard semiconductor wire-metallic superconductor configuration. The effects of short-range Coulomb interaction, which is incorporated using a purely repulsive δ-function to model the strong screening effect due to the presence of the superconductor, are determined within a Hartree-Fock approximation of the effective Bogoliubov-De Gennes Hamiltonian that describes the low-energy physics of the wire. Through a numerical diagonalization procedure we obtain interaction corrections to the single particle eigenstates and calculate the extended topological phase diagram in terms of the chemical potential and the Zeeman energy. We find that, for a fixed Zeeman energy, the interaction shifts the phase boundaries to a higher chemical potential, whereas for a fixed chemical potential this shift can occur either at lower or higher Zeeman energies. These effects can be interpreted as a renormalization of the g-factor due to the interaction. The minimum Zeeman energy needed to realize Majorana fermions decreases with the increasing strength of the Coulomb repulsion. Furthermore, we find that in wires with multi-band occupancy this effect can be enhanced by increasing the chemical potential, i.e. by occupying higher energy bands.
Bound Electron States in Skew-symmetric Quantum Wire Intersections
2014-01-01
in transistors, solar cells , LEDs, and diode lasers. They have also investigated quantum dots as agents for medical imaging and as possible qubits in... solar cells .” Chemical reviews 110.11 (2010): 6873-6890. [9] Bonadeo, Nicolas H., et al. ”Coherent optical control of the quantum state of a single...dots on GaAs /InP , (inset) a single InAs quantum dot. two reasons. First, the superposition of the ground and excited states de- phases more slowly in
Band-structure-corrected local density approximation study of semiconductor quantum dots and wires
NASA Astrophysics Data System (ADS)
Li, Jingbo; Wang, Lin-Wang
2005-09-01
This paper presents results of ab initio accuracy thousand atom calculations of colloidal quantum dots and wires using the charge patching method. We have used density functional theory under local density approximation (LDA), and we have corrected the LDA bulk band structures by modifying the nonlocal pseudopotentials, so that their effective masses agree with experimental values. We have systematically studied the electronic states of group III-V (GaAs, InAs, InP, GaN, AlN, and InN) and group II-VI (CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, and ZnO) systems. We have also calculated the electron-hole Coulomb interactions in these systems. We report the exciton energies as functions of the quantum dot sizes and quantum wire diameters for all the above materials. We found generally good agreements between our calculated results and experimental measurements. For CdSe and InP, the currently calculated results agree well with the previously calculated results using semiempirical pseudopotentials. The ratios of band-gap-increases between quantum wires and dots are material-dependent, but a majority of them are close to 0.586, as predicted by the simple effective-mass model. Finally, the size dependence of 1Se-1Pe transition energies of CdSe quantum dots agrees well with the experiment. Our results can be used as benchmarks for future experiments and calculations.
Spin-orbit interaction effects on the electronic structure of coaxial quantum well wires
NASA Astrophysics Data System (ADS)
Ghafari, A.; Vaseghi, B.; Rezaei, G.; Taghizadeh, S. F.; Karimi, M. J.
2017-01-01
The finite element method and effective mass approximation are used to investigate the simultaneous effects of SOI, electric and magnetic fields and quantum geometry on the energy and wave function of a cylindrical quantum well wire. Calculations are performed for a typical GaAs /AlxGa1-xAs cylindrical quantum well wire. It is found that SOI, external fields and layers thickness have considerable effects on the energy eigenvalues and functions of the system. Moreover it is shown that the tunneling effect and energy level anti-crossing depend on the SOI and external factors. We have proposed the SOI strength as an agent to control the tunneling effect in typical nanostructures.
Effect of Γ-X band mixing on the donor binding energy in a Quantum Wire
NASA Astrophysics Data System (ADS)
Vijaya Shanthi, R.; Jayakumar, K.; Nithiananthi, P.
2015-02-01
To invoke the technological applications of heterostructure semiconductors like Quantum Well (QW), Quantum Well Wire (QWW) and Quantum Dot (QD), it is important to understand the property of impurity energy which is responsible for the peculiar electronic & optical behavior of the Low Dimensional Semiconductor Systems (LDSS). Application of hydrostatic pressure P>35kbar drastically alters the band offsets leading to the crossover of Γ band of the well & X band of the barrier resulting in an indirect transition of the carrier and this effect has been studied experimentally and theoretically in a QW structure. In this paper, we have investigated the effect of Γ-X band mixing due to the application of hydrostatic pressure in a GaAs/AlxGa1-xAs QWW system. The results are presented and discussed for various widths of the wire.
Rashba-Zeeman-effect-induced spin filtering energy windows in a quantum wire
Xiao, Xianbo Nie, Wenjie; Chen, Zhaoxia; Zhou, Guanghui; Li, Fei
2014-06-14
We perform a numerical study on the spin-resolved transport in a quantum wire (QW) under the modulation of both Rashba spin-orbit coupling (SOC) and a perpendicular magnetic field by using the developed Usuki transfer-matrix method in combination with the Landauer-Büttiker formalism. Wide spin filtering energy windows can be achieved in this system for unpolarized spin injection. In addition, both the width of energy window and the magnitude of spin conductance within these energy windows can be tuned by varying Rashba SOC strength, which can be apprehended by analyzing the energy dispersions and spin-polarized density distributions inside the QW, respectively. Further study also demonstrates that these Rashba-SOC-controlled spin filtering energy windows show a strong robustness against disorders. These findings may not only benefit to further understand the spin-dependent transport properties of a QW in the presence of external fields but also provide a theoretical instruction to design a spin filter device.
NASA Astrophysics Data System (ADS)
Han, Yu; Li, Qiang; Lau, Kei May
2016-12-01
We report the characteristics of indium gallium arsenide stacked quantum structures inside planar indium phosphide nanowires grown on exact (001) silicon substrates. The morphological evolution of the indium phosphide ridge buffers inside sub-micron trenches has been studied, and the role of inter-facet diffusion in this process is discussed. Inside a single indium phosphide nanowire, we are able to stack quantum structures including indium gallium arsenide flat quantum wells, quasi-quantum wires, quantum wires, and ridge quantum wells. Room temperature photoluminescence measurements reveal a broadband emission spectrum centered at 1550 nm. Power dependent photoluminescence analysis indicates the presence of quasi-continuum states. This work thus provides insights into the design and growth process control of multiple quantum wells in wire structures for high performance nanowire lasers on a silicon substrate with 1550 nm band emission.
Preferential sites for InAsP/InP quantum wire nucleation using molecular dynamics
NASA Astrophysics Data System (ADS)
Nuñez-Moraleda, Bernardo; Pizarro, Joaquin; Guerrero, Elisa; Guerrero-Lebrero, Maria P.; Yáñez, Andres; Molina, Sergio Ignacio; Galindo, Pedro Luis
2014-11-01
In this paper, stress fields at the surface of the capping layer of self-assembled InAsP quantum wires grown on an InP (001) substrate have been determined from atomistic models using molecular dynamics and Stillinger-Weber potentials. To carry out these calculations, the quantum wire compositional distribution was extracted from previous works, where the As and P distributions were determined by electron energy loss spectroscopy and high-resolution aberration-corrected Z-contrast imaging. Preferential sites for the nucleation of wires on the surface of the capping layer were studied and compared with (i) previous simulations using finite element analysis to solve anisotropic elastic theory equations and (ii) experimentally measured locations of stacked wires. Preferential nucleation sites of stacked wires were determined by the maximum stress location at the MD model surface in good agreement with experimental results and those derived from finite element analysis. This indicates that MD simulations based on empirical potentials provide a suitable and flexible tool to study strain dependent atom processes.
Optically excited structural transition in atomic wires on surfaces at the quantum limit
NASA Astrophysics Data System (ADS)
Frigge, T.; Hafke, B.; Witte, T.; Krenzer, B.; Streubühr, C.; Samad Syed, A.; Mikšić Trontl, V.; Avigo, I.; Zhou, P.; Ligges, M.; von der Linde, D.; Bovensiepen, U.; Horn-von Hoegen, M.; Wippermann, S.; Lücke, A.; Sanna, S.; Gerstmann, U.; Schmidt, W. G.
2017-03-01
Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In-In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic). This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors.
Optically excited structural transition in atomic wires on surfaces at the quantum limit.
Frigge, T; Hafke, B; Witte, T; Krenzer, B; Streubühr, C; Samad Syed, A; Mikšić Trontl, V; Avigo, I; Zhou, P; Ligges, M; von der Linde, D; Bovensiepen, U; Horn-von Hoegen, M; Wippermann, S; Lücke, A; Sanna, S; Gerstmann, U; Schmidt, W G
2017-03-29
Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In-In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic). This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors.
Magnetoconductance of interacting electrons in quantum wires in the integer quantum Hall regime.
NASA Astrophysics Data System (ADS)
Zozoulenko, Igor; Ihnatsenka, Siarhei
2008-03-01
We present systematic quantitative description of the magnetoconductance of the split-gate quantum wires. Accounting for the exchange and correlation interactions within the spin density function theory (DFT) leads to the lifting of the spin degeneracy and formation of the spin-resolved plateaus at odd values of e^2/h. We show that the width of the odd conductance steps in the spin DFT calculations is equal to the width of the transition intervals between the conductance steps for the spinless Hartree electrons. A detailed analysis of the structure of compressible/incompressible strips and the evolution of the Hartree and the spin-DFT subband structure provides an explanation of this finding. Our spin-DFT calculations reproduce not only qualitatively, but rather quantitatively all the features in the magnetoconductance observed in the experiment [1] including the unexpected effect of the collapse of the odd conductance plateaus at lower fields. [1] I. P. Radu, J. B. Miller, S. Amasha, E. Levenson-Falk, D. M. Zumbuhl, M. A. Kastner, C. M. Marcus, L. N. Pfeiffer, and K. W. West, unpublished.
InAs/InP single quantum wire formation and emission at 1.5 {mu}m
Alen, B.; Fuster, D.; Gonzalez, Y.; Gonzalez, L.; Martinez-Pastor, J.
2006-12-04
Isolated InAs/InP self-assembled quantum wires have been grown using in situ accumulated stress measurements to adjust the optimal InAs thickness. Atomic force microscopy imaging shows highly asymmetric nanostructures with average length exceeding more than ten times their width. High resolution optical investigation of as-grown samples reveals strong photoluminescence from individual quantum wires at 1.5 {mu}m. Additional sharp features are related to monolayer fluctuations of the two-dimensional InAs layer present during the early stages of the quantum wire self-assembling process.
Measuring hole spin states of single quantum dot in germanium hut wire
NASA Astrophysics Data System (ADS)
Li, Shu-Xiao; Li, Yan; Gao, Fei; Xu, Gang; Li, Hai-Ou; Cao, Gang; Xiao, Ming; Wang, Ting; Zhang, Jian-Jun; Guo, Guo-Ping
2017-03-01
As a group IV material with confined holes, the germanium hut wire is considered a promising candidate for achieving fast electrically controlled spin qubits. Here we fabricated a single quantum dot device on a germanium hut wire and a standard charge stability diagram with excited states was observed by DC transport measurements. By analyzing the Zeeman splitting behaviors of each state, we chose a window for distinguishing different hole parities and spin states, launching the first step towards a useful spin qubit. Effective g-factors around 4.3 for both even and odd hole number states were also extracted.
Scalable quantum computer architecture with coupled donor-quantum dot qubits
Schenkel, Thomas; Lo, Cheuk Chi; Weis, Christoph; Lyon, Stephen; Tyryshkin, Alexei; Bokor, Jeffrey
2014-08-26
A quantum bit computing architecture includes a plurality of single spin memory donor atoms embedded in a semiconductor layer, a plurality of quantum dots arranged with the semiconductor layer and aligned with the donor atoms, wherein a first voltage applied across at least one pair of the aligned quantum dot and donor atom controls a donor-quantum dot coupling. A method of performing quantum computing in a scalable architecture quantum computing apparatus includes arranging a pattern of single spin memory donor atoms in a semiconductor layer, forming a plurality of quantum dots arranged with the semiconductor layer and aligned with the donor atoms, applying a first voltage across at least one aligned pair of a quantum dot and donor atom to control a donor-quantum dot coupling, and applying a second voltage between one or more quantum dots to control a Heisenberg exchange J coupling between quantum dots and to cause transport of a single spin polarized electron between quantum dots.
Thermoelectrics with Coulomb-coupled quantum dots
NASA Astrophysics Data System (ADS)
Thierschmann, Holger; Sánchez, Rafael; Sothmann, Björn; Buhmann, Hartmut; Molenkamp, Laurens W.
2016-12-01
In this article we review the thermoelectric properties of three terminal devices with Coulomb-coupled quantum dots (QDs) as observed in recent experiments [1,2]. The system we consider consists of two Coulomb-blockade QDs, one of which can exchange electrons with only a single reservoir (heat reservoir), while the other dot is tunnel coupled with two reservoirs at a lower temperature (conductor). The heat reservoir and the conductor interact only via the Coulomb coupling of the quantum dots. It has been found that two regimes have to be considered. In the first one, the heat flow between the two systems is small. In this regime, thermally driven occupation fluctuations of the hot QD modify the transport properties of the conductor system. This leads to an effect called thermal gating. Experiments have shown how this can be used to control charge flow in the conductor by means of temperature in a remote reservoir. We further substantiate the observations with model calculations, and implications for the realisation of an all-thermal transistor are discussed. In the second regime, the heat flow between the two systems is relevant. Here the system works as a nanoscale heat engine, as proposed recently (Sánchez and Büttiker [3]). We review the conceptual idea, its experimental realisation and the novel features arising in this new kind of thermoelectric device such as decoupling of heat and charge flow.
Lateral Fano resonances and Kondo effect in the strong coupling regime of a T -coupled quantum dot
NASA Astrophysics Data System (ADS)
Franco, R.; Figueira, M. S.; Anda, E. V.
2006-05-01
We study the electronic transport through a quantum wire (QW), modeled by a tight-binding linear chain, with a side-coupled quantum dot (QD). We obtain the conductance with a strong Fano antiresonance. The calculated density of states shows that this behavior is associated to a many-body renormalized QD resonant level Ef˜ at the edge of the conduction band (CB) strongly hybridized with the Van Hove singularity of the one-dimensional density of states of the lead. Different from the Fano antiresonances experimentally found when this system is at the Kondo regime, this phenomenon appears above the Kondo temperature. It is due to the quantum interference between the ballistic channel and a thermal activated channel created by the QD resonance at the vicinity of the bottom of the CB.
Quantum Otto cycle efficiency on coupled qudits.
Ivanchenko, E A
2015-09-01
Properties of the coupled particles with spin 3/2 (quartits) in a constant magnetic field, as a working substance in the quantum Otto cycle of the heat engine, are considered. It is shown that this system as a converter of heat energy in work (i) shows the efficiency 1 at the negative absolute temperatures of heat baths, (ii) at the temperatures of the opposite sign the efficiency approaches 1, (iii) at the positive temperatures of heat baths antiferromagnetic interaction raises efficiency threefold in comparison with uncoupled particles.
Performance of Pickup Coil Made of Litz Wire and Coupled to HTS SQUID
NASA Astrophysics Data System (ADS)
Enpuku, K.; Momotomi, R.; Mihaya, T.; Matsuo, M.; Yoshida, T.; Kandori, A.; Tanabe, K.; Adachi, S.; Tsukamoto, A.
Performance of a copper pickup coil coupled to HTS SQUID was studied. The coil was made of Litz wire to prevent the eddy current loss at high frequencies, and was cooled at T = 77 K to reduce the thermal noise. The coil with average diameter D = 50 mm and number of turns N = 150 was made with the Litz wire, and was resonantly coupled to HTS SQUID made of ramp-edge junction. The 59-turn input coil was made of superconducting thin film and coupled to the SQUID with flip chip coupling scheme, which gave the mutual inductance of M=1700 pH. The field noise of the magnetometer was measured to be about 1 fT/Hz1/2 for f > 20 kHz. The experimental results agreed reasonably well with the designed values.
Hybrid modelling of near-field coupling onto grounded wire under ultra-short duration perturbation
NASA Astrophysics Data System (ADS)
Ravelo, B.; Liu, Y.
2014-10-01
A time-frequency (TF) hybrid model (HM) for investigating the interaction between EM near-field (NF) aggression and grounded wire is addressed. The HM is based on the combination of techniques for extracting the EM NF radiated by electronic structures and the calculation of electrical disturbances across the wire due to EM coupling. The computation method is fundamentally inspired from transmission line (TL) theory under EM illumination. The methodology including flow chart interpreting the routine algorithm based on the combination of frequency and time domain approaches is featured. An experimental result showing the EM coupling between patch antenna-wire from 1.5-3.5GHz reveals the efficiency of the HM in frequency domain. The relevance of this HM was illustrated with a structure comprised of 20cm aggressor and 5cm victim I-shaped wires placed above a planar ground plane. The aggressor was excited with 40ns duration perturbation signal. After Matlab implementation of the HM, the disturbance voltages across the extremity of the victim wire were extracted. This simple and fast HM is useful for the EMC engineering during the design and fabrication phases of electrical and electronic systems.
Self-assembled quantum dot-sensitized multivalent DNA photonic wires.
Boeneman, Kelly; Prasuhn, Duane E; Blanco-Canosa, Juan B; Dawson, Philip E; Melinger, Joseph S; Ancona, Mario; Stewart, Michael H; Susumu, Kimihiro; Huston, Alan; Medintz, Igor L
2010-12-29
Combining the inherent scaffolding provided by DNA structure with spatial control over fluorophore positioning allows the creation of DNA-based photonic wires with the capacity to transfer excitation energy over distances greater than 150 Å. We demonstrate hybrid multifluorophore DNA-photonic wires that both self-assemble around semiconductor quantum dots (QDs) and exploit their unique photophysical properties. In this architecture, the QDs function as both central nanoscaffolds and ultraviolet energy harvesting donors that drive Förster resonance energy transfer (FRET) cascades through the DNA wires with emissions that approach the near-infrared. To assemble the wires, DNA fragments labeled with a series of increasingly red-shifted acceptor-dyes were hybridized in a predetermined linear arrangement to a complementary DNA template that was chemoselectively modified with a hexahistidine-appended peptide. The peptide portion facilitated metal-affinity coordination of multiple hybridized DNA-dye structures to a central QD completing the final nanocrystal-DNA photonic wire structure. We assembled several such hybrid structures where labeled-acceptor dyes were excited by the QDs and arranged to interact with each other via consecutive FRET processes. The inherently facile reconfiguration properties of this design allowed testing of alternate formats including the addition of an intercalating dye located in the template DNA or placement of multiple identical dye acceptors that engaged in homoFRET. Lastly, a photonic structure linking the central QD with multiple copies of DNA hybridized with 4-sequentially arranged acceptor dyes and demonstrating 4-consecutive energy transfer steps was examined. Step-by-step monitoring of energy transfer with both steady-state and time-resolved spectroscopy allowed efficiencies to be tracked through the structures and suggested that acceptor dye quantum yields are the predominant limiting factor. Integrating such DNA-based photonic
Electron interactions and lasing in high quality GaAs single quantum wires
NASA Astrophysics Data System (ADS)
Akiyama, Hidefumi
2002-03-01
Since the first observation of ground-state lasing in quantum wire lasers(W. Wegscheider, L. N Pfeiffer, M. M Dignam, A. Pinczuk, K. W West, S. L McCall, and R. Hull, Phys. Rev. Lett. 71), 4071 (1993)., questions about existence of band-gap renormalization and contribution of excitons to gain in lasing have been hotly argued but remain unsolved for about a decade. Here, we study these problems in highly-uniform T-shaped quantum wires (T-wires) of 14nm x 6nm cross-sectional size and lasers containing these T-wires, fabricated by the cleaved-edge overgrowth method with molecular-beam epitaxy and a recently developed annealing technique(M. Yoshita, H. Akiyama, L. N. Pfeiffer, and K. W. West, Jpn. J. Appl. Phys. 40), L252 (2001).. We studied PL of modulation-doped single T-wire structures with tunable 1-D electron density by electrical gating to study many-body electron interaction effects. It shows PL of 1-D neutral excitons and charged excitons at low densities, which evolves as the density increases to band-to-band optical recombination of single holes and an electron plasma with significant band-gap renormalization. In undoped twenty-T-wire samples, we found clear signatures of 1-D free excitons and 1-D continuum states in PLE spectra, and biexcitons in strongly pumped PL. We then studied twenty-T-wire lasers via optical pumping. Lasing by T-wires was observed up to about 100 K. Lasing energy was not at the free exciton energy, but at the low-energy tail of biexcitons. Therefore, origin of gain for lasing is attributed not to free excitons, but most probably to biexcitons. We finally realized a single-T-wire laser. In a laser bar of 500μm optical cavity with mirrors coated by gold, lasing was observed for 5-60 K via optical pumping. The threshold power was as low as 5 mW at 5 K, which is equivalent to 3 mA of current injection in generating electron-hole pairs in the device.
Anisotropic Formation of Quantum Turbulence Generated by a Vibrating Wire in Superfluid 4He
NASA Astrophysics Data System (ADS)
Yano, H.; Ogawa, K.; Chiba, Y.; Obara, K.; Ishikawa, O.
2016-12-01
To investigate the formation of quantum turbulence in superfluid 4He, we have studied the emission of vortex rings with a ring size of larger than 38 μm in diameter from turbulence generated by a vibrating wire. The emission rate of vortex rings from a turbulent region remains low until the beginning of high-rate emissions, suggesting that some of the vortex lines produced by the wire combine to form a vortex tangle, until an equilibrium is established between the rate of vortex line combination with the tangle and dissociation. The formation times of equilibrium turbulence are proportional to ɛ^{-1.2} and ɛ^{-0.6} in the directions perpendicular and parallel to the vibrating direction of the generator, respectively, indicating the anisotropic formation of turbulence. Here, ɛ is the generation power of the turbulence. This power dependence may be associated with the characteristics of quantum turbulence with a constant energy flux.
NASA Astrophysics Data System (ADS)
Gold, A.; Ghazali, A.
1990-04-01
We present a theoretical investigation of the electronic properties of a quasi-one-dimensional electron system at very low temperature. For a cylindrical quantum wire the electron-impurity interaction and the electron-electron interaction is calculated for a two-subband model. Our analytical results for the electron-impurity and the electron-electron interaction are in good agreement with the exact results for our model. Analytical results for the band bending due to the filling of the lowest subband are evaluated. Within our analytical results we discuss various aspects of the electronic properties of the semiconductor quantum wire: screening (intrasubband and intersubband plasmons), shallow impurity states (screened and unscreened), and mobility (ionized-impurity scattering and interface-roughness scattering). Analytical expressions are given for the dispersion of plasmons, the binding energies of shallow impurities, and the mobility. Our results on intersubband plasmons are compared with experiments.
Band filling effects on temperature performance of intermediate band quantum wire solar cells
Kunets, Vas. P. Furrow, C. S.; Ware, M. E.; Souza, L. D. de; Benamara, M.; Salamo, G. J.; Mortazavi, M.
2014-08-28
Detailed studies of solar cell efficiency as a function of temperature were performed for quantum wire intermediate band solar cells grown on the (311)A plane. A remotely doped one-dimensional intermediate band made of self-assembled In{sub 0.4}Ga{sub 0.6}As quantum wires was compared to an undoped intermediate band and a reference p-i-n GaAs sample. These studies indicate that the efficiencies of these solar cells depend on the population of the one-dimensional band by equilibrium free carriers. A change in this population by free electrons under various temperatures affects absorption and carrier transport of non-equilibrium carriers generated by incident light. This results in different efficiencies for both the doped and undoped intermediate band solar cells in comparison with the reference GaAs p-i-n solar cell device.
Effective field theory for the quantum electrodynamics of a graphene wire
Faccioli, P.; Lipparini, E.
2009-07-15
We study the low-energy quantum electrodynamics of electrons and holes in a thin graphene wire. We develop an effective field theory (EFT) based on an expansion in p/p{sub T}, where p{sub T} is the typical momentum of electrons and holes in the transverse direction, while p are the momenta in the longitudinal direction. We show that, to the lowest order in (p/p{sub T}), our EFT theory is formally equivalent to the exactly solvable Schwinger model. By exploiting such an analogy, we find that the ground state of the quantum wire contains a condensate of electron-hole pairs. The excitation spectrum is saturated by electron-hole collective bound states, and we calculate the dispersion law of such modes. We also compute the dc conductivity per unit length at zero chemical potential and find g{sub s}(e{sup 2}/h), where g{sub s}=4 is the degeneracy factor.
Nuclear Quantum Effects in H(+) and OH(-) Diffusion along Confined Water Wires.
Rossi, Mariana; Ceriotti, Michele; Manolopoulos, David E
2016-08-04
The diffusion of protons and hydroxide ions along water wires provides an efficient mechanism for charge transport that is exploited by biological membrane channels and shows promise for technological applications such as fuel cells. However, what is lacking for a better control and design of these systems is a thorough theoretical understanding of the diffusion process at the atomic scale. Here we focus on two aspects of this process that are often disregarded because of their high computational cost: the use of first-principles potential energy surfaces and the treatment of the nuclei as quantum particles. We consider proton and hydroxide ions in finite water wires using density functional theory augmented with an apolar cylindrical confining potential. We employ machine learning techniques to identify the charged species, thus obtaining an agnostic definition that takes explicitly into account the delocalization of the charge in the Grotthus-like mechanism. We include nuclear quantum effects (NQEs) through the thermostated ring polymer molecular dynamics method and model finite system size effects by considering Langevin dynamics on the potential of mean force of the charged species, allowing us to extract the same "universal" diffusion coefficient from simulations with different wire sizes. In the classical case, diffusion coefficients depend significantly on the potential energy surface, in particular on how dispersion forces modulate water-water distances. NQEs, however, make the diffusion less sensitive to the underlying potential and geometry of the wire.
Parabolic tailored-potential quantum-wires grown in inverted pyramids
NASA Astrophysics Data System (ADS)
Lazarev, M.; Szeszko, J.; Rudra, A.; Karlsson, K. F.; Kapon, E.
2015-03-01
Quasi-one-dimensional AlGaAs quantum wires (QWRs) with parabolic heterostructure profiles along their axis were fabricated using metallorganic vapor phase epitaxy (MOVPE) on patterned (111)B GaAs substrates. Tailoring of the confined electronic states via modification in the parabolic potential profile is demonstrated using model calculations and photoluminescence spectroscopy. These novel nanostructures are useful for studying the optical properties of systems with dimensionality between zero and one.
Double quantum wire Aharonov-Bohm interferometers for possible LN 2 temperature operation
NASA Astrophysics Data System (ADS)
Bandyopadhyay, S.; Porod, W.
In this paper, we discuss the design of semiconductor electrostatic and magnetostatic Aharonov-Bohm interferometers that could operate at liquid nitrogen temperature. We find that for elevated temperature operation, one dimensional structures constructed from quantum wires are invariably the only choice, especially when transport is diffusive instead of ballistic. We have proposed such a structure which can be fabricated by present day technology. It may exhibit large conductance modulation in an electric field at 77 K and is an ideal configuration for "Quantum Interference Transistors" (QUITS) based on the electrostatic Aharonov-Bohm effect.
Spatially indirect excitons in coupled quantum wells
Lai, Chih-Wei Eddy
2004-03-01
Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer)^{2} were
NASA Astrophysics Data System (ADS)
Sahoo, Sharmistha; Zhang, Zhao; Teo, Jeffrey
Time reversal symmetric topological superconductors in three spatial dimensions carry gapless surface Majorana fermions. They are robust against any time reversal symmetric single-body perturbation weaker than the bulk energy gap. We mimic the massless surface Majorana's by coupled wire models in two spatial dimensions. We introduce explicit many-body interwire interactions that preserve time reversal symmetry and give energy gaps to all low energy degrees of freedom. The gapping 4-fermion interactions are constructed by interwire Kac-Moody current backscattering and rely on the fractionalization or conformal embedding of the Majorana wires.
Interaction effects in a microscopic quantum wire model with strong spin-orbit interaction
NASA Astrophysics Data System (ADS)
Winkler, G. W.; Ganahl, M.; Schuricht, D.; Evertz, H. G.; Andergassen, S.
2017-06-01
We investigate the effect of strong interactions on the spectral properties of quantum wires with strong Rashba spin-orbit (SO) interaction in a magnetic field, using a combination of matrix product state and bosonization techniques. Quantum wires with strong Rashba SO interaction and magnetic field exhibit a partial gap in one-half of the conducting modes. Such systems have attracted wide-spread experimental and theoretical attention due to their unusual physical properties, among which are spin-dependent transport, or a topological superconducting phase when under the proximity effect of an s-wave superconductor. As a microscopic model for the quantum wire we study an extended Hubbard model with SO interaction and Zeeman field. We obtain spin resolved spectral densities from the real-time evolution of excitations, and calculate the phase diagram. We find that interactions increase the pseudo gap at k = 0 and thus also enhance the Majorana-supporting phase and stabilize the helical spin order. Furthermore, we calculate the optical conductivity and compare it with the low energy spiral Luttinger liquid result, obtained from field theoretical calculations. With interactions, the optical conductivity is dominated by an excotic excitation of a bound soliton-antisoliton pair known as a breather state. We visualize the oscillating motion of the breather state, which could provide the route to their experimental detection in e.g. cold atom experiments.
Negatively charged donors in parabolic quantum-well wires under magnetic fields
NASA Astrophysics Data System (ADS)
Zhai, Li-Xue; Liu, Jian-Jun
2007-09-01
The ground state of a negatively charged donor (D-) in a parabolic GaAs quantum-well wire in the presence of a magnetic field is investigated using the finite difference method within the quasi-one-dimensional effective potential model. The magnetic effects on the binding energies of the ground state of a D- center are calculated for various parabolic potentials. The distance between the electrons and the donor ion and the distance between the two electrons are also calculated, respectively, as a function of the strength of the parabolic potential and the magnetic field. We find that the interplay of the spatial confinement and the magnetic confinement of electrons in quantum-well wires leads to complicated behavior of the binding energies of the D- center and that the increase of the electron-donor ion attraction dominates the increase of the electron-electron repulsion as the spatial and magnetic confinement increases for the ground state of a D- center in a parabolic quantum-well wire.
Classical and quantum distinctions between weak and strong coupling
NASA Astrophysics Data System (ADS)
Rahimzadeh-Kalaleh Rodriguez, Said
2016-03-01
Coupled systems subject to dissipation exhibit two different regimes known as weak coupling and strong coupling. Two damped coupled harmonic oscillators (CHOs) constitute a model system where the key features of weak and strong coupling can be identified. Several of these features are common to classical and quantum systems, as a number of quantum-classical correspondences have shown. However, the condition defining the boundary between weak and strong coupling is distinct in classical and quantum formalisms. Here we describe the origin of two widely used definitions of strong coupling. Using a classical CHO model, we show that energy exchange cycles and avoided resonance crossings signal the onset of strong coupling according to one criterion. From the classical CHO model we derive a non-Hermitian Hamiltonian describing open quantum systems. Based on the analytic properties of the Hamiltonian, we identify the boundary between weak and strong coupling with a different feature: a non-Hermitian degeneracy known as the exceptional point. For certain parameter ranges the classical and quantum criterion for strong coupling coincide; for other ranges they do not. Examples of systems in strong coupling according to one or another criterion, but not both, are illustrated. The framework here presented is suitable for introducing graduate or advanced undegraduate students to the basic properties of strongly coupled systems, as well as to the similarities and subtle differences between classical and quantum descriptions of coupled dissipative systems.
NASA Astrophysics Data System (ADS)
Gutiérrez, R.; Caetano, R.; Woiczikowski, P. B.; Kubar, T.; Elstner, M.; Cuniberti, G.
2010-02-01
Charge transport through a short DNA oligomer (Dickerson dodecamer (DD)) in the presence of structural fluctuations is investigated using a hybrid computational methodology based on a combination of quantum mechanical electronic structure calculations and classical molecular dynamics (MD) simulations with a model Hamiltonian approach. Based on a fragment orbital description, the DNA electronic structure can be coarse-grained in a very efficient way. The influence of dynamical fluctuations, arising either from the solvent fluctuations or from base-pair vibrational modes, can be taken into account in a straightforward way through the time series of the effective DNA electronic parameters, evaluated at snapshots along the MD trajectory. We show that charge transport can be promoted through the coupling to solvent fluctuations, which gate the on-site energies along the DNA wire.
Transport of high intensity laser-generated hot electrons in cone coupled wire targets
NASA Astrophysics Data System (ADS)
Beg, Farhat
2008-04-01
In this talk, we present results from a series of experiments where cone-wire targets were employed both to assess hot electron coupling efficiency, and to reveal the source temperature of the hot electrons. Experiments were performed on the petawatt laser at the Rutherford Appleton Laboratory. A 500J, 1ps laser (I ˜ 4 x 10^20 W/cm-2) was focused by an f/3 off-axis parabolic mirror into hollow aluminum cones joined at their tip to Cu wires of diameters from 10 to 40 μm. The three main diagnostics fielded were a copper Kalpha Bragg crystal imager, a single hit CCD camera spectrometer and a Highly Oriented Pyrolytic Graphite (HOPG) spectrometer. The resulting data were cross-calibrated to obtain the absolute Kalpha yield. Comparison of the axially diminishing absolute Cu Kα intensity with modeling shows that the penetration of the hot electrons is consistent with one dimensional ohmic potential limited transport (1/e length ˜ 100 μm). The laser coupling efficiency to electron energy within the wire is shown to be proportional to the cross sectional area of the wire, reaching 15% for 40 μm wires. We find that the hot electron temperature within the wire was <=750 keV, significantly lower than that predicted by the ponderomotive scaling. A comparison of the experimental results with 2D hybrid PIC simulations using e-PLAS code will be presented and relevance to Fast Ignition will be discussed at the meeting. *In collaboration with J.A. King, M.H. Key, K.U. Akli, R.R. Freeman, J. Green, S. P. Hatchett, D. Hey, P. Jaanimagi, J. Koch, K. L. Lancaster, T. Ma, A.J. MacKinnon, A. MacPhee, R. Mason, P.A. Norreys, P.K Patel, T. Phillips, R. Stephens, W. Theobald, R.P.J. Town, M. Wei, L. Van Woerkom, B. Zhang.
Kinematic projective quantum states for loop quantum gravity coupled to tensor fields
NASA Astrophysics Data System (ADS)
Okołów, Andrzej
2017-04-01
We present a construction of kinematic quantum states for theories of tensor fields of an arbitrary sort. The construction is based on projective techniques by Kijowski. Applying projective quantum states for Loop Quantum Gravity (LQG) obtained by Lanéry and Thiemann we construct quantum states for LQG coupled to tensor fields.
Quantum wire structures by MBE overgrowth on a cleaved edge
NASA Astrophysics Data System (ADS)
Pfeiffer, Loren; Störmer, H. L.; West, K.; Baldwin, K. W.
1991-05-01
We have recently demonstrated the existence of a high mobility (6.1×10 5 cm 2/V·s) two-dimensional electron gas (2DEG) at the (110) vicinal surface formed by cleaving [L. Pfeiffer et al., Appl. Phys. Letters 56 (1990) 1697] a (100) GaAs wafer. We have now expanded this work to modulation-doped overgrowth on the cleaved edge of a multiperiod superlattice. We report here the first observation of the quantum Hall characteristics in such a two-dimensional system containing an atomically precise 71 Å GaAs by 31 Å Al 0.24Ga 0.76As compositional superlattice. The onset of Shubnikov-De Haas oscillations occurs at only 3000 G, implying the Landau cyclotron orbits are phase coherent over diameters as large as 5000 Å, corresponding to more than 200 GaAs/AlGaAs interface crossings.
New insights into ETS-10 and titanate quantum wire: a comprehensive characterization.
Jeong, Nak Cheon; Lee, Young Ju; Park, Jung-Hyun; Lim, Hyunjin; Shin, Chae-Ho; Cheong, Hyeonsik; Yoon, Kyung Byung
2009-09-16
The titanate quantum wires in ETS-10 crystals remain intact during ion exchange of the pristine cations (Na(+)(0.47) + K(+)(0.53)) with M(n+) ions (M(n+) = Na(+), K(+), Mg(2+), Ca(2+), Sr(2+), Ba(2+), Pb(2+), Cd(2+), Zn(2+)) and during reverse exchange of the newly exchanged cations with Na(+). The binding energies of O(1s) and Ti(2p) decrease as the electronegativity of the cation decreases, and they are inversely proportional to the negative partial charge of the framework oxygen [-delta(O(f))]. At least five different oxygen species were identified, and their binding energies (526.1-531.9 eV) indicate that the titanate-forming oxides are much more basic than those of aluminosilicate zeolites (530.2-533.3 eV), which explains the vulnerability of the quantum wire to acids and oxidants. The chemical shifts of the five NMR-spectroscopically nonequivalent Si sites, delta(I(A)), delta(I(B)), delta(II(A)), delta(II(B)), and delta(III), shift downfield as -delta(O(f)) increases, with slopes of 2.5, 18.6, 133.5, 216.3, and 93.8 ppm/[-delta(O(f))], respectively. The nonuniform responses of the chemical shifts to -delta(O(f)) arise from the phenomenon that the cations in the 12-membered-ring channels shift to the interiors of the cages surrounded by four seven-membered-ring windows. On the basis of the above, we assign delta(I(A)), delta(I(B)), delta(II(A)), and delta(II(B)) to the chemical shifts arising from Si(12,12), Si(12,7), Si(7,12), and Si(7,7) atoms, respectively. The frequency of the longitudinal stretching vibration of the titanate quantum wire increases linearly and the bandwidth decreases nonlinearly with increasing -delta(O(f)), indicating that the titanate quantum wire resembles a metallic carbon nanotube. As the degree of hydration increases, the vibrational frequency shifts linearly to higher frequencies while the bandwidth decreases. We identified another normal mode of vibration of the quantum wire, which vibrates in the region of 274-280 cm(-1). In the
Junctions of Multiple Quantum Wires with Different Luttinger Parameters
2012-08-22
each of the charge and spin sectors. We present numerical density-matrix- renormalization -group calculations of the conductance of a junction of two...invariant BCs for three TLLs. Such asymmetric fixed points have been identified using perturbative renormalization group analysis in the weak coupling regime...21,57,58 Another important result of this paper is a direct numerical verification, through density-matrix renormalization group (DMRG) computations
Dissipative Quantum Tunneling of a Single Defect in a Submicron Bismuth Wire Below 1 K
NASA Astrophysics Data System (ADS)
Chun, Kookjin
The quantum mechanical problem of a particle tunneling in a double-well potential is of great theoretical and experimental interest. Interaction of the tunneling system with a dissipative environment can have a striking effect on the tunneling dynamics. A very interesting case is that of ohmic dissipation, which occurs when an atom tunnels in a metal in the presence of conduction electrons. We have studied the electrical resistance of submicron Bi wires at low temperature. Due to quantum interference of the conduction electrons, the resistance is highly sensitive to the motion of even a single scattering center. We observe discrete switching of the resistance due to the motion of bistable defects in the sample. We have measured the tunneling rates of a particular defect over the temperature range 0.1-2 K and magnetic field range 0-7 T. The energy asymmetry, varepsilon, of this defect varied over the range 40-420 mK depending on the value of the magnetic field. The temperature dependence of the tunneling rates is qualitatively different for the cases k_{B}T << varepsilon and k_{B}T gg varepsilon . We observe that for k_{B }T << varepsilon, the fast rate (transition rate from upper state to lower state) is roughly temperature independent and the slow rate (transition from lower state to upper state) decreases exponentially, as expected from a simple picture of spontaneous emission and stimulated absorption. When k_{B }T gg varepsilon, however, both rates increase as the temperature is lowered, as predicted by dissipative quantum tunneling theory. We fit our data to the theory and discuss the defect-electron bath coupling parameter alpha, and the renormalized tunneling matrix element Delta_{ rm r}. We have also studied the effect of Joule heating on the dynamics of the defect in the same sample. The ratio of the fast and slow transition rates of a defect depends on temperature through the detailed balance relation, gamma_{f}/ gamma_{s} e^ {varepsilon / k_{B}T}. We
The standard model coupled to quantum gravitodynamics
NASA Astrophysics Data System (ADS)
Aldabe, Fermin
2017-01-01
We show that the renormalizable SO(4)× U(1)× SU(2)× SU(3) Yang-Mills coupled to matter and the Higgs field fits all the experimentally observed differential cross sections known in nature. This extended Standard Model reproduces the experimental gravitational differential cross sections without resorting to the graviton field and instead by exchanging SO(4) gauge fields. By construction, each SO(4) generator in quantum gravitodynamics does not commute with the Dirac gamma matrices. This produces additional interactions absent to non-Abelian gauge fields in the Standard Model. The contributions from these new terms yield differential cross sections consistent with the Newtonian and post-Newtonian interactions derived from General Relativity. Dimensional analysis of the Lagrangian shows that all its terms have total dimensionality four or less and therefore that all physical quantities in the theory renormalize by finite amounts. These properties make QGD the only renormalizable four-dimensional theory describing gravitational interactions.
Long-distance coherent coupling in a quantum dot array.
Braakman, F R; Barthelemy, P; Reichl, C; Wegscheider, W; Vandersypen, L M K
2013-06-01
Controlling long-distance quantum correlations is central to quantum computation and simulation. In quantum dot arrays, experiments so far rely on nearest-neighbour couplings only, and inducing long-distance correlations requires sequential local operations. Here, we show that two distant sites can be tunnel-coupled directly. The coupling is mediated by virtual occupation of an intermediate site, with a strength that is controlled via the energy detuning of this site. It permits a single charge to oscillate coherently between the outer sites of a triple dot array without passing through the middle, as demonstrated through the observation of Landau-Zener-Stückelberg interference. The long-distance coupling significantly improves the prospects of fault-tolerant quantum computation using quantum dot arrays, and opens up new avenues for performing quantum simulations in nanoscale devices.
Quantum dissipative effect of one dimension coupled anharmonic oscillator
Sulaiman, A.; Zen, Freddy P.
2015-04-16
Quantum dissipative effect of one dimension coupled anharmonic oscillator is investigated. The systems are two coupled harmonic oscillator with the different masses. The dissipative effect is studied based on the quantum state diffusion formalism. The result show that the anharmonic effect increase the amplitude but the lifetime of the oscillation depend on the damping coefficient and do not depend on the temperature.
NASA Astrophysics Data System (ADS)
Danga, J. E.; Kenfack, S. C.; Fai, L. C.
2016-05-01
Landau-Zener-Stückelberg interferometry is extensively investigated in a 3D heterostructure magnetic quantum wire. Local magnetic fields are used to coherently manipulate and control a qubit’s quantum state. For our numerical calculations, a parabolic confinement is assumed. Energy eigenvalues, non-adiabatic and adiabatic transition probabilities are calculated from the diabatic and adiabatic bases for two-level systems. Here, we show that the spatial crossing between interspin levels becomes a spatial anticrossing if the two spin states are coupled by external fields, and that consequently, due to the spin dependence of the harmonic confinement, it will undergo Landau-Zener-Stückelberg interference. It is shown that the system undergoes nonadiabatic Landau-Zener dynamics for a strong confinement in a strong external field, whereas a weak external field induces adiabatic Landau-Zener transition dynamics. Our system allows the coupling strength between the level states at the anti(crossing) point to be modulated. This system allows one to tune the wire’s parabolic confinement potential using experimentally accessible parameters.
UV Nano-Lights: Nonlinear Quantum Dot-Plasmon Coupling
2014-08-01
Final 3. DATES COVERED (From - To) 11-Mar-2013 to 10-Mar-2014 4. TITLE AND SUBTITLE UV Nano-Lights: Nonlinear Quantum Dot- Plasmon ...Nonlinear Quantum Dot- Plasmon Coupling 5a. CONTRACT NUMBER FA2386-13-1-4016 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 61102F 6. AUTHOR(S) Eric...nonlinear emission from Quantum Dots through Plasmon Coupling PERIOD OF PERFORMANCE 11 March 2013 - 11 March 2014 RECIPIENT PRINCIPAL
Spin-orbit-coupled quantum gases
NASA Astrophysics Data System (ADS)
Radic, Juraj
The dissertation explores the effects of synthetic spin-orbit coupling on the behaviour of quantum gases in several different contexts. We first study realistic methods to create vortices in spin-orbit-coupled (SOC) Bose-Einstein condensates (BEC). We propose two different methods to induce thermodynamically stable static vortex configurations: (1) to rotate both the Raman lasers and the anisotropic trap; and (2) to impose a synthetic Abelian field on top of synthetic spin-orbit interactions. We solve the Gross-Pitaevskii equation for several experimentally relevant regimes and find new interesting effects such as spatial separation of left- and right-moving spin-orbit-coupled condensates, and the appearance of unusual vortex arrangements. Next we consider cold atoms in an optical lattice with synthetic SOC in the Mott-insulator regime. We calculate the parameters of the corresponding tight-binding model and derive the low-energy spin Hamiltonian which is a combination of Heisenberg model, quantum compass model and Dzyaloshinskii-Moriya interaction. We find that the Hamiltonian supports a rich classical phase diagram with collinear, spiral and vortex phases. Next we study the time evolution of the magnetization in a Rashba spin-orbit-coupled Fermi gas, starting from a fully-polarized initial state. We model the dynamics using a Boltzmann equation, which we solve in the Hartree-Fock approximation. The resulting non-linear system of equations gives rise to three distinct dynamical regimes controlled by the ratio of interaction and spin-orbit-coupling strength lambda: for small lambda, the magnetization decays to zero. For intermediate lambda, it displays undamped oscillations about zero and for large lambda, a partially magnetized state is dynamically stabilized. Motivated by an interesting stripe phase which appears in BEC with SOC [Li et al., Phys. Rev. Lett. 108, 225301 (2011)], we study the finite-temperature phase diagram of a pseudospin-1/2 Bose gas with
Trapping of an electron in coupled quantum dots in graphene
NASA Astrophysics Data System (ADS)
Hewageegana, Prabath; Apalkov, Vadym
2009-03-01
Due to Klein’s tunneling the electronic states of a quantum dot in graphene have finite widths and an electron in quantum dot has a finite trapping time. This property introduces a special type of interdot coupling in a system of many quantum dots in graphene. The interdot coupling is realized not as a direct tunneling between quantum dots but as coupling through the continuum states of graphene. As a result the interdot coupling modifies both the positions and the widths of the energy levels of the quantum dot system. We study the system of quantum dots in graphene theoretically by analyzing the complex energy spectra of the quantum dot system. We show that in a double-dot system some energy levels become strongly localized with an infinite trapping time. Such strongly localized states are achieved only at one value of the interdot separation. We also study a periodic array of quantum dots in graphene within a tight-binding mode for a quantum dot system. The values of the hopping integrals in the tight-binding model are found from the expression for the energy spectra of the double quantum dot system. In the array of quantum dots the states with infinitely large trapping time are realized at all values of interdot separation smaller than some critical value. Such states have nonzero wave vectors.
Landau quantized dynamics and spectra for group-VI dichalcogenides, including a model quantum wire
NASA Astrophysics Data System (ADS)
Horing, Norman J. M.
2017-06-01
This work is concerned with the derivation of the Green's function for Landau-quantized carriers in the Group-VI dichalcogenides. In the spatially homogeneous case, the Green's function is separated into a Peierls phase factor and a translationally invariant part which is determined in a closed form integral representation involving only elementary functions. The latter is expanded in an eigenfunction series of Laguerre polynomials. These results for the retarded Green's function are presented in both position and momentum representations, and yet another closed form representation is derived in circular coordinates in terms of the Bessel wave function of the second kind (not to be confused with the Bessel function). The case of a quantum wire is also addressed, representing the quantum wire in terms of a model one-dimensional δ (x ) -potential profile. This retarded Green's function for propagation directly along the wire is determined exactly in terms of the corresponding Green's function for the system without the δ (x ) -potential, and the Landau quantized eigenenergy dispersion relation is examined. The thermodynamic Green's function for the dichalcogenide carriers in a normal magnetic field is formulated here in terms of its spectral weight, and its solution is presented in a momentum/integral representation involving only elementary functions, which is subsequently expanded in Laguerre eigenfunctions and presented in both momentum and position representations.
Higher-order mesoscopic fluctuations in quantum wires: Conductance and current cumulants
NASA Astrophysics Data System (ADS)
Stenberg, Markku P. V.; Särkkä, Jani
2006-07-01
We study conductance cumulants ⟪gn⟫ and current cumulants Cj related to heat and electrical transport in coherent mesoscopic quantum wires near the diffusive regime. We consider the asymptotic behavior in the limit where the number of channels and the length of the wire in the units of the mean free path are large but the bare conductance is fixed. A recursion equation unifying the descriptions of the standard and Bogoliubov-de Gennes (BdG) symmetry classes is presented. We give values and come up with a novel scaling form for the higher-order conductance cumulants. In the BdG wires, in the presence of time-reversal symmetry, for the cumulants higher than the second it is found that there may be only contributions which depend nonanalytically on the wire length. This indicates that diagrammatic or semiclassical pictures do not adequately describe higher-order spectral correlations. Moreover, we obtain the weak-localization corrections to Cj with j⩽10 .
NASA Astrophysics Data System (ADS)
Tsukui, Tetsuya; Oda, Shunri
1995-02-01
We have proposed the trench-oxide metal-oxide-semiconductor (MOS) structure as a novel quantum wire device. In this paper we present results of computer simulation based on a self-consistent system and calculated quantized electron distribution and capacitance-voltage (C-V) characteristics. We have also fabricated the quantum wire MOS structure using electron beam lithography and electron cyclotron resonance reactive ion etching method and carried out measurements of C-V characteristics at 0.55 K. Possible evidence of one-dimensional quantum effect is obtained for the first time from C-V measurements using the 28 nm-wide trench-oxide structure.
Chida, K.; Yamauchi, Y.; Arakawa, T.; Kobayashi, K.; Ono, T.; Hashisaka, M.; Nakamura, S.; Machida, T.
2013-12-04
We performed the resistively-detected nuclear magnetic resonance (RDNMR) to study the electron spin polarization in the non-equilibrium quantum Hall regime. By measuring the Knight shift, we derive source-drain bias voltage dependence of the electron spin polarization in quantum wires. The electron spin polarization shows minimum value around the threshold voltage of the dynamic nuclear polarization.
Origins and optimization of entanglement in plasmonically coupled quantum dots
Otten, Matthew; Larson, Jeffrey; Min, Misun; Wild, Stefan M.; Pelton, Matthew; Gray, Stephen K.
2016-08-11
In this paper, a system of two or more quantum dots interacting with a dissipative plasmonic nanostructure is investigated in detail by using a cavity quantum electrodynamics approach with a model Hamiltonian. We focus on determining and understanding system configurations that generate multiple bipartite quantum entanglements between the occupation states of the quantum dots. These configurations include allowing for the quantum dots to be asymmetrically coupled to the plasmonic system. Analytical solution of a simplified limit for an arbitrary number of quantum dots and numerical simulations and optimization for the two- and three-dot cases are used to develop guidelines for maximizing the bipartite entanglements. For any number of quantum dots, we show that through simple starting states and parameter guidelines, one quantum dot can be made to share a strong amount of bipartite entanglement with all other quantum dots in the system, while entangling all other pairs to a lesser degree.
Origins and optimization of entanglement in plasmonically coupled quantum dots
Otten, Matthew; Larson, Jeffrey; Min, Misun; ...
2016-08-11
In this paper, a system of two or more quantum dots interacting with a dissipative plasmonic nanostructure is investigated in detail by using a cavity quantum electrodynamics approach with a model Hamiltonian. We focus on determining and understanding system configurations that generate multiple bipartite quantum entanglements between the occupation states of the quantum dots. These configurations include allowing for the quantum dots to be asymmetrically coupled to the plasmonic system. Analytical solution of a simplified limit for an arbitrary number of quantum dots and numerical simulations and optimization for the two- and three-dot cases are used to develop guidelines formore » maximizing the bipartite entanglements. For any number of quantum dots, we show that through simple starting states and parameter guidelines, one quantum dot can be made to share a strong amount of bipartite entanglement with all other quantum dots in the system, while entangling all other pairs to a lesser degree.« less
Origins and optimization of entanglement in plasmonically coupled quantum dots
NASA Astrophysics Data System (ADS)
Otten, Matthew; Larson, Jeffrey; Min, Misun; Wild, Stefan M.; Pelton, Matthew; Gray, Stephen K.
2016-08-01
A system of two or more quantum dots interacting with a dissipative plasmonic nanostructure is investigated in detail by using a cavity quantum electrodynamics approach with a model Hamiltonian. We focus on determining and understanding system configurations that generate multiple bipartite quantum entanglements between the occupation states of the quantum dots. These configurations include allowing for the quantum dots to be asymmetrically coupled to the plasmonic system. Analytical solution of a simplified limit for an arbitrary number of quantum dots and numerical simulations and optimization for the two- and three-dot cases are used to develop guidelines for maximizing the bipartite entanglements. For any number of quantum dots, we show that through simple starting states and parameter guidelines, one quantum dot can be made to share a strong amount of bipartite entanglement with all other quantum dots in the system, while entangling all other pairs to a lesser degree.
Ultranarrow resonance in Coulomb drag between quantum wires at coinciding densities
NASA Astrophysics Data System (ADS)
Dmitriev, A. P.; Gornyi, I. V.; Polyakov, D. G.
2016-08-01
We investigate the influence of the chemical potential mismatch Δ (different electron densities) on Coulomb drag between two parallel ballistic quantum wires. For pair collisions, the drag resistivity ρD(Δ ) shows a peculiar anomaly at Δ =0 with ρD being finite at Δ =0 and vanishing at any nonzero Δ . The "bodyless" resonance in ρD(Δ ) at zero Δ is only broadened by processes of multiparticle scattering. We analyze Coulomb drag for finite Δ in the presence of both two- and three-particle scattering within the kinetic equation framework, focusing on a Fokker-Planck picture of the interaction-induced diffusion in momentum space of the double-wire system. We describe the dependence of ρD on Δ for both weak and strong intrawire equilibration due to three-particle scattering.
Nonlinear spectra of spinons and holons in short GaAs quantum wires
Moreno, M; Ford, C. J. B.; Jin, Y.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Tsyplyatyev, O.; Schofield, A. J.
2016-01-01
One-dimensional electronic fluids are peculiar conducting systems, where the fundamental role of interactions leads to exotic, emergent phenomena, such as spin-charge (spinon-holon) separation. The distinct low-energy properties of these 1D metals are successfully described within the theory of linear Luttinger liquids, but the challenging task of describing their high-energy nonlinear properties has long remained elusive. Recently, novel theoretical approaches accounting for nonlinearity have been developed, yet the rich phenomenology that they predict remains barely explored experimentally. Here, we probe the nonlinear spectral characteristics of short GaAs quantum wires by tunnelling spectroscopy, using an advanced device consisting of 6000 wires. We find evidence for the existence of an inverted (spinon) shadow band in the main region of the particle sector, one of the central predictions of the new nonlinear theories. A (holon) band with reduced effective mass is clearly visible in the particle sector at high energies. PMID:27627993
Electron states and electron Raman scattering in a semiconductor step-quantum well wire
NASA Astrophysics Data System (ADS)
Betancourt-Riera, Ri.; Betancourt-Riera, Re.; Munguía-Rodríguez, M.
2017-06-01
The differential cross-section for an electron Raman scattering process in a semiconductor GaAs / AlGaAs step-quantum well wire is calculated and expressions for the electron states are presented. The system is modeled by considering T = 0 K and also by a single parabolic conduction band, which is split into a sub-band system due to confinement. The net Raman gain for an electron Raman scattering process is obtained. Also, the emission spectra for several scattering configurations are discussed, and the interpretation of the singularities found in the spectra is given. The results obtained in this study are compared with those obtained for other structures, and so it has been demonstrated that the wire shows greater efficiency.
UV Nano Lights - Nonlinear Quantum Dot-Plasmon Coupling
2016-06-20
AFRL-AFOSR-JP-TR-2016-0072 UV Nano- Lights - Nonlinear Quantum Dot-Plasmon Coupling Eric Waclawik QUEENSLAND UNIVERSITY OF TECHNOLOGY Final Report 06...Final 3. DATES COVERED (From - To) 03 Feb 2014 to 02 Feb 2016 4. TITLE AND SUBTITLE UV Nano- Lights - Nonlinear Quantum Dot-Plasmon Coupling 5a...coupled to gold (Au) nanoparticles, were examined both theoretically and experimentally. This work was performed to establish if confined light fields
Scalable quantum memory in the ultrastrong coupling regime.
Kyaw, T H; Felicetti, S; Romero, G; Solano, E; Kwek, L-C
2015-03-02
Circuit quantum electrodynamics, consisting of superconducting artificial atoms coupled to on-chip resonators, represents a prime candidate to implement the scalable quantum computing architecture because of the presence of good tunability and controllability. Furthermore, recent advances have pushed the technology towards the ultrastrong coupling regime of light-matter interaction, where the qubit-resonator coupling strength reaches a considerable fraction of the resonator frequency. Here, we propose a qubit-resonator system operating in that regime, as a quantum memory device and study the storage and retrieval of quantum information in and from the Z2 parity-protected quantum memory, within experimentally feasible schemes. We are also convinced that our proposal might pave a way to realize a scalable quantum random-access memory due to its fast storage and readout performances.
Scalable quantum memory in the ultrastrong coupling regime
Kyaw, T. H.; Felicetti, S.; Romero, G.; Solano, E.; Kwek, L.-C.
2015-01-01
Circuit quantum electrodynamics, consisting of superconducting artificial atoms coupled to on-chip resonators, represents a prime candidate to implement the scalable quantum computing architecture because of the presence of good tunability and controllability. Furthermore, recent advances have pushed the technology towards the ultrastrong coupling regime of light-matter interaction, where the qubit-resonator coupling strength reaches a considerable fraction of the resonator frequency. Here, we propose a qubit-resonator system operating in that regime, as a quantum memory device and study the storage and retrieval of quantum information in and from the Z2 parity-protected quantum memory, within experimentally feasible schemes. We are also convinced that our proposal might pave a way to realize a scalable quantum random-access memory due to its fast storage and readout performances. PMID:25727251
Single and coupled quantum wells: SiGe
NASA Astrophysics Data System (ADS)
Usami, N.; Shiraki, Y.
This document is part of subvolume C3 'Optical Properties' of volume 34 'Semiconductor quantum structures' of Landolt-Börnstein, Group III, Condensed Matter, on the optical properties of quantum structures based on group IV semiconductors. It discusses single and coupled quantum wells based on SiGe. Topics include the photoluminescence from SiGe/Si quantum wells (spectral features, dependence on excitation power and temperature), effects of quantum confinement, post-growth annealing, electric fields and external stress, the Fermi-edge singularity, time-resolved photoluminescence, growth mode transition, type-II strained Si quantum wells, coupled quantum wells, electroluminescence, interband absorption and intraband absorption, second-harmonic generation, and phonon modes.
Ab initio transport calculations of molecular wires with electron-phonon couplings
NASA Astrophysics Data System (ADS)
Hirose, Kenji; Kobayashi, Nobuhiko
2009-03-01
Understanding of electron transport through nanostructures becomes important with the advancement of fabrication process to construct atomic-scale devices. Due to the drastic change of transport properties by contact conditions to electrodes in local electric fields, first-principles calculation approaches are indispensable to understand and characterize the transport properties of nanometer-scale molecular devices. Here we study the transport properties of molecular wires between metallic electrodes, especially focusing on the effects of contacts to electrodes and of the electron-phonon interactions. We use an ab initio calculation method based on the scattering waves, which are obtained by the recursion-transfer-matrix (RTM) method, combined with non-equilibrium Green's function (NEGF) method including the electron-phonon scatterings. We find that conductance shows exponential behaviors as a function of the length of molecular wires due to tunneling process determined by the HOMO-LUMO energy gap. From the voltage drop behaviors inside the molecular wires, we show that the contact resistances are dominant source for the bias drop and thus are related to local heating. We will present the electron-phonon coupling effects at contact on the inelastic scattering and discuss on the local heating and local temperature, comparing them with those of metallic atomic wires.
T-shaped GaAs quantum-wire lasers and the exciton Mott transition.
Yoshita, M; Liu, S M; Okano, M; Hayamizu, Y; Akiyama, H; Pfeiffer, L N; West, K W
2007-07-25
T-shaped GaAs quantum-wire (T-wire) lasers fabricated by the cleaved-edge overgrowth method with molecular beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal are measured to study the laser device physics and fundamental many-body physics in clean one-dimensional (1D) systems. A current-injection T-wire laser that has 20 periods of T-wires in the active region and a 0.5 mm long cavity with high-reflection coatings shows a low threshold current of 0.27 mA at 30 K. The origin of the laser gain above the lasing threshold is studied with the high-quality T-wire lasers by means of optical pumping. The lasing energy is about 5 meV below the photoluminescence (PL) peak of free excitons, and is on the electron-hole (e-h) plasma PL band at a high e-h carrier density. The observed energy shift excludes the laser gain due to free excitons, and it suggests a contribution from the e-h plasma instead. A systematic micro-PL study reveals that the PL evolves with the e-h density from a sharp exciton peak, via a biexciton peak, to an e-h-plasma PL band. The data demonstrate an important role of biexcitons in the exciton Mott transition. Comparison with microscopic theories points out some problems in the picture of the exciton Mott transition.
Correlated electrons in coupled quantum dots and related phenomena
NASA Astrophysics Data System (ADS)
Ugajin, Ryuichi
1998-01-01
Three topics related to correlated electrons in coupled quantum dots are discussed. The first is quasi-resonance between multi-electron states, which causes hitherto unremarked types of resonant absorption in coupled quantum dots. The second is electron tunneling through a Hubbard gap, which is induced by an increase in the density of electrons in a quantum-dot chain under an overall confining potential. The third is Mott transition in a two-dimensional quantum-dot array induced by an external electric field. In this system, the metal-insulator transition goes through a heavy electron phase in which the density of correlated electrons fluctuates.
Kushwaha, Manvir S
2011-09-28
We report on the theoretical investigation of the elementary electronic excitations in a quantum wire made up of vertically stacked self-assembled InAs/GaAs quantum dots. The length scales (of a few nanometers) involved in the experimental setups prompt us to consider an infinitely periodic system of two-dimensionally confined (InAs) quantum dot layers separated by GaAs spacers. The resultant quantum wire is characterized by a two-dimensional harmonic confining potential in the x-y plane and a periodic (Kronig-Penney) potential along the z (or the growth) direction within the tight-binding approximation. Since the wells and barriers are formed from two different materials, we employ the Bastard's boundary conditions in order to determine the eigenfunctions along the z direction. These wave functions are then used to generate the Wannier functions, which, in turn, constitute the legitimate Bloch functions that govern the electron dynamics along the direction of periodicity. Thus, the Bloch functions and the Hermite functions together characterize the whole system. We then make use of the Bohm-Pines' (full) random-phase approximation in order to derive a general nonlocal, dynamic dielectric function. Thus, developed theoretical framework is then specified to work within a (lowest miniband and) two-subband model that enables us to scrutinize the single-particle as well as collective responses of the system. We compute and discuss the behavior of the eigenfunctions, band-widths, density of states, Fermi energy, single-particle and collective excitations, and finally size up the importance of studying the inverse dielectric function in relation with the quantum transport phenomena. It is remarkable to notice how the variation in the barrier- and well-widths can allow us to tailor the excitation spectrum in the desired energy range. Given the advantage of the vertically stacked quantum dots over the planar ones and the foreseen applications in the single-electron devices
Spin and charge pumping in a quantum wire: the role of spin-flip scattering and Zeeman splitting
NASA Astrophysics Data System (ADS)
Kwapiński, T.; Taranko, R.
2011-10-01
We investigate theoretically charge and spin pumps based on a linear configuration of quantum dots (quantum wire) which are disturbed by an external time-dependent perturbation. This perturbation forms an impulse which moves as a train pulse through the wire. It is found that the charge pumped through the system depends non-monotonically on the wire length, N. In the presence of the Zeeman splitting pure spin current flowing through the wire can be generated in the absence of charge current. Moreover, we observe electron pumping in a direction which does not coincide with the propagation direction of the pulse and the spin pumping direction (spin-charge separation). Additionally, on-site spin-flip processes significantly influence electron transport through the system and can also reverse the charge current direction.
Picosecond excitonic absorption recovery of 100 nm GaAs/AlGaAs narrow multiple quantum-well wires
NASA Astrophysics Data System (ADS)
Tackeuchi, Atsushi; Kitada, Hideki; Arimoto, Hiroshi; Sugiyama, Yoshihiro; Endoh, Akira; Nakata, Yoshiaki; Inata, Tsuguo; Muto, Shunichi
1991-08-01
We report the time-resolved absorption measurement of narrow multiple quantum-well (MQW) wires to investigate their fast recoveries from excitonic absorption bleaching. Wires down to 130 nm were fabricated from MQWs using focused ion beam lithography and electron cyclotron-resonance chlorine-plasma etching. In this structure, the photoexcited carriers diffuse toward the sidewalls and recombine on the surface of the sidewalls. We show that the strong optical nonlinearity of excitons is preserved, even in wires of 130 nm width, and having a fast recovery time in the picosecond region. We also briefly discuss the possibility of making quantum wires which have a faster recovery time and larger optical nonlinearity.
NASA Astrophysics Data System (ADS)
Sarikurt, Sevil; Sakiroglu, Serpil; Akgungor, Kadir; Sokmen, Ismail
2015-03-01
We have investigated the effect of spin-orbit (SO) coupling on the energy level spectrum and spin texturing of parabolically confined quantum wire that is subjected to an externally applied perpendicular magnetic field. Additionally we have also taken into account exchange-correlation contribution.Highly accurate numerical calculations have been carried out by finite element method. Our results have been revealed that the interplay of the SO coupling with effective magnetic field significantly modifies the band structure, producing additional subband extrema and energy gaps. Energy subband structure varies depending on which type of SO coupling strength is considered and also the magnitude of SO coupling. We also obtain that spatial modulation of spin density along the wire width can be considerably modified by SO coupling strength, magnetic field and charge carrier concentration. Besides, we have observed that the presence of exchange-correlation contribution leads to a softening behavior in the local maxima at subbands and shifts all energy subbands to lower energy values. Numerical results point out that the combined effect of exchange-correlation and SO coupling produces asymmetry in the dispersion relations. Supported by Scientific and Technological Research Council of Turkey.
Shot noise in a quantum dot system coupled with Majorana bound states.
Chen, Qiao; Chen, Ke-Qiu; Zhao, Hong-Kang
2014-08-06
We investigate the spectral density of shot noise and current for the system of a quantum dot coupled to Majorana bound states (MBS) employing the nonequilibrium Green's function. The Majorana bound states at the end of the wire strongly affect the shot noise. There are two types of coupling in the system: dot-MBS and MBS-MBS coupling. The curves of shot noise and current versus coupling strength have novel steps owing to the energy-level splitting caused by dot-MBS coupling. The magnitude of these steps increases with the strength of dot-MBS coupling λ but decreases with the strength of MBS-MBS coupling. The steps shift toward the large ∣eV∣ region as λ or ϵ(M) increases. In addition, dot-MBS coupling enhances the shot noise while MBS-MBS coupling suppresses the shot noise. In the absence of MBS-MBS coupling, a sharp jump emerges in the curve of the Fano factor at zero bias owing to the differential conductance being reduced by a factor of 1/2. This provides a novel technique for the detection of Majorana fermions.
Shot noise in a quantum dot system coupled with Majorana bound states
NASA Astrophysics Data System (ADS)
Chen, Qiao; Chen, Ke-Qiu; Zhao, Hong-Kang
2014-08-01
We investigate the spectral density of shot noise and current for the system of a quantum dot coupled to Majorana bound states (MBS) employing the nonequilibrium Green’s function. The Majorana bound states at the end of the wire strongly affect the shot noise. There are two types of coupling in the system: dot-MBS and MBS-MBS coupling. The curves of shot noise and current versus coupling strength have novel steps owing to the energy-level splitting caused by dot-MBS coupling. The magnitude of these steps increases with the strength of dot-MBS coupling λ but decreases with the strength of MBS-MBS coupling. The steps shift toward the large ∣eV∣ region as λ or ɛM increases. In addition, dot-MBS coupling enhances the shot noise while MBS-MBS coupling suppresses the shot noise. In the absence of MBS-MBS coupling, a sharp jump emerges in the curve of the Fano factor at zero bias owing to the differential conductance being reduced by a factor of 1/2. This provides a novel technique for the detection of Majorana fermions.
NASA Astrophysics Data System (ADS)
Zhang, Li; Shi, Jun-jie
2005-06-01
Within the framework of the dielectric continuum approximation and Loudon's uniaxial crystal model, the interface optical (IO) phonon modes and the corresponding Fröhlich electron phonon interaction Hamiltonian in a wurtzite AlN/GaN/AlN quantum well wire (QWW) are derived and studied. Numerical calculations are mainly focused on the frequency dispersion of the IO phonons and electron phonon interaction coupling function. Results reveal that, in general, there are four branches of IO phonon modes in the systems. The dispersions of the four branches of IO phonon modes are obvious only when the axial direction wave number kz or the azimuthal quantum number m is small. The degenerating behaviour of the IO phonon modes in wurtzite QWW has also been observed for small kz or m. When kz or m are relatively large, with the increasing of them, the frequencies of these IO phonon modes converge to the two definite limiting frequencies in wurtzite single planar heterostructure, and this feature has been explained reasonably from the mathematical and physical viewpoints. The calculations of the electron phonon coupling function show that, though some branches of IO phonon modes exchange their localized positions with each other at a large m, there always exist two branches of IO phonon modes localized on each interface. The high-frequency IO phonon modes compared with the low-frequency ones play a more important role in the electron phonon interaction. Detailed comparison of the dispersion behaviours of the IO phonons and electron IO phonon couplings properties in wurtzite QWWs with those in zinc-blende QWWs has also been made.
Zhang, Tong-Yi; Zhao, Wei; Liu, Xue-Ming
2009-08-19
We have made a thorough theoretical investigation of the interplay of spin-orbit interactions (SOIs) resulting from Rashba, Dresselhaus and the lateral parabolic confining potential on the energy dispersion relation of the spin subbands in a parabolic quantum wire. The influence of an applied external magnetic field is also discussed. We show the interplay of different types of SOI, as well as the Zeeman effect, leads to rather complex and intriguing electrosubbands for different spin branches. The effect of different coupling strengths and different magnetic field strengths is also investigated.
Novel T-shaped GaSb/InAsN quantum wire for mid-infrared laser applications
NASA Astrophysics Data System (ADS)
Ridene, Said
2017-10-01
In this work, we investigate GaSb /InAs1-xNx T-shaped quantum wire active region in mid-infrared laser. Multi-band k.p model and variational formalism are applied to find the confinement energies, the band structures, and optical gain. We then present a method of numerical calculation that is suited to any T-shaped quantum wire. By tuning the quantum wire thickness, the TE- and TM-polarized optical gain up to 21 ×103 cm-1 can be obtained for λ = 3.11 μm at room temperature (RT), which is very promising to serve as an alternative active region for high-efficiency mid-infrared laser applications.
Wire-shaped quantum dots-sensitized solar cells based on nanosheets and nanowires.
Chen, Haining; Zhu, Liqun; Wang, Meng; Liu, Huicong; Li, Weiping
2011-11-25
Wire-shaped quantum dots-sensitized solar cells (WS-QDSCs) based on nanosheets and nanowires were fabricated and investigated for this paper. The nanosheets grown on stainless steel (SS) wire by electrodeposition were mainly composed of Zn₅(OH)₈Cl₂·H₂O and most of the Zn₅(OH)₈Cl₂·H₂O was converted to ZnO by post-treatment, and ZnO nanowires were directly grown on SS wire by the hydrothermal method. CdS QDs were deposited on nanosheets and nanowires by successive ionic layer adsorption and reaction method. The results of photoelectrochemical performance indicated that WS-QDSCs showed a similar conversion efficiency in polysulfide and Na₂SO₄ electrolytes, while the WS-QDSCs based on the Cu2S counter electrode achieved much higher performance than those based on SS and Cu counter electrodes. By optimizing electrodeposition duration, the WS-QDSCs based on nanosheets presented the highest conversion efficiency of 0.60% for the duration of 20 min. Performance comparison indicated that the WS-QDSC based on nanosheets showed very superior performance to that based on the nanowires with similar film thickness.
Conformal loop quantum gravity coupled to the standard model
NASA Astrophysics Data System (ADS)
Campiglia, Miguel; Gambini, Rodolfo; Pullin, Jorge
2017-01-01
We argue that a conformally invariant extension of general relativity coupled to the standard model is the fundamental theory that needs to be quantized. We show that it can be treated by loop quantum gravity techniques. Through a gauge fixing and a modified Higgs mechanism particles acquire mass and one recovers general relativity coupled to the standard model. The theory suggests new views with respect to the definition of the Hamiltonian constraint in loop quantum gravity, the semi-classical limit and the issue of finite renormalization in quantum field theory in quantum space-time. It also gives hints about the elimination of ambiguities that arise in quantum field theory in quantum space-time in the calculation of back-reaction.
NASA Astrophysics Data System (ADS)
Kushwaha, Manvir S.
2013-04-01
The nanofabrication technology has taught us that an m-dimensional confining potential imposed upon an n-dimensional electron gas paves the way to a quasi-(n-m)-dimensional electron gas, with m ⩽ n and 1 ⩽ n, m ⩽ 3. This is the road to the (semiconducting) quasi-n dimensional electron gas systems we have been happily traversing on now for almost two decades. Achieving quasi-one dimensional electron gas (Q-1DEG) [or quantum wire(s) for more practical purposes] led us to some mixed moments in this journey: while the reduced phase space for the scattering led us believe in the route to the faster electron devices, the proximity to the 1D systems left us in the dilemma of describing it as a Fermi liquid or as a Luttinger liquid. No one had ever suspected the potential of the former, but it took quite a while for some to convince the others on the latter. A realistic Q-1DEG system at the low temperatures is best describable as a Fermi liquid rather than as a Luttinger liquid. In the language of condensed matter physics, a critical scrutiny of Q-1DEG systems has provided us with a host of exotic (electronic, optical, and transport) phenomena unseen in their higher- or lower-dimensional counterparts. This has motivated us to undertake a systematic investigation of the inelastic electron scattering (IES) and the inelastic light scattering (ILS) from the elementary electronic excitations in quantum wires. We begin with the Kubo's correlation functions to derive the generalized dielectric function, the inverse dielectric function, and the Dyson equation for the dynamic screened potential in the framework of Bohm-Pines' random-phase approximation. These fundamental tools then lead us to develop methodically the theory of IES and ILS for the Q-1DEG systems. As an application of the general formal results, which know no bounds regarding the subband occupancy, we compute the density of states, the Fermi energy, the full excitation spectrum [comprised of intrasubband and
Quantum transport through disordered 1D wires: Conductance via localized and delocalized electrons
Gopar, Víctor A.
2014-01-14
Coherent electronic transport through disordered systems, like quantum wires, is a topic of fundamental and practical interest. In particular, the exponential localization of electron wave functions-Anderson localization-due to the presence of disorder has been widely studied. In fact, Anderson localization, is not an phenomenon exclusive to electrons but it has been observed in microwave and acoustic experiments, photonic materials, cold atoms, etc. Nowadays, many properties of electronic transport of quantum wires have been successfully described within a scaling approach to Anderson localization. On the other hand, anomalous localization or delocalization is, in relation to the Anderson problem, a less studied phenomenon. Although one can find signatures of anomalous localization in very different systems in nature. In the problem of electronic transport, a source of delocalization may come from symmetries present in the system and particular disorder configurations, like the so-called Lévy-type disorder. We have developed a theoretical model to describe the statistical properties of transport when electron wave functions are delocalized. In particular, we show that only two physical parameters determine the complete conductance distribution.
Charge carrier relaxation in InGaAs-GaAs quantum wire modulation-doped heterostructures.
Kondratenko, S V; Iliash, S A; Mazur, Yu I; Kunets, V P; Benamara, M; Salamo, G J
2017-09-15
The time dependencies of the carrier relaxation in modulation-doped InGaAs-GaAs low-dimensional structures with quantum wires have been studied as functions of temperature and light excitation levels. The photoconductivity (PC) relaxation follows a stretched exponent with decay constant, which depends on the morphology of InGaAs epitaxial layers, presence of deep traps, and energy disorder due to inhomogeneous distribution of size and composition. A hopping model, where electron tunnels between bands of localized states, gives appropriate interpretation for temperature-independent PC decay across the temperature range 150-290 K. At low temperatures (T < 150 K), multiple trapping-retrapping via 1D states of InGaAs quantum wires (QWRs), sub-bands of two-dimensional electron gas of modulation-doped n-GaAs spacers, as well as defect states in the GaAs environment are the dominant relaxation mechanism. The PC and photoluminescence transients for samples with different morphologies of the InGaAs nanostructures are compared. The relaxation rates are found to be largely dependent on energy disorder due to inhomogeneous distribution of strain, nanostructure size and composition, and piezoelectric fields in and around nanostructures, which have a strong impact on efficiency of carrier exchange between bands of the InGaAs QWRs, GaAs spacers, or wetting layers; presence of local electric fields; and deep traps.
Magnetotransport in p-type Ge quantum well narrow wire arrays
Newton, P. J. Llandro, J.; Mansell, R.; Barnes, C. H. W.; Holmes, S. N.; Morrison, C.; Foronda, J.; Myronov, M.; Leadley, D. R.
2015-04-27
We report magnetotransport measurements of a SiGe heterostructure containing a 20 nm p-Ge quantum well with a mobility of 800 000 cm{sup 2} V{sup −1} s{sup −1}. By dry etching arrays of wires with widths between 1.0 μm and 3.0 μm, we were able to measure the lateral depletion thickness, built-in potential, and the phase coherence length of the quantum well. Fourier analysis does not show any Rashba related spin-splitting despite clearly defined Shubnikov-de Haas oscillations being observed up to a filling factor of ν = 22. Exchange-enhanced spin-splitting is observed for filling factors below ν = 9. An analysis of boundary scattering effects indicates lateral depletion of the hole gas by 0.5 ± 0.1 μm from the etched germanium surface. The built-in potential is found to be 0.25 ± 0.04 V, presenting an energy barrier for lateral transport greater than the hole confinement energy. A large phase coherence length of 3.5 ± 0.5 μm is obtained in these wires at 1.7 K.
Magnetotransport in p-type Ge quantum well narrow wire arrays
NASA Astrophysics Data System (ADS)
Newton, P. J.; Llandro, J.; Mansell, R.; Holmes, S. N.; Morrison, C.; Foronda, J.; Myronov, M.; Leadley, D. R.; Barnes, C. H. W.
2015-04-01
We report magnetotransport measurements of a SiGe heterostructure containing a 20 nm p-Ge quantum well with a mobility of 800 000 cm2 V-1 s-1. By dry etching arrays of wires with widths between 1.0 μm and 3.0 μm, we were able to measure the lateral depletion thickness, built-in potential, and the phase coherence length of the quantum well. Fourier analysis does not show any Rashba related spin-splitting despite clearly defined Shubnikov-de Haas oscillations being observed up to a filling factor of ν = 22. Exchange-enhanced spin-splitting is observed for filling factors below ν = 9. An analysis of boundary scattering effects indicates lateral depletion of the hole gas by 0.5 ± 0.1 μm from the etched germanium surface. The built-in potential is found to be 0.25 ± 0.04 V, presenting an energy barrier for lateral transport greater than the hole confinement energy. A large phase coherence length of 3.5 ± 0.5 μm is obtained in these wires at 1.7 K.
Charge carrier relaxation in InGaAs-GaAs quantum wire modulation-doped heterostructures
NASA Astrophysics Data System (ADS)
Kondratenko, S. V.; Iliash, S. A.; Mazur, Yu I.; Kunets, V. P.; Benamara, M.; Salamo, G. J.
2017-09-01
The time dependencies of the carrier relaxation in modulation-doped InGaAs-GaAs low-dimensional structures with quantum wires have been studied as functions of temperature and light excitation levels. The photoconductivity (PC) relaxation follows a stretched exponent with decay constant, which depends on the morphology of InGaAs epitaxial layers, presence of deep traps, and energy disorder due to inhomogeneous distribution of size and composition. A hopping model, where electron tunnels between bands of localized states, gives appropriate interpretation for temperature-independent PC decay across the temperature range 150-290 K. At low temperatures (T < 150 K), multiple trapping-retrapping via 1D states of InGaAs quantum wires (QWRs), sub-bands of two-dimensional electron gas of modulation-doped n-GaAs spacers, as well as defect states in the GaAs environment are the dominant relaxation mechanism. The PC and photoluminescence transients for samples with different morphologies of the InGaAs nanostructures are compared. The relaxation rates are found to be largely dependent on energy disorder due to inhomogeneous distribution of strain, nanostructure size and composition, and piezoelectric fields in and around nanostructures, which have a strong impact on efficiency of carrier exchange between bands of the InGaAs QWRs, GaAs spacers, or wetting layers; presence of local electric fields; and deep traps.
Single to quadruple quantum dots with tunable tunnel couplings
Takakura, T.; Noiri, A.; Obata, T.; Yoneda, J.; Yoshida, K.; Otsuka, T.; Tarucha, S.
2014-03-17
We prepare a gate-defined quadruple quantum dot to study the gate-tunability of single to quadruple quantum dots with finite inter-dot tunnel couplings. The measured charging energies of various double dots suggest that the dot size is governed by the gate geometry. For the triple and quadruple dots, we study the gate-tunable inter-dot tunnel couplings. For the triple dot, we find that the effective tunnel coupling between side dots significantly depends on the alignment of the center dot potential. These results imply that the present quadruple dot has a gate performance relevant for implementing spin-based four-qubits with controllable exchange couplings.
Khordad, R. Bahramiyan, H.
2014-03-28
In this paper, optical phonon modes are studied within the framework of dielectric continuum approach for parallelogram and triangular quantum wires, including the derivation of the electron-phonon interaction Hamiltonian and a discussion on the effects of this interaction on the electronic energy levels. The polaronic energy shift is calculated for both ground-state and excited-state electron energy levels by applying the perturbative approach. The effects of the electron-phonon interaction on the expectation value of r{sup 2} and diamagnetic susceptibility for both quantum wires are discussed.
Mihaljevic, Miodrag J.
2007-05-15
It is shown that the security, against known-plaintext attacks, of the Yuen 2000 (Y00) quantum-encryption protocol can be considered via the wire-tap channel model assuming that the heterodyne measurement yields the sample for security evaluation. Employing the results reported on the wire-tap channel, a generic framework is proposed for developing secure Y00 instantiations. The proposed framework employs a dedicated encoding which together with inherent quantum noise at the attacker's side provides Y00 security.
Effective quantum dynamics of interacting systems with inhomogeneous coupling
Lopez, C. E.; Retamal, J. C.; Christ, H.; Solano, E.
2007-03-15
We study the quantum dynamics of a single mode (particle) interacting inhomogeneously with a large number of particles and introduce an effective approach to find the accessible Hilbert space, where the dynamics takes place. Two relevant examples are given: the inhomogeneous Tavis-Cummings model (e.g., N atomic qubits coupled to a single cavity mode, or to a motional mode in trapped ions) and the inhomogeneous coupling of an electron spin to N nuclear spins in a quantum dot.
Coupling polariton quantum boxes in sub-wavelength grating microcavities
Zhang, Bo; Wang, Zhaorong; Deng, Hui; Brodbeck, Sebastian; Kamp, Martin; Schneider, Christian; Höfling, Sven
2015-02-02
We report the construction of decoupled, coupled, and quasi-one dimensional polariton systems from zero dimensional polariton quantum boxes using microcavities with sub-wavelength gratings as the top mirror. By designing the tethering patterns around the suspended sub-wavelength gratings, we control the coupling between individual quantum boxes through different optical potentials. Energy levels and real-space or momentum space distributions of the confined modes were measured, which agreed well with simulations.
Hyper-parallel photonic quantum computation with coupled quantum dots
Ren, Bao-Cang; Deng, Fu-Guo
2014-01-01
It is well known that a parallel quantum computer is more powerful than a classical one. So far, there are some important works about the construction of universal quantum logic gates, the key elements in quantum computation. However, they are focused on operating on one degree of freedom (DOF) of quantum systems. Here, we investigate the possibility of achieving scalable hyper-parallel quantum computation based on two DOFs of photon systems. We construct a deterministic hyper-controlled-not (hyper-CNOT) gate operating on both the spatial-mode and the polarization DOFs of a two-photon system simultaneously, by exploiting the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics (QED). This hyper-CNOT gate is implemented by manipulating the four qubits in the two DOFs of a two-photon system without auxiliary spatial modes or polarization modes. It reduces the operation time and the resources consumed in quantum information processing, and it is more robust against the photonic dissipation noise, compared with the integration of several cascaded CNOT gates in one DOF. PMID:24721781
Quantum transport in coupled resonators enclosed synthetic magnetic flux
NASA Astrophysics Data System (ADS)
Jin, L.
2016-07-01
Quantum transport properties are instrumental to understanding quantum coherent transport processes. Potential applications of quantum transport are widespread, in areas ranging from quantum information science to quantum engineering, and not restricted to quantum state transfer, control and manipulation. Here, we study light transport in a ring array of coupled resonators enclosed synthetic magnetic flux. The ring configuration, with an arbitrary number of resonators embedded, forms a two-arm Aharonov-Bohm interferometer. The influence of magnetic flux on light transport is investigated. Tuning the magnetic flux can lead to resonant transmission, while half-integer magnetic flux quantum leads to completely destructive interference and transmission zeros in an interferometer with two equal arms.
Current quantization due to single-electron transfer in Si-wire charge-coupled devices
NASA Astrophysics Data System (ADS)
Fujiwara, Akira; Zimmerman, Neil M.; Ono, Yukinori; Takahashi, Yasuo
2004-02-01
We observe a quantized current due to single-electron transfer in a small charge-coupled device, which consists of a narrow Si-wire channel with fine gates; the gate is used to form a tunable barrier potential. By modulating two barrier potentials under the fine gates with phase-shifted pulse voltages, quantized numbers of electrons are injected into and extracted from the charge island sandwiched by the two barriers. Current plateaus due to single-electron transfer are clearly observed at 20 K with frequencies up to 100 MHz and a current level of 16 pA.
Majorana Fermion Induced Non-local Current Correlations in Spin-orbit Coupled Superconducting Wires
NASA Astrophysics Data System (ADS)
Liu, Jie; Zhang, Fu-Chun; Law, K. T.
2014-03-01
The observation of zero bias conductance peaks in semiconductor wire-superconductor heterostructures has generated great interest, and there is a hot debate on whether the observation is associated with Majorana Fermions (MFs) or other effects which enhance local Andreev reflections. In this work, we study the transport of a normal lead/semiconductor wire-superconductor heterostructure /normal lead junction. We show that when MF end states from the two ends of the wire are strongly coupled, the MF end states can suppress local Andreev reflections and strongly enhance crossed Andreev reflections (CARs), in which an electron from one lead is reflected as a hole in a different lead. In the CAR dominated regime, the current-current correlations between the two leads are strongly enhanced. Moreover, the Fano factor of a normal lead, which is the ratio of the shot noise to the average current, is reduced from 2e to e. Since the CAR associated effects are non-local effects and they cannot be induced by processes which enhance local Andreev reflections, therefore, the measurement of Fano factors and current-current correlations of the normal leads can be used to identify MFs.
Coupled Langmuir oscillations in 2-dimensional quantum plasmas
Akbari-Moghanjoughi, M.
2014-03-15
In this work, we present a hydrodynamic model to study the coupled quantum electron plasma oscillations (QEPO) for two dimensional (2D) degenerate plasmas, which incorporates all the essential quantum ingredients such as the statistical degeneracy pressure, electron-exchange, and electron quantum diffraction effect. Effects of diverse physical aspects like the electronic band-dispersion effect, the electron exchange-correlations and the quantum Bohm-potential as well as other important plasma parameters such as the coupling parameter (plasma separation) and the plasma electron number-densities on the linear response of the coupled system are investigated. By studying three different 2D plasma coupling types, namely, graphene-graphene, graphene-metalfilm, and metalfilm-metalfilm coupling configurations, it is remarked that the collective quantum effects can influence the coupled modes quite differently, depending on the type of the plasma configuration. It is also found that the slow and fast QEPO frequency modes respond very differently to the change in plasma parameters. Current findings can help in understanding of the coupled density oscillations in multilayer graphene, graphene-based heterojunctions, or nanofabricated integrated circuits.
Dynamical spin-spin coupling of quantum dots
NASA Astrophysics Data System (ADS)
Grigoryan, Vahram; Xiao, Jiang; A spintronics Group Team
2014-03-01
We carried out a nested Schrieffer-Wolff transformation of an Anderson two-impurity Hamiltonian to study the spin-spin coupling between two dynamical quantum dots under the influence of rotating transverse magnetic field. As a result of the rotating field, we predict a novel Ising type spin-spin coupling mechanism between quantum dots, whose strength is tunable via the magnitude of the rotating field. Due to its dynamical origin, this new coupling mechanism is qualitatively different from the all existing static couplings such as RKKY, while the strength could be comparable to the strength of the RKKY coupling. The dynamical coupling with the intristic RKKY coupling enables to construct a four level system of maximally entangled Bell states in a controllable manner. This work was supported by the special funds for the Major State Basic Research Project of China (No. 2011CB925601) and the National Natural Science Foundation of China (Grants No. 11004036 and No. 91121002).
High photoresponse in room temperature quantum cascade detector based on coupled quantum well design
NASA Astrophysics Data System (ADS)
Dougakiuchi, Tatsuo; Fujita, Kazuue; Hirohata, Toru; Ito, Akio; Hitaka, Masahiro; Edamura, Tadataka
2016-12-01
We report high photoresponse measured in a room temperature quantum cascade detector (QCD) based on a coupled quantum well design that operates with a peak response wavelength of 5.4 μm. The coupled quantum well design is expected to produce higher photocurrents when compared with device active regions that use a combination of simple quantum wells. The coupled quantum well QCD demonstrated high responsivity of 22 mA/W at room temperature with a commonly used 45° wedge-based light coupling configuration. Application of a waveguide configuration to the proposed QCD yielded an elevated responsivity of ˜130 mA/W and a specific detectivity (D*) of 1.1 × 108 cm W-1 Hz1/2 at room temperature.
Recovering classical dynamics from coupled quantum systems through continuous measurement
Ghose, Shohini; Alsing, Paul; Deutsch, Ivan; Bhattacharya, Tanmoy; Habib, Salman; Jacobs, Kurt
2003-05-01
We study the role of continuous measurement in the quantum to classical transition for a system with coupled internal (spin) and external (motional) degrees of freedom. Even when the measured motional degree of freedom can be treated classically, entanglement between spin and motion causes strong measurement back action on the quantum spin subsystem so that classical trajectories are not recovered in this mixed quantum-classical regime. The measurement can extract localized quantum trajectories that behave classically only when the internal action also becomes large relative to ({Dirac_h}/2{pi})
Interaction of solitons with a string of coupled quantum dots
Kumar, Vijendra Swami, O. P. Nagar, A. K.; Taneja, S.
2016-05-06
In this paper, we develop a theory for discrete solitons interaction with a string of coupled quantum dots in view of the local field effects. Discrete nonlinear Schrodinger (DNLS) equations are used to describe the dynamics of the string. Numerical calculations are carried out and results are analyzed with the help of matlab software. With the help of numerical solutions we demonstrate that in the quantum dots string, Rabi oscillations (RO) are self trapped into stable bright Rabi solitons. The Rabi oscillations in different types of nanostructures have potential applications to the elements of quantum logic and quantum memory.
Jeans self gravitational instability of strongly coupled quantum plasma
Sharma, Prerana; Chhajlani, R. K.
2014-07-15
The Jeans self-gravitational instability is studied for quantum plasma composed of weakly coupled degenerate electron fluid and non-degenerate strongly coupled ion fluid. The formulation for such system is done on the basis of two fluid theory. The dynamics of weakly coupled degenerate electron fluid is governed by inertialess momentum equation. The quantum forces associated with the quantum diffraction effects and the quantum statistical effects act on the degenerate electron fluid. The strong correlation effects of ion are embedded in generalized viscoelastic momentum equation including the viscoelasticity and shear viscosities of ion fluid. The general dispersion relation is obtained using the normal mode analysis technique for the two regimes of propagation, i.e., hydrodynamic and kinetic regimes. The Jeans condition of self-gravitational instability is also obtained for both regimes, in the hydrodynamic regime it is observed to be affected by the ion plasma oscillations and quantum parameter while in the kinetic regime in addition to ion plasma oscillations and quantum parameter, it is also affected by the ion velocity which is modified by the viscosity generated compressional effects. The Jeans critical wave number and corresponding critical mass are also obtained for strongly coupled quantum plasma for both regimes.
[Arc spectrum diagnostic and heat coupling mechanism analysis of double wire pulsed MIG welding].
Liu, Yong-qiang; Li, Huan; Yang, Li-jun; Zheng, Kai; Gao, Ying
2015-01-01
A double wire pulsed MIG welding test system was built in the present paper, in order to analyze the heat-coupling mechanism of double wire pulsed MIG welding, and study are temperature field. Spectroscopic technique was used in diagnostic analysis of the are, plasma radiation was collected by using hollow probe method to obtain the arc plasma optical signal The electron temperature of double wire pulsed MIG welding arc plasma was calculated by using Boltzmann diagram method, the electron temperature distribution was obtained, a comprehensive analysis of the arc was conducted combined with the high speed camera technology and acquisition means of electricity signal. The innovation of this paper is the combination of high-speed camera image information of are and optical signal of arc plasma to analyze the coupling mechanism for dual arc, and a more intuitive analysis for are temperature field was conducted. The test results showed that a push-pull output was achieved and droplet transfer mode was a drop in a pulse in the welding process; Two arcs attracted each other under the action of a magnetic field, and shifted to the center of the arc in welding process, so a new heat center was formed at the geometric center of the double arc, and flowing up phenomenon occurred on the arc; Dual arc electronic temperature showed an inverted V-shaped distribution overall, and at the geometric center of the double arc, the arc electron temperature at 3 mm off the workpiece surface was the highest, which was 16,887.66 K, about 4,900 K higher than the lowest temperature 11,963.63 K.
Spillmann, Christopher M; Ancona, Mario G; Buckhout-White, Susan; Algar, W Russ; Stewart, Michael H; Susumu, Kimihiro; Huston, Alan L; Goldman, Ellen R; Medintz, Igor L
2013-08-27
Assembling DNA-based photonic wires around semiconductor quantum dots (QDs) creates optically active hybrid architectures that exploit the unique properties of both components. DNA hybridization allows positioning of multiple, carefully arranged fluorophores that can engage in sequential energy transfer steps while the QDs provide a superior energy harvesting antenna capacity that drives a Förster resonance energy transfer (FRET) cascade through the structures. Although the first generation of these composites demonstrated four-sequential energy transfer steps across a distance >150 Å, the exciton transfer efficiency reaching the final, terminal dye was estimated to be only ~0.7% with no concomitant sensitized emission observed. Had the terminal Cy7 dye utilized in that construct provided a sensitized emission, we estimate that this would have equated to an overall end-to-end ET efficiency of ≤ 0.1%. In this report, we demonstrate that overall energy flow through a second generation hybrid architecture can be significantly improved by reengineering four key aspects of the composite structure: (1) making the initial DNA modification chemistry smaller and more facile to implement, (2) optimizing donor-acceptor dye pairings, (3) varying donor-acceptor dye spacing as a function of the Förster distance R0, and (4) increasing the number of DNA wires displayed around each central QD donor. These cumulative changes lead to a 2 orders of magnitude improvement in the exciton transfer efficiency to the final terminal dye in comparison to the first-generation construct. The overall end-to-end efficiency through the optimized, five-fluorophore/four-step cascaded energy transfer system now approaches 10%. The results are analyzed using Förster theory with various sources of randomness accounted for by averaging over ensembles of modeled constructs. Fits to the spectra suggest near-ideal behavior when the photonic wires have two sequential acceptor dyes (Cy3 and Cy3.5) and
Sukegawa, Junpei; Schubert, Christina; Zhu, Xiaozhang; Tsuji, Hayato; Guldi, Dirk M; Nakamura, Eiichi
2014-10-01
Electron transfer (ET) is a fundamental process in a wide range of biological systems, photovoltaics and molecular electronics. Therefore to understand the relationship between molecular structure and ET properties is of prime importance. For this purpose, photoinduced ET has been studied extensively using donor-bridge-acceptor molecules, in which π-conjugated molecular wires are employed as bridges. Here, we demonstrate that carbon-bridged oligo-p-phenylenevinylene (COPV), which is both rigid and flat, shows an 840-fold increase in the ET rate compared with the equivalent flexible molecular bridges. A 120-fold rate enhancement is explained in terms of enhanced electronic coupling between the electron donor and the electron acceptor because of effective conjugation through the COPVs. The remainder of the rate enhancement is explained by inelastic electron tunnelling through COPV caused by electron-vibration coupling, unprecedented for organic molecular wires in solution at room temperature. This type of nonlinear effect demonstrates the versatility and potential practical utility of COPVs in molecular device applications.
A scanning transmon qubit for strong coupling circuit quantum electrodynamics.
Shanks, W E; Underwood, D L; Houck, A A
2013-01-01
Like a quantum computer designed for a particular class of problems, a quantum simulator enables quantitative modelling of quantum systems that is computationally intractable with a classical computer. Superconducting circuits have recently been investigated as an alternative system in which microwave photons confined to a lattice of coupled resonators act as the particles under study, with qubits coupled to the resonators producing effective photon-photon interactions. Such a system promises insight into the non-equilibrium physics of interacting bosons, but new tools are needed to understand this complex behaviour. Here we demonstrate the operation of a scanning transmon qubit and propose its use as a local probe of photon number within a superconducting resonator lattice. We map the coupling strength of the qubit to a resonator on a separate chip and show that the system reaches the strong coupling regime over a wide scanning area.
Broadband room temperature strong coupling between quantum dots and metamaterials.
Indukuri, Chaitanya; Yadav, Ravindra Kumar; Basu, J K
2017-08-17
Herein, we report the first demonstration of room temperature enhanced light-matter coupling in the visible regime for metamaterials using cooperative coupled quasi two dimensional quantum dot assemblies located at precise distances from the hyperbolic metamaterial (HMM) templates. The non-monotonic variation of the magnitude of strong coupling, manifested in terms of strong splitting of the photoluminescence of quantum dots, can be explained in terms of enhanced LDOS near the surface of such metamaterials as well as the plasmon mediated super-radiance of closely spaced quantum dots (QDs). Our methodology of enhancing broadband, room temperature, light-matter coupling in the visible regime for metamaterials opens up new possibilities of utilising these materials for a wide range of applications including QD based thresholdless nanolasers and novel metamaterial based integrated photonic devices.
Characteristics of surface plasmon coupled quantum well infrared photodetectors
NASA Astrophysics Data System (ADS)
Hsu, Wei-Cheng; Ling, Hong-Shi; Wang, Shiang-Yu; Lee, Chien-Ping
2017-06-01
Quantum Well Infrared Photodetectors (QWIPs) with different structures were characterized for the study of surface plasmon wave coupling. Detailed comparisons between surface plasmon coupled and etched grating coupled devices were investigated. A bias dependence for the enhancement of the responsivity of surface plasmon coupled devices was found, especially for the samples with non-uniform quantum wells. The non-uniform QWIPs with surface plasmon coupling showed an asymmetric enhancement with respect to the bias directions. Stronger enhancements were shown under the biases when a higher effective electric field region is close to the collector. The change of the photocarrier escape probability due to the narrow coupling bandwidth of the surface plasmon wave is attributed to this unexpected bias dependence.
Wang, Lin-Wang; Sun, Jianwei; Wang, Lin-Wang; Buhro, William E.
2008-07-11
High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range of 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi-nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure, and grow along the [002] direction (parallel to the c axis). The size dependence of the band gaps in the wires are determined from the absorption spectra, and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire band gaps converge. The origin and magnitude of this threshold diameter is discussed.
Sun, Jianwei; Wang, Lin-Wang; Buhro, William E
2008-06-25
High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure and grow along the [002] direction (parallel to the c axis). The size dependence of the effective band gaps in the wires is determined from the absorption spectra and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the effective band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire effective band gaps converge. The origin and magnitude of this threshold diameter are discussed.
Ultrafast quantum computation in ultrastrongly coupled circuit QED systems
NASA Astrophysics Data System (ADS)
Wang, Yimin; Guo, Chu; Zhang, Guo-Qiang; Wang, Gangcheng; Wu, Chunfeng
2017-03-01
The latest technological progress of achieving the ultrastrong-coupling regime in circuit quantum electrodynamics (QED) systems has greatly promoted the developments of quantum physics, where novel quantum optics phenomena and potential computational benefits have been predicted. Here, we propose a scheme to accelerate the nontrivial two-qubit phase gate in a circuit QED system, where superconducting flux qubits are ultrastrongly coupled to a transmission line resonator (TLR), and two more TLRs are coupled to the ultrastrongly-coupled system for assistant. The nontrivial unconventional geometric phase gate between the two flux qubits is achieved based on close-loop displacements of the three-mode intracavity fields. Moreover, as there are three resonators contributing to the phase accumulation, the requirement of the coupling strength to realize the two-qubit gate can be reduced. Further reduction in the coupling strength to achieve a specific controlled-phase gate can be realized by adding more auxiliary resonators to the ultrastrongly-coupled system through superconducting quantum interference devices. We also present a study of our scheme with realistic parameters considering imperfect controls and noisy environment. Our scheme possesses the merits of ultrafastness and noise-tolerance due to the advantages of geometric phases.
Ultrafast quantum computation in ultrastrongly coupled circuit QED systems.
Wang, Yimin; Guo, Chu; Zhang, Guo-Qiang; Wang, Gangcheng; Wu, Chunfeng
2017-03-10
The latest technological progress of achieving the ultrastrong-coupling regime in circuit quantum electrodynamics (QED) systems has greatly promoted the developments of quantum physics, where novel quantum optics phenomena and potential computational benefits have been predicted. Here, we propose a scheme to accelerate the nontrivial two-qubit phase gate in a circuit QED system, where superconducting flux qubits are ultrastrongly coupled to a transmission line resonator (TLR), and two more TLRs are coupled to the ultrastrongly-coupled system for assistant. The nontrivial unconventional geometric phase gate between the two flux qubits is achieved based on close-loop displacements of the three-mode intracavity fields. Moreover, as there are three resonators contributing to the phase accumulation, the requirement of the coupling strength to realize the two-qubit gate can be reduced. Further reduction in the coupling strength to achieve a specific controlled-phase gate can be realized by adding more auxiliary resonators to the ultrastrongly-coupled system through superconducting quantum interference devices. We also present a study of our scheme with realistic parameters considering imperfect controls and noisy environment. Our scheme possesses the merits of ultrafastness and noise-tolerance due to the advantages of geometric phases.
Ultrafast quantum computation in ultrastrongly coupled circuit QED systems
Wang, Yimin; Guo, Chu; Zhang, Guo-Qiang; Wang, Gangcheng; Wu, Chunfeng
2017-01-01
The latest technological progress of achieving the ultrastrong-coupling regime in circuit quantum electrodynamics (QED) systems has greatly promoted the developments of quantum physics, where novel quantum optics phenomena and potential computational benefits have been predicted. Here, we propose a scheme to accelerate the nontrivial two-qubit phase gate in a circuit QED system, where superconducting flux qubits are ultrastrongly coupled to a transmission line resonator (TLR), and two more TLRs are coupled to the ultrastrongly-coupled system for assistant. The nontrivial unconventional geometric phase gate between the two flux qubits is achieved based on close-loop displacements of the three-mode intracavity fields. Moreover, as there are three resonators contributing to the phase accumulation, the requirement of the coupling strength to realize the two-qubit gate can be reduced. Further reduction in the coupling strength to achieve a specific controlled-phase gate can be realized by adding more auxiliary resonators to the ultrastrongly-coupled system through superconducting quantum interference devices. We also present a study of our scheme with realistic parameters considering imperfect controls and noisy environment. Our scheme possesses the merits of ultrafastness and noise-tolerance due to the advantages of geometric phases. PMID:28281654
Synergistic Tunability of Fluorescence Using Coupled Quantum Dots
NASA Astrophysics Data System (ADS)
Sengupta, Sucheta; Acharya, Somobrata
2011-07-01
Size and shape controlled semiconductor quantum dots have been widely adopted in tailoring nanomaterials properties, which basically utilize band edge engineering extending the band gap to larger values than the bulk. However, the long range electronic transition tunability is difficult to achieve using size or shape controlled quantum dots. Alternatively, chemically coupled quantum dots offer a novel route for tuning long range electronic transitions of semiconductors via band offset engineering at the material interface. We report on a simple route of tailoring visible emissions over long range by chemically designing coupled dots comprising of ZnSe and CdS quantum dots. Long range tunability of visible emissions originated from strong interdot dispersion, which can not be realized from individual quantum dots. A variation of component size or extend of coupling led to the tuning of visible emission wavelengths, the intensity of which can be controlled by temperature. The first principles electronic structure calculations based on density functional theory reveals the type-II nature of chemical bonding at CdS/ZnSe interface which plays a crucial role in tailoring the long range emissions. Such novel class of asymmetric coupled structures may offer a basis for diverse set of building blocks for optoelectronic devices, ultrahigh density memories and quantum information processing.
NASA Astrophysics Data System (ADS)
Chwiej, T.
2016-10-01
We theoretically study the single electron magnetotransport in GaAs and InGaAs vertically stacked bilayer nanowires. In considered geometry, the tilted magnetic field is always perpendicular to the main (transport) axis of the quantum wire and, therefore its transverse and vertical components allow separately for changing the magnitude of intralayer and interlayer subbands mixing. We study the changes introduced to energy dispersion relation E(k) by tilted magnetic field of strength up to several tesla and analyze their origins for symmetric as well as asymmetric confining potentials in the growth direction. Calculated energy dispersion relations are thereafter used to show that the value of a conductance of the bilayer nanowire may abruptly rise as well as fall by few conductance quanta when the Fermi energy in nanosystem is changed. It is also shown that such conductance oscillations, in conjunction with spin Zeeman effect, may give a moderately spin polarized current in the bilayer nanowire.
Effective electron mass in quantum wires of III-V, ternary and quaternary materials.
Paitya, N; Ghatak, K P
2012-12-01
In this paper, an attempt is made to study the effective electron mass (EEM) in Quantum wires (QWs) of III-V, ternary and quaternary materials on the basis of three and two band models of Kane within the framework of k x p formalism. It has been found, taking QWs of InAs, InSb, GaAs, Hg(1-x)Cd(x)Te and In(1-x)Ga(x)As(1-y)P(t) that the 1D EEM increases with electron concentration per unit length and decreases with increasing film thickness respectively. For ternary and quaternary materials the EEM increases with increase in alloy composition. Under certain special conditions all the results for all the 1-D materials get simplified into the well known parabolic energy bands and thus confirming the compatibility test. The results of this paper find two applications in the fields of nanoscience and technology.
NASA Astrophysics Data System (ADS)
Giuliano, Domenico; Nava, Andrea
2015-09-01
Making a combined use of bosonization and fermionization techniques, we build nonlocal transformations between dual fermion operators, describing junctions of strongly interacting spinful one-dimensional quantum wires. Our approach allows for trading strongly interacting (in the original coordinates) fermionic Hamiltonians for weakly interacting (in the dual coordinates) ones. It enables us to generalize to the strongly interacting regime the fermionic renormalization group approach to weakly interacting junctions. As a result, on one hand, we are able to pertinently complement the information about the phase diagram of the junction obtained within the bosonization approach; on the other hand, we map out the full crossover of the conductance tensors between any two fixed points in the phase diagram connected by a renormalization group trajectory.
Quantum transport in coupled resonators enclosed synthetic magnetic flux
Jin, L.
2016-07-15
Quantum transport properties are instrumental to understanding quantum coherent transport processes. Potential applications of quantum transport are widespread, in areas ranging from quantum information science to quantum engineering, and not restricted to quantum state transfer, control and manipulation. Here, we study light transport in a ring array of coupled resonators enclosed synthetic magnetic flux. The ring configuration, with an arbitrary number of resonators embedded, forms a two-arm Aharonov–Bohm interferometer. The influence of magnetic flux on light transport is investigated. Tuning the magnetic flux can lead to resonant transmission, while half-integer magnetic flux quantum leads to completely destructive interference and transmission zeros in an interferometer with two equal arms. -- Highlights: •The light transport is investigated through ring array of coupled resonators enclosed synthetic magnetic field. •Aharonov–Bohm ring interferometer of arbitrary configuration is investigated. •The half-integer magnetic flux quantum leads to destructive interference and transmission zeros for two-arm at equal length. •Complete transmission is available via tuning synthetic magnetic flux.
Slow light in evanescently coupled optical cavities containing quantum dots
NASA Astrophysics Data System (ADS)
Ergecen, Emre
2014-05-01
Ability to tune the group velocity of a light pulse is of great importance for optical communication applications and realization of quantum information processing. Tunability of group velocity can be achieved by using either optical or electronic resonances. Tunability of an optical resonance depends on the change in refractive index of the cavity material. However, since electro-optical coefficients of non-engineered materials are quite small, the tuning range of optical resonances by electric field is narrow. This makes tuning by electric field impractical for most applications. Quantum dot (QD) coupled to a photonic crystal cavity is a useful hybrid system exhibiting nonlinear features. In this work, we analyze the use of quantum dot - optical cavity hybrid systems to engineer nonlinear waveguides susceptible to electric fields. We start by theoretically analyzing the optical pulse propagation at low-photon number excitation limit in a periodically arranged strongly coupled quantum dot - photonic crystal system. A one dimensional periodic array of evanescently coupled photonic cavities (coupled resonator optical waveguides, CROWs) containing non-interacting quantum dots allows us to tune the group velocity and the bandwidth of the pulse by adjusting the cavity/QD coupling. Tunable group velocity can be achieved by applying an external electric field which will result in a significant decrease in the cavity/QD coupling because of DC Stark effect. We also show that, using this approach, light pulses can be slowed down or stored by compressing the pulse bandwidth adiabatically and reversibly. Adiabatic bandwidth compression can be achieved by slowly decreasing the coupling strength when the light pulse is inside the coupled resonator optical waveguide. The energy splitting and the coupling constant after applying electric field is calculated by using perturbation theory for two level systems. With our approach, nonlinear materials highly susceptible to electric
Field effect in the quantum Hall regime of a high mobility graphene wire
Barraud, C. E-mail: clement.barraud@univ-paris-diderot.fr; Choi, T.; Ihn, T.; Ensslin, K.; Butti, P.; Shorubalko, I.; Taniguchi, T.; Watanabe, K.
2014-08-21
In graphene-based electronic devices like in transistors, the field effect applied thanks to a gate electrode allows tuning the charge density in the graphene layer and passing continuously from the electron to the hole doped regime across the Dirac point. Homogeneous doping is crucial to understand electrical measurements and for the operation of future graphene-based electronic devices. However, recently theoretical and experimental studies highlighted the role of the electrostatic edge due to fringing electrostatic field lines at the graphene edges [P. Silvestrov and K. Efetov, Phys. Rev. B 77, 155436 (2008); F. T. Vasko and I. V. Zozoulenko, Appl. Phys. Lett. 97, 092115 (2010)]. This effect originates from the particular geometric design of the samples. A direct consequence is a charge accumulation at the graphene edges giving a value for the density, which deviates from the simple picture of a plate capacitor and also varies along the width of the graphene sample. Entering the quantum Hall regime would, in principle, allow probing this accumulation thanks to the extreme sensitivity of this quantum effect to charge density and the charge distribution. Moreover, the presence of an additional and counter-propagating edge channel has been predicted [P. Silvestrov and K. Efetov, Phys. Rev. B 77, 155436 (2008)] giving a fundamental aspect to this technological issue. In this article, we investigate this effect by tuning a high mobility graphene wire into the quantum Hall regime in which charge carriers probe the electrostatic potential at high magnetic field close to the edges. We observe a slight deviation to the linear shift of the quantum Hall plateaus with magnetic field and we study its evolution for different filling factors, which correspond to different probed regions in real space. We discuss the possible origins of this effect including an increase of the charge density towards the edges.
QUANTUM MODE-COUPLING THEORY: Formulation and Applications to Normal and Supercooled Quantum Liquids
NASA Astrophysics Data System (ADS)
Rabani, Eran; Reichman, David R.
2005-05-01
We review our recent efforts to formulate and study a mode-coupling approach to real-time dynamic fluctuations in quantum liquids. Comparison is made between the theory and recent neutron scattering experiments performed on liquid ortho-deuterium and para-hydrogen. We discuss extensions of the theory to supercooled and glassy states where quantum fluctuations compete with thermal fluctuations. Experimental scenarios for quantum glassy liquids are briefly discussed.
Completely CMOS compatible SiN-waveguide-based fiber coupling structure for Si wire waveguides.
Maegami, Yuriko; Okano, Makoto; Cong, Guangwei; Ohno, Morifumi; Yamada, Koji
2016-07-25
For Si wire waveguides, we designed a highly efficient fiber coupling structure consisting of a Si inverted taper waveguide and a CMOS-compatible thin SiN waveguide with an SiO_{2} spacer inserted between them. By using a small SiN waveguide with a 310 nm-square core, the optical field can be expanded to correspond to a fiber with a 4.0-μm mode field diameter. A coupled waveguide system with the SiN waveguide and Si taper waveguide can provide low-loss and low-polarization-dependent mode conversion. Both losses in fiber-SiN waveguide coupling and SiN-Si waveguide mode conversion are no more than 1 dB in a wide wavelength bandwidth from 1.36 μm to 1.65 μm. Through a detailed analysis of the effective refractive indices in the coupled waveguide system, we can understand mode conversion accurately and also derive guidelines for reducing the polarization dependence and for shortening device length.
NV-based quantum memories coupled to photonic integrated circuits
NASA Astrophysics Data System (ADS)
Mouradian, Sara; Schröder, Tim; Zheng, Jiabao; Lu, Tsung-Ju; Choi, Hyeongrak; Wan, Noel; Walsh, Michael; Bersin, Eric; Englund, Dirk
2016-09-01
The negatively charged nitrogen vacancy (NV) center in diamond is a promising solid-state quantum memory. However, developing networks comprising such quantum memories is limited by the fabrication yield of the quantum nodes and the collection efficiency of indistinguishable photons. In this letter, we report on advances on a hybrid quantum system that allows for scalable production of networks, even with low-yield node fabrication. Moreover, an NV center in a simple single mode diamond waveguide is shown in simulation and experiment to couple well to a single mode SiN waveguide with a simple adiabatic taper for optimal mode transfer. In addition, cavity enhancement of the zero phonon line of the NV center with a resonance coupled to the waveguide mode allows a simulated <1800 fold increase in the collection of photon states coherent with the state of the NV center into a single frequency and spatial mode.
Pyramidal GaAs/AlzGa1-zAs quantum wire/dot systems with controlled heterostructure potential
NASA Astrophysics Data System (ADS)
Zhu, Q.; Ganière, J. D.; He, Z. B.; Karlsson, K. F.; Byszewski, M.; Pelucchi, E.; Rudra, A.; Kapon, E.
2010-10-01
The structural and optical properties of controlled-heterostructure-potential, low-dimensional GaAs/AlGaAs nanostructures self-formed during organometallic chemical vapor deposition in tetrahedral pyramids etched in (111)B-GaAs substrates, are investigated using electron microscopy, cathodoluminescence, photoluminescence (PL), photon correlation spectroscopy, and theoretical modeling. Quantum wires/dots with AlGaAs cores with growth-controlled dimensions are formed, with a system of well-defined, low-dimensional nanostructure barriers around them. Transitions between carrier states confined in the AlGaAs quantum wires and dots are identified in the PL spectra, with features in good agreement with model calculations. Emission of single-photons and bunched-photon pairs is observed using temporal photon correlation spectroscopy. This self-formed nanostructure system provides new ways for shaping low-dimensional quantum structures and their heterostructure environment.
Performance of a quantum heat engine at strong reservoir coupling.
Newman, David; Mintert, Florian; Nazir, Ahsan
2017-03-01
We study a quantum heat engine at strong coupling between the system and the thermal reservoirs. Exploiting a collective coordinate mapping, we incorporate system-reservoir correlations into a consistent thermodynamic analysis, thus circumventing the usual restriction to weak coupling and vanishing correlations. We apply our formalism to the example of a quantum Otto cycle, demonstrating that the performance of the engine is diminished in the strong coupling regime with respect to its weakly coupled counterpart, producing a reduced net work output and operating at a lower energy conversion efficiency. We identify costs imposed by sudden decoupling of the system and reservoirs around the cycle as being primarily responsible for the diminished performance, and we define an alternative operational procedure which can partially recover the work output and efficiency. More generally, the collective coordinate mapping holds considerable promise for wider studies of thermodynamic systems beyond weak reservoir coupling.
NASA Astrophysics Data System (ADS)
Al-Qahtani, Saad Mubarak; Abdullah, Abdulmuin Mostafa A.; Nishat, Md. Rezaul Karim; Ahmed, Shaikh S.
2017-03-01
A multiscale computational study is performed to investigate how electronic structure, optical transitions, and terminal characteristics of nanostructured ZnO/MgO disk-in-wire emitters are governed by an intricate coupling of size-quantization, atomicity, and built-in structural and polarization fields. As for the models, an 8-band sp3 (with spin) atomistic tight-binding basis set was used to construct the Hamiltonian of the device in wurtzite crystal symmetry. Strain and the associated distortions of bond directions and bond lengths were modeled via the valence force-field (VFF) molecular mechanics framework. Specifically, in this work, a recently proposed ab initio based diameter-dependent model for the piezoelectric fields was implemented, which, as compared to the conventional diameter-independent model, was found to curb the influence of spontaneous (pyroelectric) polarization significantly. This particular finding is further illustrated through the calculation of electronic bandgap and localization of wavefunctions, optical emission characteristics, and the internal quantum efficiency of the device.
Quantum interference induced photon blockade in a coupled single quantum dot-cavity system.
Tang, Jing; Geng, Weidong; Xu, Xiulai
2015-03-18
We propose an experimental scheme to implement a strong photon blockade with a single quantum dot coupled to a nanocavity. The photon blockade effect can be tremendously enhanced by driving the cavity and the quantum dot simultaneously with two classical laser fields. This enhancement of photon blockade is ascribed to the quantum interference effect to avoid two-photon excitation of the cavity field. Comparing with Jaynes-Cummings model, the second-order correlation function at zero time delay g((2))(0) in our scheme can be reduced by two orders of magnitude and the system sustains a large intracavity photon number. A red (blue) cavity-light detuning asymmetry for photon quantum statistics with bunching or antibunching characteristics is also observed. The photon blockade effect has a controllable flexibility by tuning the relative phase between the two pumping laser fields and the Rabi coupling strength between the quantum dot and the pumping field. Moreover, the photon blockade scheme based on quantum interference mechanism does not require a strong coupling strength between the cavity and the quantum dot, even with the pure dephasing of the system. This simple proposal provides an effective way for potential applications in solid state quantum computation and quantum information processing.
Quantum Interference Induced Photon Blockade in a Coupled Single Quantum Dot-Cavity System
Tang, Jing; Geng, Weidong; Xu, Xiulai
2015-01-01
We propose an experimental scheme to implement a strong photon blockade with a single quantum dot coupled to a nanocavity. The photon blockade effect can be tremendously enhanced by driving the cavity and the quantum dot simultaneously with two classical laser fields. This enhancement of photon blockade is ascribed to the quantum interference effect to avoid two-photon excitation of the cavity field. Comparing with Jaynes-Cummings model, the second-order correlation function at zero time delay g(2)(0) in our scheme can be reduced by two orders of magnitude and the system sustains a large intracavity photon number. A red (blue) cavity-light detuning asymmetry for photon quantum statistics with bunching or antibunching characteristics is also observed. The photon blockade effect has a controllable flexibility by tuning the relative phase between the two pumping laser fields and the Rabi coupling strength between the quantum dot and the pumping field. Moreover, the photon blockade scheme based on quantum interference mechanism does not require a strong coupling strength between the cavity and the quantum dot, even with the pure dephasing of the system. This simple proposal provides an effective way for potential applications in solid state quantum computation and quantum information processing. PMID:25783560
Reentrant enhancement of quantum fluctuations for symmetric environmental coupling.
Cuccoli, Alessandro; Del Sette, Niccolò; Vaia, Ruggero
2010-04-01
The system-plus-reservoir (SPR) model is the most common and effective approach to study quantum dissipative effects. Indeed, it makes quantization possible by considering the whole energy-conserving system, while the reservoir's degrees of freedom, assumed to be harmonic, can be traced out by the path-integral technique, leading to a formulation that only includes the system of interest. In the standard SPR model the environment is only coupled with the system's coordinate and turns out to quench its quantum fluctuations. However, there are physical systems coupled with an environment whose "coordinates" and "momenta" can be completely interchangeable (e.g., magnets), so an SPR coupling must symmetrically affect both canonical variables. In this paper such a general environmental coupling is studied in the case of a harmonic oscillator. It is found that quantum fluctuations are generally enhanced by environmental coupling with an unexpected nonmonotonic behavior. This leads one to speculate about the possibility that spin-lattice coupling could drive the two-dimensional Heisenberg antiferromagnet to reach its quantum-critical point.
The nonuniform spin polarization in the square-shaped 1D wire induced by spin-orbit coupling
NASA Astrophysics Data System (ADS)
Val'kov, V. V.; Fedoseev, A. D.
2017-10-01
It is shown that the Rashba spin-orbit coupling induces the spatially nonuniform spin state in the square-shaped 1D wire. The electron states of this type are characterized with spin orientation changing according to the harmonic motion along the square side. The period of the oscillation is determined only by the spin-orbital coupling and the hopping parameters ratio. The modulation of spin orientation is caused by step-like changing of Rashba field direction. The obtained results were generalized on the case of polygon-shaped wire.
Magnetoresistance and planar Hall effects in submicron exchange-coupled NiO/Fe19Ni81 wires
NASA Astrophysics Data System (ADS)
Nemoto, A.; Otani, Y.; Kim, S. G.; Fukamichi, K.; Kitakami, O.; Shimada, Y.
1999-06-01
Magnetization reversal processes of submicron NiO/Fe19Ni81 exchange-coupled Hall cross with a variable width in the range from 0.2 to 0.4 μm were studied by the magnetoresistivity and the planar Hall effect measurements. The magnetization reversal was found to take place via a coherent rotation in the Hall cross, suggesting that the size of the antiferromagnetic domain is regulated by the wire width. The magnitude of the exchange coupling field Hex varied in proportion to the inverse wire width.
Quantum simulations of strongly coupled quark-gluon plasma
NASA Astrophysics Data System (ADS)
Filinov, V. S.; Ivanov, Yu. B.; Bonitz, M.; Levashov, P. R.; Fortov, V. E.
2012-06-01
A strongly coupled quark-gluon plasma (QGP) of heavy constituent quasi-particles is studied by a path-integral Monte-Carlo method. This approach is a quantum generalization of the classical molecular dynamics by Gelman, Shuryak, and Zahed. It is shown that this method is able to reproduce the QCD lattice equation of state. The results indicate that the QGP reveals liquid-like rather than gaslike properties. Quantum effects turned out to be of prime importance in these simulations.
NASA Astrophysics Data System (ADS)
Abdelrehim, Adel A. A.; Ghafouri-Shiraz, H.
2016-09-01
In this paper, three dimensional periodic structure composed of circular split ring resonators and thin wires is used to improve the performance of a microstrip patch antenna. The three dimensional periodic structure is placed at the top of the patch within a specific separation distance to construct the proposed antenna. The radiated electromagnetic waves intensity of the proposed antenna is improved compared with the conventional patch antenna due to the electric and magnetic coupling enhancements. These enhancements occur between the patch and the periodic structure resonators and between the different resonator pairs of the periodic structure. As a result, the electric and the magnetic fields at the top of the patch are improved, the radiated electromagnetic beam size reduces which results in a highly focused beam and hence the antenna directivity and gain are improved, while the beam are is reduced. The proposed antenna has been designed and simulated using CST microwave studio at 10 GHz. An infinite two dimensional periodicity unit cell of circular split ring resonator and thin wire is designed to resonate at a 10 GHz and simulated in CST software, the scattering parameters are extracted, the results showed that the infinite periodicity two dimensional structure has a pass band frequency response of good transmission and reflection characteristics around 10 GHz. The infinite periodicity of the two dimensional periodic structure is then truncated and multi layers of such truncated structure is used to construct a three dimensional periodic structure. A parametric analysis has been performed on the proposed antenna incorporated with the three dimensional periodic structure. The impacts of the separation distance between the patch and three dimensional periodic structures and the size of the three dimensional periodic structure on the radiation and impedance matching parameters of the proposed antenna are studied. For experimental verification, the proposed
NASA Astrophysics Data System (ADS)
Ludwig, Andreas W. W.
1996-03-01
We review recent exact results (Work done in collaboration with Paul Fendley and Hubert Saleur, Physics Dept., University of Southern California, Los Angeles, CA 90089-0484.) for transport properties through a local impurity in a Luttinger liquid. These represent ``spectroscopic probes'' of the Luttinger non-Fermi-liquid state. Edge modes occuring in the fractional quantum Hall effect provide realizations of Luttinger liquids, insensitive to disorder. The linear-response conductance through a point contact in the ν =1/3 quantum Hall state has been predicted to be a universal function of temperature and point-contact interaction strength, independent of sample-specific details of the device. Our exact result for this scaling function is in quantitative agreement with experimental measurements (F.P. Milliken, C.P. Umbach and R.A. Webb, preprint.). The theoretical advance made in this work(P. Fendley, A.W.W. Ludwig and H. Saleur, Phys. Rev. Lett. 74) (1995) 3005; 75 (1995) 2196; Phys. Rev. B 52 (1995) 8934. is the computation of exact transport properties from the Bethe ansatz; in the past, the Bethe ansatz was useful mainly for thermodynamic quantities. We utilize an exact kinetic transport equation in a particular quasiparticle basis of the Luttinger liquid dictated by the integrability of the point-contact interaction. Since this equation is also valid out of equilibrium, we obtain also non-equilibrium quantum transport properties in this fully interacting system. In particular, we also present universal exact results for the I(V) characteristics and the DC shot noise of the point contact. The differential conductance develops a peak beyond a critical value e V/kB T >7.18868 of driving.
Coupling of individual quantum emitters to channel plasmons
Bermúdez-Ureña, Esteban; Gonzalez-Ballestero, Carlos; Geiselmann, Michael; Marty, Renaud; Radko, Ilya P.; Holmgaard, Tobias; Alaverdyan, Yury; Moreno, Esteban; García-Vidal, Francisco J.; Bozhevolnyi, Sergey I.; Quidant, Romain
2015-01-01
Efficient light-matter interaction lies at the heart of many emerging technologies that seek on-chip integration of solid-state photonic systems. Plasmonic waveguides, which guide the radiation in the form of strongly confined surface plasmon-polariton modes, represent a promising solution to manipulate single photons in coplanar architectures with unprecedented small footprints. Here we demonstrate coupling of the emission from a single quantum emitter to the channel plasmon polaritons supported by a V-groove plasmonic waveguide. Extensive theoretical simulations enable us to determine the position and orientation of the quantum emitter for optimum coupling. Concomitantly with these predictions, we demonstrate experimentally that 42% of a single nitrogen-vacancy centre emission efficiently couples into the supported modes of the V-groove. This work paves the way towards practical realization of efficient and long distance transfer of energy for integrated solid-state quantum systems. PMID:26249363
Optically Mapping the Electronic Structure of Coupled Quantum Dots
2008-02-10
frequency- tunable titanium–sapphire laser . The photoluminescence signal was spectrally dispersed with a 0.75m monochromator equipped with a 1,200mm−1...923–926 (2000). 9. Drexler, H., Leonard, D., Hansen, W., Kotthaus, J. P. & Petroff, P. M. Spectroscopy of quantum levels in charge- tunable InGaAs...Beirne, G. J. et al. Quantum light emission of two lateral tunnel-coupled (In,Ga)As/GaAs quantum dots controlled by a tunable static electric field. Phys
Quantum chaos in an ultrastrongly coupled bosonic junction.
Naether, Uta; García-Ripoll, Juan José; Mazo, Juan José; Zueco, David
2014-02-21
The semiclassical and quantum dynamics of two ultrastrongly coupled nonlinear resonators cannot be explained using the discrete nonlinear Schrödinger equation or the Bose-Hubbard model, respectively. Instead, a model beyond the rotating wave approximation must be studied. In the semiclassical limit this model is not integrable and becomes chaotic for a finite window of parameters. For the quantum dimer we find corresponding regions of stability and chaos. The more striking consequence for both semiclassical and quantum chaos is that the tunneling time between the sites becomes unpredictable. These results, including the transition to chaos, can be tested in experiments with superconducting microwave resonators.
Monolithically integrated single quantum dots coupled to bowtie nanoantennas
NASA Astrophysics Data System (ADS)
Lyamkina, A. A.; Schraml, K.; Regler, A.; Schalk, M.; Bakarov, A. K.; Toropov, A. I.; Moshchenko, S. P.; Kaniber, Michael
2016-12-01
Deterministically integrating semiconductor quantum emitters with plasmonic nano-devices paves the way towards chip-scale integrable, true nanoscale quantum photonics technologies. For this purpose, stable and bright semiconductor emitters are needed, which moreover allow for CMOS-compatibility and optical activity in the telecommunication band. Here, we demonstrate strongly enhanced light-matter coupling of single near-surface ($<10\\,nm$) InAs quantum dots monolithically integrated into electromagnetic hot-spots of sub-wavelength sized metal nanoantennas. The antenna strongly enhances the emission intensity of single quantum dots by up to $\\sim16\\times$, an effect accompanied by an up to $3.4\\times$ Purcell-enhanced spontaneous emission rate. Moreover, the emission is strongly polarised along the antenna axis with degrees of linear polarisation up to $\\sim85\\,\\%$. The results unambiguously demonstrate the efficient coupling of individual quantum dots to state-of-the-art nanoantennas. Our work provides new perspectives for the realisation of quantum plasmonic sensors, step-changing photovoltaic devices, bright and ultrafast quantum light sources and efficent nano-lasers.
Macroscopic Quantum Phenomena from the Correlation, Coupling and Criticality Perspectives
NASA Astrophysics Data System (ADS)
Chou, C. H.; Hu, B. L.; Subaşi, Y.
2011-12-01
In this sequel paper we explore how macroscopic quantum phenomena can be measured or understood from the behavior of quantum correlations which exist in a quantum system of many particles or components and how the interaction strengths change with energy or scale, under ordinary situations and when the system is near its critical point. We use the nPI (master) effective action related to the Boltzmann-BBGKY / Schwinger-Dyson hierarchy of equations as a tool for systemizing the contributions of higher order correlation functions to the dynamics of lower order correlation functions. Together with the large N expansion discussed in our first paper [1] we explore 1) the conditions whereby an H-theorem is obtained, which can be viewed as a signifier of the emergence of macroscopic behavior in the system. We give two more examples from past work: 2) the nonequilibrium dynamics of N atoms in an optical lattice under the large Script N (field components), 2PI and second order perturbative expansions, illustrating how N and Script N enter in these three aspects of quantum correlations, coherence and coupling strength. 3) the behavior of an interacting quantum system near its critical point, the effects of quantum and thermal fluctuations and the conditions under which the system manifests infrared dimensional reduction. We also discuss how the effective field theory concept bears on macroscopic quantum phenomena: the running of the coupling parameters with energy or scale imparts a dynamical-dependent and an interaction-sensitive definition of 'macroscopia'.
NASA Astrophysics Data System (ADS)
Bouazra, A.; Nasrallah, S. Abdi-Ben; Said, M.
2016-01-01
In this work, we propose an efficient method to investigate optical properties as well as their dependence on geometrical parameters in InAs/InAlAs quantum wires. The used method is based on the coordinate transformation and the finite difference method. It provides sufficient accuracy, stability and flexibility with respect to the size and shape of the quantum wire. The electron and hole energy levels as well as their corresponding wave functions are investigated for different shape of quantum wires. The optical transition energies, the emission wavelengths and the oscillator strengths are also studied.
Searching for robust quantum memories in many coupled oscillators
NASA Astrophysics Data System (ADS)
Bosco de Magalhães, A. R.
2011-11-01
The relation between microscopic symmetries in the system-environment interaction and the emergence of robust states is studied for many linearly coupled harmonic oscillators. Different types of symmetry, which are introduced into the model as terms in the coupling constants between each system's oscillator and a common reservoir, lead to distinct robust modes. Since these modes are partially or completely immune to the symmetric part of the environmental noise, they are good candidates for building quantum memories. A comparison of the model investigated here, with bilinear system-reservoir coupling, and a model where such coupling presents an exponential dependence on the variables of interest is performed.
Optical investigation of the one-dimensional confinement effects in narrow GaAs/GaAlAs quantum wires
NASA Astrophysics Data System (ADS)
Birotheau, L.; Izrael, A.; Marzin, J. Y.; Azoulay, R.; Thierry-Mieg, V.; Ladan, F. R.
1992-12-01
We show optical data obtained at 8 K on narrow GaAs/GaAlAs quantum wires, with width down to 15 nm, fabricated by reactive ion etching and metal organic chemical vapor deposition overgrowth. Lateral confinement energies (up to 23 meV) and polarization effects are evidenced in the photoluminescence excitation spectra. These experimental results are in good agreement with calculated absorption spectra, which include the effects of wire width fluctuations, yielding, for our fabrication technique, a value of ±5 nm for these size fluctuations.
Quantized conductance in up to 20 μm long shallow etched GaAs/AlGaAs quantum wires
NASA Astrophysics Data System (ADS)
Worschech, L.; Beuscher, F.; Forchel, A.
1999-07-01
Conductance quantization is observed in up to 20 μm long and 135 nm wide quantum wires fabricated by wet chemical etching of modulation-doped GaAs/AlGaAs heterostructures. With increasing wire length, the higher plateaus in the conductance vanish, whereas a fundamental step persists up to a length comparable to the transport mean free path of the two-dimensional layers. Via magnetic depopulation of the one-dimensional (1D) subbands a lateral depletion length of wdep=15 nm is estimated for a gate voltage of 1.1 V. By temperature dependent measurements the energy spacings between the 1D subbands are investigated.
Hope, J J; Olsen, M K
2001-04-09
We show that in certain parameter regimes there is a macroscopic dynamical breakdown of the Gross-Pitaevskii equation. Stochastic field equations for coupled atomic and molecular condensates are derived using the functional positive- P representation. These equations describe the full quantum state of the coupled condensates and include the commonly used Gross-Pitaevskii equation as the noiseless limit. The full quantum theory includes the spontaneous processes which will become significant when the atomic population is low. The experimental signature of the quantum effects will be the time scale of the revival of the atomic population after a near total conversion to the molecular condensate.
Zheng, Kai; Li, Huan; Yang, Li-Jun; Gu, Xiao-Yan; Gao, Ying
2013-04-01
The plasma radiation of laser-double wire hybrid welding was collected by using fiber spectrometer, the coupling mechanism of arc with laser was studied through high-speed photography during welding process, and the temperature of hybrid plasma was calculated by using the method of Boltzmann plot. The results indicated that with laser hybrid, luminance was enhanced; radiation intensity became stronger; arc was attracted to the laser point; cross section contracted and arc was more stable. The laser power, welding current and arc-arc distance are important factors that have great influence on electron temperature. Increase in the laser power, amplification of welding current and reduction of arc-arc distance can all result in the rise of temperature.
Alvaro, M; Bonilla, L L; Carretero, M; Melnik, R V N; Prabhakar, S
2013-08-21
In this paper we develop a kinetic model for the analysis of semiconductor superlattices, accounting for quantum effects. The model consists of a Boltzmann-Poisson type system of equations with simplified Bhatnagar-Gross-Krook collisions, obtained from the general time-dependent Schrödinger-Poisson model using Wigner functions. This system for superlattice transport is supplemented by the quantum mechanical part of the model based on the Ben-Daniel-Duke form of the Schrödinger equation for a cylindrical superlattice of finite radius. The resulting energy spectrum is used to characterize the Fermi-Dirac distribution that appears in the Bhatnagar-Gross-Krook collision, thereby coupling the quantum mechanical and kinetic parts of the model. The kinetic model uses the dispersion relation obtained by the generalized Kronig-Penney method, and allows us to estimate radii of quantum wire superlattices that have the same miniband widths as in experiments. It also allows us to determine more accurately the time-dependent characteristics of superlattices, in particular their current density. Results, for several experimentally grown superlattices, are discussed in the context of self-sustained coherent oscillations of the current density which are important in an increasing range of current and potential applications.
Intra-wire coupling in segmented Ni/Cu nanowires deposited by electrodeposition
NASA Astrophysics Data System (ADS)
Sergelius, Philip; Lee, Ji Hyun; Fruchart, Olivier; Shaker Salem, Mohamed; Allende, Sebastian; Alejandro Escobar, Roberto; Gooth, Johannes; Zierold, Robert; Toussaint, Jean-Christophe; Schneider, Sebastian; Pohl, Darius; Rellinghaus, Bernd; Martin, Sylvain; Garcia, Javier; Reith, Heiko; Spende, Anne; Toimil-Molares, Maria-Eugenia; Altbir, Dora; Cowburn, Russel; Görlitz, Detlef; Nielsch, Kornelius
2017-02-01
Segmented magnetic nanowires are a promising route for the development of three dimensional data storage techniques. Such devices require a control of the coercive field and the coupling mechanisms between individual magnetic elements. In our study, we investigate electrodeposited nanomagnets within host templates using vibrating sample magnetometry and observe a strong dependence between nanowire length and coercive field (25 nm-5 μm) and diameter (25-45 nm). A transition from a magnetization reversal through coherent rotation to domain wall propagation is observed at an aspect ratio of approximately 2. Our results are further reinforced via micromagnetic simulations and angle dependent hysteresis loops. The found behavior is exploited to create nanowires consisting of a fixed and a free segment in a spin-valve like structure. The wires are released from the membrane and electrically contacted, displaying a giant magnetoresistance effect that is attributed to individual switching of the coupled nanomagnets. We develop a simple analytical model to describe the observed switching phenomena and to predict stable and unstable regimes in coupled nanomagnets of certain geometries.
Intra-wire coupling in segmented Ni/Cu nanowires deposited by electrodeposition.
Sergelius, Philip; Lee, Ji Hyun; Fruchart, Olivier; Salem, Mohamed Shaker; Allende, Sebastian; Escobar, Roberto Alejandro; Gooth, Johannes; Zierold, Robert; Toussaint, Jean-Christophe; Schneider, Sebastian; Pohl, Darius; Rellinghaus, Bernd; Martin, Sylvain; Garcia, Javier; Reith, Heiko; Spende, Anne; Toimil-Molares, Maria-Eugenia; Altbir, Dora; Cowburn, Russel; Görlitz, Detlef; Nielsch, Kornelius
2017-02-10
Segmented magnetic nanowires are a promising route for the development of three dimensional data storage techniques. Such devices require a control of the coercive field and the coupling mechanisms between individual magnetic elements. In our study, we investigate electrodeposited nanomagnets within host templates using vibrating sample magnetometry and observe a strong dependence between nanowire length and coercive field (25 nm-5 μm) and diameter (25-45 nm). A transition from a magnetization reversal through coherent rotation to domain wall propagation is observed at an aspect ratio of approximately 2. Our results are further reinforced via micromagnetic simulations and angle dependent hysteresis loops. The found behavior is exploited to create nanowires consisting of a fixed and a free segment in a spin-valve like structure. The wires are released from the membrane and electrically contacted, displaying a giant magnetoresistance effect that is attributed to individual switching of the coupled nanomagnets. We develop a simple analytical model to describe the observed switching phenomena and to predict stable and unstable regimes in coupled nanomagnets of certain geometries.
Quantum Yield of Single Surface Plasmons Generated by a Quantum Dot Coupled with a Silver Nanowire.
Li, Qiang; Wei, Hong; Xu, Hongxing
2015-12-09
The interactions between surface plasmons (SPs) in metal nanostructures and excitons in quantum emitters (QEs) lead to many interesting phenomena and potential applications that are strongly dependent on the quantum yield of SPs. The difficulty in distinguishing all the possible exciton recombination channels hinders the experimental determination of SP quantum yield. Here, we experimentally measured for the first time the quantum yield of single SPs generated by the exciton-plasmon coupling in a system composed of a single quantum dot and a silver nanowire (NW). By utilizing the SP guiding property of the NW, the decay rates of all the exciton recombination channels, i.e., direct free space radiation channel, SP generation channel, and nonradiative damping channel, are quantitatively obtained. It is determined that the optimum emitter-NW coupling distance for the largest SP quantum yield is about 10 nm, resulting from the different distance-dependent decay rates of the three channels. These results are important for manipulating the coupling between plasmonic nanostructures and QEs and developing on-chip quantum plasmonic devices for potential nanophotonic and quantum information applications.
Correlated dynamics of a Rabi oscillation and a quantum tunneling in coupled quantum dots
NASA Astrophysics Data System (ADS)
Xie, Weidong; Chu, Bingxin; Duan, Suqing; Xie, Yan; Chu, Weidong; Yang, Ning; Zhao, Xian-Geng
2015-08-01
We couple the Rabi oscillation in a double quantum dot (DQD) with the quantum tunneling in another DQD by Coulomb interaction between the neighboring dots. Such a coupling leads to correlation of the Rabi oscillating electron and the quantum tunneling one, and gives a tendency of synchronizing them under appropriate Rabi frequency ΩR and tunneling rate Tc. The correlated oscillation is shown clearly in the tunneling current. As ΩR =Tc, the Rabi oscillation and the quantum tunneling reach their strongest correlation and the two electrons finish their complete transitions simultaneously. And then, a single optical signal accomplishes a gang control of two electrons. This result encourages superior design of two-qubit quantum gates based on correlated DQDs.
Collective behavior of quantum resonators coupled to a metamaterial
NASA Astrophysics Data System (ADS)
Felbacq, Didier; Rousseau, Emmanuel
2016-09-01
We study a device that consist of quantum resonators coupled to a mesoscopic photonic structure, such as a metasurface or a 2D metamaterial. For metasurfaces, we use surface Bloch modes in order to reach various coupling regimes between the metasurface and a quantum emitter, modelized semi-classically by an oscillator. Using multiple scattering theory and complex plane techniques, we show that the coupling can be characterized by means of a pole-and-zero structure. The regime of strong coupling is shown to be reached when the pole-and- zero pair is broken. For 2D metamaterial, we show the possibility of controlling optically the opening or closing of a gap.
NASA Astrophysics Data System (ADS)
Tsukui, Tetsuya; Oda, Shunri
1993-12-01
We propose “trench-oxide metal-oxide-semiconductor (MOS)” structures as a novel formation method of silicon-based low-dimensional quantum structures, which are considered to be basic elements of future ultrahigh-speed and ultralarge-scale integrated devices. In this method, the applied gate voltage forms the potential well confined in an additional direction defined by ultrafine “trenches” on the oxide layer of the MOS structure. We characterize “trench-oxide MOS” quantum wire structures by two-dimensional numerical calculation of the shape of the potential well, the subband energy levels and the electron density, and investigate the possibility of the experimental observation of quantized density of states peculiar to quantum wires, by measuring capacitance-gate voltage (C-V) characteristics of “trench-oxide MOS capacitors.” We also have successfully fabricated “trench-oxide MOS” quantum wires with the width of 16 nm using electron beam (EB) lithography and electron cyclotron resonance reactive ion etching (ECR-RIE).
Quantum Brayton cycle with coupled systems as working substance.
Huang, X L; Wang, L C; Yi, X X
2013-01-01
We explore the quantum version of the Brayton cycle with a composite system as the working substance. The actual Brayton cycle consists of two adiabatic and two isobaric processes. Two pressures can be defined in our isobaric process; one corresponds to the external magnetic field (characterized by F(x)) exerted on the system, while the other corresponds to the coupling constant between the subsystems (characterized by F(y)). As a consequence, we can define two types of quantum Brayton cycle for the composite system. We find that the subsystem experiences a quantum Brayton cycle in one quantum Brayton cycle (characterized by F(x)), whereas the subsystem's cycle is quantum Otto cycle in another Brayton cycle (characterized by F(y)). The efficiency for the composite system equals to that for the subsystem in both cases, but the work done by the total system is usually larger than the sum of the work done by the two subsystems. The other interesting finding is that for the cycle characterized by F(y), the subsystem can be a refrigerator, while the total system is a heat engine. The result in this paper can be generalized to a quantum Brayton cycle with a general coupled system as the working substance.
Silicon quantum processor with robust long-distance qubit couplings.
Tosi, Guilherme; Mohiyaddin, Fahd A; Schmitt, Vivien; Tenberg, Stefanie; Rahman, Rajib; Klimeck, Gerhard; Morello, Andrea
2017-09-06
Practical quantum computers require a large network of highly coherent qubits, interconnected in a design robust against errors. Donor spins in silicon provide state-of-the-art coherence and quantum gate fidelities, in a platform adapted from industrial semiconductor processing. Here we present a scalable design for a silicon quantum processor that does not require precise donor placement and leaves ample space for the routing of interconnects and readout devices. We introduce the flip-flop qubit, a combination of the electron-nuclear spin states of a phosphorus donor that can be controlled by microwave electric fields. Two-qubit gates exploit a second-order electric dipole-dipole interaction, allowing selective coupling beyond the nearest-neighbor, at separations of hundreds of nanometers, while microwave resonators can extend the entanglement to macroscopic distances. We predict gate fidelities within fault-tolerance thresholds using realistic noise models. This design provides a realizable blueprint for scalable spin-based quantum computers in silicon.Quantum computers will require a large network of coherent qubits, connected in a noise-resilient way. Tosi et al. present a design for a quantum processor based on electron-nuclear spins in silicon, with electrical control and coupling schemes that simplify qubit fabrication and operation.
Resonant soft x-ray scattering from La1-xSrxMnO3 quantum wire arrays
NASA Astrophysics Data System (ADS)
Wang, Shuai; Smadici, Serban; Lee, James; Odlyzko, Michael; Zhai, Xiaofang; Eckstein, James; Shah, Amish; Zuo, Jian-Min; Abbamonte, Peter; Bhattacharya, Anand
2009-03-01
Any finite sized, patterned system with an energy gap is expected to have elementary excitations that are characteristic of its boundary. To test this idea we have fabricated large arrays (>60000 elements) of colossal magnetoresistance- phase La2/3 Sr1/3 MnO3 quantum wires. These wires are 80 nm in width so have properties that are dominated by edge effects. We used resonant soft x-ray scattering (RSXS) and SQUID magnetometry to study their magnetic properties. We found that patterning lowers the Curie temperature and suppresses the degree of magnetization. RSXS studies show diffraction maxima from the wire period, as well as temperature-dependent diffuse scattering. We will discuss these results in the context of combined structural and magnetic disorder. Funding #: DOE grants DE-FG02-07ER46453 and DE-FG02-06ER46285
Bell states and entanglement dynamics on two coupled quantum molecules
Oliveira, P.A.; Sanz, L.
2015-05-15
This work provides a complete description of entanglement properties between electrons inside coupled quantum molecules, nanoestructures which consist of two quantum dots. Each electron can tunnel between the two quantum dots inside the molecule, being also coupled by Coulomb interaction. First, it is shown that Bell states act as a natural basis for the description of this physical system, defining the characteristics of the energy spectrum and the eigenstates. Then, the entanglement properties of the eigenstates are discussed, shedding light on the roles of each physical parameters on experimental setup. Finally, a detailed analysis of the dynamics shows the path to generate states with a high degree of entanglement, as well as physical conditions associated with coherent oscillations between separable and Bell states.
Decoherence of quantum superpositions through coupling to engineered reservoirs
Myatt; King; Turchette; Sackett; Kielpinski; Itano; Monroe; Wineland
2000-01-20
The theory of quantum mechanics applies to closed systems. In such ideal situations, a single atom can, for example, exist simultaneously in a superposition of two different spatial locations. In contrast, real systems always interact with their environment, with the consequence that macroscopic quantum superpositions (as illustrated by the 'Schrodinger's cat' thought-experiment) are not observed. Moreover, macroscopic superpositions decay so quickly that even the dynamics of decoherence cannot be observed. However, mesoscopic systems offer the possibility of observing the decoherence of such quantum superpositions. Here we present measurements of the decoherence of superposed motional states of a single trapped atom. Decoherence is induced by coupling the atom to engineered reservoirs, in which the coupling and state of the environment are controllable. We perform three experiments, finding that the decoherence rate scales with the square of a quantity describing the amplitude of the superposition state.
Spectral classification of coupling regimes in the quantum Rabi model
NASA Astrophysics Data System (ADS)
Rossatto, Daniel Z.; Villas-Bôas, Celso J.; Sanz, Mikel; Solano, Enrique
2017-07-01
The quantum Rabi model is in the scientific spotlight due to the recent theoretical and experimental progress. Nevertheless, a full-fledged classification of its coupling regimes remains as a relevant open question. We propose a spectral classification dividing the coupling regimes into three regions based on the validity of perturbative criteria on the quantum Rabi model, which allows us the use of exactly solvable effective Hamiltonians. These coupling regimes are (i) the perturbative ultrastrong coupling regime which comprises the Jaynes-Cummings model, (ii) a region where nonperturbative ultrastrong and nonperturbative deep strong coupling regimes coexist, and (iii) the perturbative deep strong coupling regime. We show that this spectral classification depends not only on the ratio between the coupling strength and the natural frequencies of the unperturbed parts, but also on the energy to which the system can access. These regimes additionally discriminate the completely different behaviors of several static physical properties, namely the total number of excitations, the photon statistics of the field, and the cavity-qubit entanglement. Finally, we explain the dynamical properties which are traditionally associated with the deep strong coupling regime, such as the collapses and revivals of the state population, in the frame of the proposed spectral classification.
Saravanan, S.; Peter, A. John
2016-05-23
Intense high frequency laser field induced electronic and optical properties of heavy hole exciton in the InAs{sub 0.8}P{sub 0.2}/InP quantum wire is studied taking into account the geometrical confinement effect. Laser field related exciton binding energies and the optical band gap in the InAs{sub 0.8}P{sub 0.2}/InP quantum well wire are investigated. The optical gain, for the interband optical transition, as a function of photon energy, in the InAs{sub 0.8}P{sub 0.2}/InP quantum wire, is obtained in the presence of intense laser field. The compact density matrix method is employed to obtain the optical gain. The obtained optical gain in group III-V narrow quantum wire can be applied for achieving the preferred telecommunication wavelength.
Novel mid-IR quantum cascade laser waveguide coupling techniques
NASA Astrophysics Data System (ADS)
Shyu, David; Choa, Fow-Sen; Chen, Xing; Trivedi, Sudhir
2011-02-01
Signal modulations and coherent signal detections are important for both communications and remote sensing applications. Photonic integration reduces size, weight, cost, and improves the performance of these systems as demonstrated by integrated DFB-laser modulators, receivers and transceivers at near-IR wavelength regions. However, due to the difficulties of waveguide coupling, photonic integration in the mid-IR wavelength range is under-developed. We developed a technique that can monitor mid-IR coupling based on the negative photoconductivity in quantum cascaded (QC) materials that can simplify and optimize laser waveguide coupling in the mid-IR wavelength range.
Self-Organized InGaAs/GaAs Quantum Wire Nanostructures Grown by Metal-Organic Vapor Phase Epitaxy
1999-06-18
quantum wells with x = 0.9 and the thickness equal to 2 and 3 monolayers (0.6 and 0.9 nm) respectively on the basis of comparing the calculated...GaAs quantum wire nanostructures grown by metal-organic vapor phase epitaxy I. A. Karpovicht, B. N. Zvonkov§, N. V. Baidus§, D. 0. Filatov.t, Yu. Yu...Organic Vapor Phase Epitaxy. Atomic Force Microscopy (AFM) studies show presence of a homogeneous system of well ordered shaped rectangular nanoislands
The circular current in a conducting mesoscopic ring with coupled quantum dot
NASA Astrophysics Data System (ADS)
Bo, Zhou
2016-11-01
In this paper, we investigate the circular current in the metallic ring coupled with two QDs connected with two ferromagnetic leads with the in-plane electric field ξ which is perpendicular to the wire applied to the ring by using the non-equilibrium Green's function technique. We find that in this systems the circular current in the ring can be tuned by the voltages between left and right electrodes, the temperature, the spin polarization, and the energy of the QDs and the electric field ɛ. With these parameters varied, the values and direction of the circular current can be controlled, so we can control the values and the directions of the magnetic field produced at the ring center. In addition, there also has the function of spin filter in the P and has the function of suppression of the circular current for large 𝒫 of the AP. So this structure has potential applications in designing spin-based quantum devices.
Storage and retrieval of light pulse in coupled quantum wells
Liu, Jibing Liu, Na; Shan, Chuanjia; Li, Hong; Liu, Tangkun; Zheng, Anshou
2016-03-15
In this paper, we propose an effective scheme to create a frequency entangled states based on bound-to-bound inter-subband transitions in an asymmetric three-coupled quantum well structure. A four-subband cascade configuration quantum well structure is illuminated with a pulsed probe field and two continuous wave control laser fields to generate a mixing field. By properly adjusting the frequency detunings and the intensity of coupling fields, the conversion efficiency can reach 100%. A maximum entangled state can be achieved by selecting a proper length of the sample. We also numerically investigate the propagation dynamics of the probe pulse and mixing pulse, the results show that two frequency components are able to exchange energy through a four-wave mixing process. Moreover, by considering special coupling fields, the storage and retrieval of the probe pulse is also numerically simulated.
Fully coupled hybrid cavity optomechanics: Quantum interferences and correlations
NASA Astrophysics Data System (ADS)
Restrepo, Juan; Favero, Ivan; Ciuti, Cristiano
2017-02-01
We present a quantum theory for a fully coupled hybrid optomechanical system where all mutual couplings between a two-level atom, a confined photon mode, and a mechanical oscillator mode are considered. In such a configuration, new quantum interference effects and correlations arise due to the interplay and competition between the different physical interactions. We present an analytical diagonalization of the related fully coupled Hamiltonian, showing the nature and energy spectra of the tripartite dressed excitations. We determine the driven-dissipative dynamics of such hybrid systems and study phonon blockade effects under resonant excitation. We also study the statistical properties of the photon emission obtained under incoherent pumping of the two-level atom, which is particularly relevant for experiments with solid-state two-level emitters.
Coupling between graphene and intersubband collective excitations in quantum wells
NASA Astrophysics Data System (ADS)
Gonzalez de la Cruz, G.
2017-08-01
Recently, strong light-matter coupling between the electromagnetic modes in plasmonic metasurfaces with quantum-engineering electronic intersubband transitions in quantum wells has been demonstrated experimentally (Benz et al., [14], Lee et al., [15]). These novel materials combining different two-dimensional electronic systems offer new opportunities for tunable optical devices and fundamental studies of collective excitations driven by interlayer Coulomb interactions. In this work, our aim is to study the plasmon spectra of a hybrid structure consisting of conventional two-dimensional electron gas (2DEG) in a semiconductor quantum well and a graphene sheet with an interlayer separation of a. This electronic bilayer structure is immersed in a nonhomgeneous dielectric background of the system. We use a simple model in which the graphene surface plasmons and both; the intrasubband and intersubband collective electron excitations in the quantum well are coupled via screened Coulomb interaction. Here we calculate the dispersion of these relativistic/nonrelativistic new plasmon modes taking into account the thickness of the quantum well providing analytical expressions in the long-wavelength limit.
Entanglement dynamics of quantum oscillators nonlinearly coupled to thermal environments
NASA Astrophysics Data System (ADS)
Voje, Aurora; Croy, Alexander; Isacsson, Andreas
2015-07-01
We study the asymptotic entanglement of two quantum harmonic oscillators nonlinearly coupled to an environment. Coupling to independent baths and a common bath are investigated. Numerical results obtained using the Wangsness-Bloch-Redfield method are supplemented by analytical results in the rotating wave approximation. The asymptotic negativity as function of temperature, initial squeezing, and coupling strength, is compared to results for systems with linear system-reservoir coupling. We find that, due to the parity-conserving nature of the coupling, the asymptotic entanglement is considerably more robust than for the linearly damped cases. In contrast to linearly damped systems, the asymptotic behavior of entanglement is similar for the two bath configurations in the nonlinearly damped case. This is due to the two-phonon system-bath exchange causing a suppression of information exchange between the oscillators via the bath in the common-bath configuration at low temperatures.
Kondo effect in coupled quantum dots: A noncrossing approximation study
NASA Astrophysics Data System (ADS)
Aguado, Ramón; Langreth, David C.
2003-06-01
The out-of-equilibrium transport properties of a double quantum dot system in the Kondo regime are studied theoretically by means of a two-impurity Anderson Hamiltonian with interimpurity hopping. The Hamiltonian, formulated in slave-boson language, is solved by means of a generalization of the noncrossing approximation (NCA) to the present problem. We provide benchmark calculations of the predictions of the NCA for the linear and nonlinear transport properties of coupled quantum dots in the Kondo regime. We give a series of predictions that can be observed experimentally in linear and nonlinear transport measurements through coupled quantum dots. Importantly, it is demonstrated that measurements of the differential conductance G=dI/dV, for the appropriate values of voltages and interdot tunneling couplings, can give a direct observation of the coherent superposition between the many-body Kondo states of each dot. This coherence can be also detected in the linear transport through the system: the curve linear conductance vs temperature is nonmonotonic, with a maximum at a temperature T* characterizing quantum coherence between both the Kondo states.
Effect of the fissile bead's and thermocouple wires' sizes on the response time of a fission couple.
Liang, Wenfeng; Lu, Yi; Li, Meng; Fan, Xiaoqiang; Lu, Wei
2014-05-01
The fission couple is proposed as a fast response miniature neutron detector in the measurement of time dependent energy depositions within the fissile material based on theoretical analysis, but the response time of a fission couple is relatively slow in practice. The time lag originated from heat transfer process was demonstrated to be the dominating factor by theoretical simulations and experimental verification in this paper. The response of a fission couple as a function of the bead size and the thermocouple wires' sizes are simulated using ANSYS workbench. The decrease of wires' diameter results in the decrease of response time, and the increase of bead's diameter leads to a slight increase of response time. During a pulse heating transient in the fuel of Chinese Fast Burst Reactor II with a FWHM of 181 μs, the time lag originated from heat transfer process is about tens of microseconds for the peaks of the change rate of temperature, and is of the order of milliseconds to achieve 85% of the temperature rise for a typical fission couple with a Φ 1 mm fissile bead and two Φ 0.05 mm thermocouple wires. The results obtained provide foundation for the optimization of fission couples.
Localized states in a semiconductor quantum ring with a tangent wire
Yang, F.; Wu, M. W.
2014-08-28
We extend a special kind of localized state trapped at the intersection due to the geometric confinement, first proposed in a three-terminal-opening T-shaped structure [L. A. Openov, Europhys. Lett. 55, 539 (2001)], into a ring geometry with a tangent connection to the wire. In this ring geometry, there exists one localized state trapped at the intersection with energy lying inside the lowest subband. We systematically study this localized state and the resulting Fano-type interference due to the coupling between this localized state and the continuum ones. It is found that the increase of inner radius of the ring weakens the coupling to the continuum ones and the asymmetric Fano dip fades away. A wide energy gap in transmission appears due to the interplay of two types of antiresonances: the Fano-type antiresonance and the structure antiresonance. The size of this antiresonance gap can be modulated by adjusting the magnetic flux. Moreover, a large transmission amplitude can be obtained in the same gap area. The strong robustness of the antiresonance gap is demonstrated and shows the feasibility of the proposed geometry for a real application.
Electromagnetic modeling of edge coupled quantum well infrared photodetectors
NASA Astrophysics Data System (ADS)
Choi, K. K.
2012-06-01
Edge coupling through a 45° facet is the standard approach in characterizing quantum well infrared photodetector materials. From the spectral responsivity, the material absorption coefficient can be deduced from a classical model. However, this classical model has not been closely examined by a rigorous theory. In this work, we apply finite element electromagnetic modeling to obtain the detector quantum efficiency under this geometry and compare the result with the classical prediction. Remarkably, we find that both models give the same result at this particular angle while they differ significantly at other angles.
Molecular nanomagnets with switchable coupling for quantum simulation
Chiesa, Alessandro; Whitehead, George F. S.; Carretta, Stefano; Carthy, Laura; Timco, Grigore A.; Teat, Simon J.; Amoretti, Giuseppe; Pavarini, Eva; Winpenny, Richard E. P.; Santini, Paolo
2014-01-01
Molecular nanomagnets are attractive candidate qubits because of their wide inter- and intra-molecular tunability. Uniform magnetic pulses could be exploited to implement one- and two-qubit gates in presence of a properly engineered pattern of interactions, but the synthesis of suitable and potentially scalable supramolecular complexes has proven a very hard task. Indeed, no quantum algorithms have ever been implemented, not even a proof-of-principle two-qubit gate. Here we show that the magnetic couplings in two supramolecular {Cr7Ni}-Ni-{Cr7Ni} assemblies can be chemically engineered to fit the above requisites for conditional gates with no need of local control. Microscopic parameters are determined by a recently developed many-body ab-initio approach and used to simulate quantum gates. We find that these systems are optimal for proof-of-principle two-qubit experiments and can be exploited as building blocks of scalable architectures for quantum simulation. PMID:25502419
Terahertz Quantum Cascade Laser With Efficient Coupling and Beam Profile
NASA Technical Reports Server (NTRS)
Chattopadhyay, Goutam; Kawamura, Jonathan H.; Lin, Robert H.; Williams, Benjamin
2012-01-01
Quantum cascade lasers (QCLs) are unipolar semiconductor lasers, where the wavelength of emitted radiation is determined by the engineering of quantum states within the conduction band in coupled multiple-quantum-well heterostructures to have the desired energy separation. The recent development of terahertz QCLs has provided a new generation of solid-state sources for radiation in the terahertz frequency range. Terahertz QCLs have been demonstrated from 0.84 to 5.0 THz both in pulsed mode and continuous wave mode (CW mode). The approach employs a resonant-phonon depopulation concept. The metal-metal (MM) waveguide fabrication is performed using Cu-Cu thermo-compression bonding to bond the GaAs/AlGaAs epitaxial layer to a GaAs receptor wafer.
Charge transfer magnetoexciton formation at vertically coupled quantum dots
2012-01-01
A theoretical investigation is presented on the properties of charge transfer excitons at vertically coupled semiconductor quantum dots in the presence of electric and magnetic fields directed along the growth axis. Such excitons should have two interesting characteristics: an extremely long lifetime and a permanent dipole moment. We show that wave functions and the low-lying energies of charge transfer exciton can be found exactly for a special morphology of quantum dots that provides a parabolic confinement inside the layers. To take into account a difference between confinement potentials of an actual structure and of our exactly solvable model, we use the Galerkin method. The density of energy states is calculated for different InAs/GaAs quantum dots’ dimensions, the separation between layers, and the strength of the electric and magnetic fields. A possibility of a formation of a giant dipolar momentum under external electric field is predicted. PMID:23092373
Molecular nanomagnets with switchable coupling for quantum simulation
Chiesa, Alessandro; Whitehead, George F. S.; Carretta, Stefano; Carthy, Laura; Timco, Grigore A.; Teat, Simon J.; Amoretti, Giuseppe; Pavarini, Eva; Winpenny, Richard E. P.; Santini, Paolo
2014-12-11
Molecular nanomagnets are attractive candidate qubits because of their wide inter- and intra-molecular tunability. Uniform magnetic pulses could be exploited to implement one- and two-qubit gates in presence of a properly engineered pattern of interactions, but the synthesis of suitable and potentially scalable supramolecular complexes has proven a very hard task. Indeed, no quantum algorithms have ever been implemented, not even a proof-of-principle two-qubit gate. In this paper we show that the magnetic couplings in two supramolecular {Cr7Ni}-Ni-{Cr7Ni} assemblies can be chemically engineered to fit the above requisites for conditional gates with no need of local control. Microscopic parameters are determined by a recently developed many-body ab-initio approach and used to simulate quantum gates. We find that these systems are optimal for proof-of-principle two-qubit experiments and can be exploited as building blocks of scalable architectures for quantum simulation.
Electric field geometries dominate quantum transport coupling in silicon nanoring
Lee, Tsung-Han E-mail: sfhu.hu@gmail.com; Hu, Shu-Fen E-mail: sfhu.hu@gmail.com
2014-03-28
Investigations on the relation between the geometries of silicon nanodevices and the quantum phenomenon they exhibit, such as the Aharonov–Bohm (AB) effect and the Coulomb blockade, were conducted. An arsenic doped silicon nanoring coupled with a nanowire by electron beam lithography was fabricated. At 1.47 K, Coulomb blockade oscillations were observed under modulation from the top gate voltage, and a periodic AB oscillation of ΔB = 0.178 T was estimated for a ring radius of 86 nm under a high sweeping magnetic field. Modulating the flat top gate and the pointed side gate was performed to cluster and separate the many electron quantum dots, which demonstrated that quantum confinement and interference effects coexisted in the doped silicon nanoring.
Quantum transport through a Coulomb blockaded quantum emitter coupled to a plasmonic dimer.
Goker, A; Aksu, H
2016-01-21
We study the electron transmission through a Coulomb blockaded quantum emitter coupled to metal nanoparticles possessing plasmon resonances by employing the time-dependent non-crossing approximation. We find that the coupling of the nanoparticle plasmons with the excitons results in a significant enhancement of the conductance through the discrete state with higher energy beyond the unitarity limit while the other discrete state with lower energy remains Coulomb blockaded. We show that boosting the plasmon-exciton coupling well below the Kondo temperature increases the enhancement adding another quantum of counductance upon saturation. Finite bias and increasing emitter resonance energy tend to reduce this enhancement. We attribute these observations to the opening of an additional transport channel via the plasmon-exciton coupling.
Collective Rabi dynamics of electromagnetically coupled quantum-dot ensembles
NASA Astrophysics Data System (ADS)
Glosser, Connor; Shanker, B.; Piermarocchi, Carlo
2017-09-01
Rabi oscillations typify the inherent nonlinearity of optical excitations in quantum dots. Using an integral kernel formulation to solve the three-dimensional Maxwell-Bloch equations in ensembles of up to 104 quantum dots, we observe features in Rabi oscillations due to the interplay of nonlinearity, nonequilibrium excitation, and electromagnetic coupling between the dots. This approach allows us to observe the dynamics of each dot in the ensemble without resorting to spatial averages. Our simulations predict synchronized multiplets of dots that exchange energy, dots that dynamically couple to screen the effect of incident external radiation, localization of the polarization due to randomness and interactions, as well as wavelength-scale regions of enhanced and suppressed polarization.
Pumped double quantum dot with spin-orbit coupling.
Khomitsky, Denis; Sherman, Eugene
2011-03-11
We study driven by an external electric field quantum orbital and spin dynamics of electron in a one-dimensional double quantum dot with spin-orbit coupling. Two types of external perturbation are considered: a periodic field at the Zeeman frequency and a single half-period pulse. Spin-orbit coupling leads to a nontrivial evolution in the spin and orbital channels and to a strongly spin- dependent probability density distribution. Both the interdot tunneling and the driven motion contribute into the spin evolution. These results can be important for the design of the spin manipulation schemes in semiconductor nanostructures.PACS numbers: 73.63.Kv,72.25.Dc,72.25.Pn.
Energy Exchange in Driven Open Quantum Systems at Strong Coupling
NASA Astrophysics Data System (ADS)
Carrega, Matteo; Solinas, Paolo; Sassetti, Maura; Weiss, Ulrich
2016-06-01
The time-dependent energy transfer in a driven quantum system strongly coupled to a heat bath is studied within an influence functional approach. Exact formal expressions for the statistics of energy dissipation into the different channels are derived. The general method is applied to the driven dissipative two-state system. It is shown that the energy flows obey a balance relation, and that, for strong coupling, the interaction may constitute the major dissipative channel. Results in analytic form are presented for the particular value K =1/2 of strong Ohmic dissipation. The energy flows show interesting behaviors including driving-induced coherences and quantum stochastic resonances. It is found that the general characteristics persists for K near 1/2 .
Analytic structure of eigenvalues of coupled quantum systems
NASA Astrophysics Data System (ADS)
Bender, Carl M.; Felski, Alexander; Hassanpour, Nima; Klevansky, S. P.; Beygi, Alireza
2017-01-01
By analytically continuing the coupling constant g of a coupled quantum theory, one can, at least in principle, arrive at a state whose energy is lower than the ground state of the theory. The idea is to begin with the uncoupled g = 0 theory in its ground state, to analytically continue around an exceptional point (square-root singularity) in the complex-coupling-constant plane, and finally to return to the point g = 0. In the course of this analytic continuation, the uncoupled theory ends up in an unconventional state whose energy is lower than the original ground-state energy. However, it is unclear whether one can use this analytic continuation to extract energy from the conventional vacuum state; this process appears to be exothermic but one must do work to vary the coupling constant g.
Electron-photon coupling in mesoscopic quantum electrodynamics
NASA Astrophysics Data System (ADS)
Cottet, A.; Kontos, T.; Douçot, B.
2015-05-01
Understanding the interaction between cavity photons and electronic nanocircuits is crucial for the development of mesoscopic quantum electrodynamics (QED). One has to combine ingredients from atomic cavity QED, such as orbital degrees of freedom, with tunneling physics and strong cavity field inhomogeneities, specific to superconducting circuit QED. It is therefore necessary to introduce a formalism which bridges between these two domains. We develop a general method based on a photonic pseudopotential to describe the electric coupling between electrons in a nanocircuit and cavity photons. In this picture, photons can induce simultaneously orbital energy shifts, tunneling, and local orbital transitions. We study in detail the elementary example of a single quantum dot with a single normal metal reservoir, coupled to a cavity. Photon-induced tunneling terms lead to a nonuniversal relation between the cavity frequency pull and the damping pull. Our formalism can also be applied to multiple quantum dot circuits, molecular circuits, quantum point contacts, metallic tunnel junctions, and superconducting nanostructures enclosing Andreev bound states or Majorana bound states, for instance.
Thermal gating of charge currents with Coulomb coupled quantum dots
NASA Astrophysics Data System (ADS)
Thierschmann, H.; Arnold, F.; Mittermüller, M.; Maier, L.; Heyn, C.; Hansen, W.; Buhmann, H.; Molenkamp, L. W.
2015-11-01
We have observed thermal gating, i.e. electrostatic gating induced by hot electrons. The effect occurs in a device consisting of two capacitively coupled quantum dots. The double dot system is coupled to a hot electron reservoir on one side (QD1), while the conductance of the second dot (QD2) is monitored. When a bias across QD2 is applied we observe a current which is strongly dependent on the temperature of the heat reservoir. This current can be either enhanced or suppressed, depending on the relative energetic alignment of the QD levels. Thus, the system can be used to control a charge current by hot electrons.
The XY model coupled to two-dimensional quantum gravity
NASA Astrophysics Data System (ADS)
Baillie, C. F.; Johnston, D. A.
1992-09-01
We perform Monte Carlo simulations using the Wolff cluster algorithm of the XY model on both fixed and dynamical phi-cubed graphs (i.e. without and with coupling to two-dimensional quantum gravity). We compare the numerical results with the theoretical expectation that the phase transition remains of KT type when the XY model is coupled to gravity. We also examine whether the universality we discovered in our earlier work on various Potts models with the same value of the central charge, c, carries over to the XY model, which has c=1.
A triangular triple quantum dot with tunable tunnel couplings
NASA Astrophysics Data System (ADS)
Noiri, A.; Kawasaki, K.; Otsuka, T.; Nakajima, T.; Yoneda, J.; Amaha, S.; Delbecq, M. R.; Takeda, K.; Allison, G.; Ludwig, A.; Wieck, A. D.; Tarucha, S.
2017-08-01
A two-dimensional arrangement of quantum dots (QDs) with finite inter-dot tunnel coupling provides a promising platform for studying complicated spin correlations as well as for constructing large-scale quantum computers. Here, we fabricate a tunnel-coupled triangular triple QD with a novel gate geometry in which three dots are defined by positively biasing the surface gates. At the same time, the small area in the center of the triangle is depleted by negatively biasing the top gate placed above the surface gates. The size of the small center depleted area is estimated from the Aharonov-Bohm oscillation measured for the triangular channel but incorporating no gate-defined dots, with a value consistent with the design. With this approach, we can bring the neighboring gate-defined dots close enough to one another to maintain a finite inter-dot tunnel coupling. We finally confirm the presence of the inter-dot tunnel couplings in the triple QD from the measurement of tunneling current through the dots in the stability diagram. We also show that the charge occupancy of each dot and that the inter-dot tunnel couplings are tunable with gate voltages.
Finite coupling effects in double quantum dots near equilibrium
NASA Astrophysics Data System (ADS)
Xu, Xiansong; Thingna, Juzar; Wang, Jian-Sheng
2017-01-01
A weak coupling quantum master equation provides reliable steady-state results only in the van Hove limit, i.e., when the system-lead coupling approaches zero. Recently, J. Thingna et al. [Phys. Rev. E 88, 052127 (2013), 10.1103/PhysRevE.88.052127] proposed an alternative approach, based on an analytic continuation of the Redfield solution, to evaluate the steady-state reduced density matrix up to second order in the system-bath coupling. The approach provides accurate results for harmonic oscillator and spin-bosonic systems. We apply this approach to study steady-state fermionic systems and the calculation on an exactly solvable double quantum dot system shows that the method is rigorously valid up to second order in system-lead coupling only near equilibrium, i.e., linear response regime. We further compare to the Redfield and the secular Redfield (Lindblad-type) master equations that are inaccurate in all parameter regimes. Lastly, we consider the nontrivial problem of strong Coulomb interaction and illustrate the interplay between system-lead coupling, interdot tunneling, and Coulomb strength that can be captured only via the analytic continuation method.
Cotunneling Drag Effect in Coulomb-Coupled Quantum Dots
NASA Astrophysics Data System (ADS)
Keller, A. J.; Lim, J. S.; Sánchez, David; López, Rosa; Amasha, S.; Katine, J. A.; Shtrikman, Hadas; Goldhaber-Gordon, D.
2016-08-01
In Coulomb drag, a current flowing in one conductor can induce a voltage across an adjacent conductor via the Coulomb interaction. The mechanisms yielding drag effects are not always understood, even though drag effects are sufficiently general to be seen in many low-dimensional systems. In this Letter, we observe Coulomb drag in a Coulomb-coupled double quantum dot and, through both experimental and theoretical arguments, identify cotunneling as essential to obtaining a correct qualitative understanding of the drag behavior.
Band structures of laterally coupled quantum dots, accounting for electromechanical effects
NASA Astrophysics Data System (ADS)
Prabhakar, Sanjay; Melnik, Roderick; Patil, Sunil
2010-09-01
In a series of recent papers we demonstrated that coupled electro-mechanical effects can lead to pronounced contributions in band structure calculations of low dimensional semiconductor nanostructures (LDSNs) such as quantum dots, wires, and even wells. Some such effects are essentially nonlinear. Both strain and piezoelectric effects have been used as tuning parameters for the optical response of LDSNs in photonics, band gap engineering and other applications. However, these effects have been largely neglected in literature while laterally coupled quantum dots (QDs) have been studied. The superposition of electron wave functions in these QDs become important in the design of optoelectronic devices as well in tayloring properties of QDs in other applications areas. At the same time, laterally grown QDs coupled with electric and mechanical fields are becoming increasingly important in many applications of LDSN-based systems, in particular where the tunneling of electron wave function through wetting layer (WL) becomes important and the distance between the dots is treated as a tuning parameter. Indeed, as electric and elastic effects are often significant in LDSNs, it is reasonable to expect that the separation between the QDs may also be used as a tuning parameter in the application of logic devices, for example, OR gates, AND gates and others. In this contribution, by using the fully coupled model of electroelasticity, we build on our previous results while analyzing the influence of these effects on optoelectronic properties of QDs. Results are reported for III-V type semiconductors with a major focus given to GaN/AlN based QD systems.
Density Matrix Embedding: A Strong-Coupling Quantum Embedding Theory.
Knizia, Gerald; Chan, Garnet Kin-Lic
2013-03-12
We extend our density matrix embedding theory (DMET) [Phys. Rev. Lett.2012, 109, 186404] from lattice models to the full chemical Hamiltonian. DMET allows the many-body embedding of arbitrary fragments of a quantum system, even when such fragments are open systems and strongly coupled to their environment (e.g., by covalent bonds). In DMET, empirical approaches to strong coupling, such as link atoms or boundary regions, are replaced by a small, rigorous quantum bath designed to reproduce the entanglement between a fragment and its environment. We describe the theory and demonstrate its feasibility in strongly correlated hydrogen ring and grid models; these are not only beyond the scope of traditional embeddings but even challenge conventional quantum chemistry methods themselves. We find that DMET correctly describes the notoriously difficult symmetric dissociation of a 4 × 3 hydrogen atom grid, even when the treated fragments are as small as single hydrogen atoms. We expect that DMET will open up new ways of treating complex strongly coupled, strongly correlated systems in terms of their individual fragments.
Quantum effects and nonlocality in strongly coupled plasmonic nanowire dimers.
Teperik, Tatiana V; Nordlander, Peter; Aizpurua, Javier; Borisov, Andrei G
2013-11-04
Using a fully quantum mechanical approach we study the optical response of a strongly coupled metallic nanowire dimer for variable separation widths of the junction between the nanowires. The translational invariance of the system allows to apply the time-dependent density functional theory (TDDFT) for nanowires of diameters up to 10 nm which is the largest size considered so far in quantum modeling of plasmonic dimers. By performing a detailed analysis of the optical extinction, induced charge densities, and near fields, we reveal the major nonlocal quantum effects determining the plasmonic modes and field enhancement in the system. These effects consist mainly of electron tunneling between the nanowires at small junction widths and dynamical screening. The TDDFT results are compared with results from classical electromagnetic calculations based on the local Drude and non-local hydrodynamic descriptions of the nanowire permittivity, as well as with results from a recently developed quantum corrected model. The latter provides a way to include quantum mechanical effects such as electron tunneling in standard classical electromagnetic simulations. We show that the TDDFT results can be thus retrieved semi-quantitatively within a classical framework. We also discuss the shortcomings of classical non-local hydrodynamic approaches. Finally, the implications of the actual position of the screening charge density at the gap interfaces are discussed in connection with plasmon ruler applications at subnanometric distances.
Slipko, Valeriy A.; Pershin, Yuriy V.
2011-10-15
In this paper we use a spin kinetic equation to study spin-polarization dynamics in one-dimensional (1D) wires and 2D channels. The spin kinetic equation is valid in both diffusive and ballistic spin transport regimes and therefore is more general than the usual spin drift-diffusion equations. In particular, we demonstrate that in infinite 1D wires with Rashba spin-orbit interaction the exponential spin-relaxation decay can be modulated by an oscillating function. In the case of spin relaxation in finite length 1D wires, it is shown that an initially homogeneous spin polarization spontaneously transforms into a persistent spin helix. We find that a propagating spin-polarization profile reflects from a system boundary and returns back to its initial position similarly to the reflectance of sound waves from an obstacle. The Green's function of the spin kinetic equation is derived for both finite and infinite 1D systems. Moreover, we demonstrate explicitly that the spin relaxation in specifically oriented 2D channels with Rashba and Dresselhaus spin-orbit interactions of equal strength occurs similarly to that in 1D wires of finite length. Finally, a simple transformation mapping 1D spin kinetic equation into the Klein-Gordon equation with an imaginary mass is found thus establishing an interesting connection between semiconductor spintronics and relativistic quantum mechanics.
Mixed quantum-classical versus full quantum dynamics: Coupled quasiparticle-oscillator system
NASA Astrophysics Data System (ADS)
Schanz, Holger; Esser, Bernd
1997-05-01
The relation between the dynamical properties of a coupled quasiparticle-oscillator system in the mixed quantum-classical and fully quantized descriptions is investigated. The system is considered as a model for applying a stepwise quantization. Features of the nonlinear dynamics in the mixed description such as the presence of a separatrix structure or regular and chaotic motion are shown to be reflected in the evolu- tion of the quantum state vector of the fully quantized system. In particular, it is demonstrated how wave packets propagate along the separatrix structure of the mixed description, and that chaotic dynamics leads to a strongly entangled quantum state vector. Special emphasis is given to viewing the system from a dyn- amical Born-Oppenheimer approximation defining integrable reference oscillators, and elucidating the role of the nonadiabatic couplings which complement this approximation into a rigorous quantization scheme.
NASA Astrophysics Data System (ADS)
Gehl, Michael; Zandbergen, Sander; Gibson, Ricky; Béchu, Muriel; Nader, Nima; Hendrickson, Joshua; Sears, Jasmine; Keiffer, Patrick; Wegener, Martin; Khitrova, Galina
2014-11-01
The coupling of radiation emitted on semiconductor inter-band transitions to resonant optical-antenna arrays allows for enhanced light-matter interaction via the Purcell effect. Semiconductor optical gain also potentially allows for loss reduction in metamaterials. Here we extend our previous work on optically pumped individual near-surface InGaAs quantum wells coupled to silver split-ring-resonator arrays to wire and square-antenna arrays. By comparing the transient pump-probe experimental results with the predictions of a simple model, we find that the effective coupling is strongest for the split rings, even though the split rings have the weakest dipole moment. The effect of the latter must thus be overcompensated by a smaller effective mode volume of the split rings. Furthermore, we also present a systematic variation of the pump-pulse energy, which was fixed in our previous experiments.
Why a magnetized quantum wire can act as an optical amplifier: a short survey
NASA Astrophysics Data System (ADS)
Kushwaha, Manvir S.
2014-10-01
This paper reviews the fundamental issues associated with the magnetoplasmon excitations investigated in a semiconducting quantum wire characterized by a harmonic confining potential and subjected to an applied (perpendicular) magnetic field. We embark on the charge-density excitations in a two-subband model within the framework of Bohm-Pines's random-phase approximation. The problem involves two length scales: {l}0 = √ {\\hbar /m*ω0} and {l}c = √ {\\hbar /m*ωc}, which characterize the strengths of the confinement and the magnetic field (B). Essentially, we focus on the device aspects of the intersubband collective (magnetoroton) excitation, which observes a negative group velocity between maxon and roton. Consequently, it leads to tachyon-like (superluminal) behavior without one's having to introduce the negative energies. Existence of the negative group velocity is a clear manifestation of a medium with population inversion brought about due to a metastable state caused by the magnetic field that satisfies the condition B > Bth; Bth being the threshold value below which the magnetoroton does not exist. The interest in negative group velocity is based on anomalous dispersion in a medium with inverted population, so that gain instead of absorption occurs at the frequencies of interest. A medium with an inverted population has the remarkable ability of amplifying a small optical signal of definite wavelength, i.e. it can serve as an active laser medium. An extensive scrutiny of the gain coefficient suggests an interesting and important application: The electronic device designed on the basis of such magnetoroton modes can act as an optical amplifier. Examining the magnetic field dependence of the life-time of magnetorotons leads us to infer that relatively smaller magnetic fields are optimal.
Electron spin rotations induced by oscillating Rashba interaction in a quantum wire
NASA Astrophysics Data System (ADS)
Pawłowski, J.; Szumniak, P.; Bednarek, S.
2016-01-01
A method and nanodevice are introduced that allows us to rotate the single electron spin confined in a gated electrostatic InSb nanowire quantum dot. The proposed method does not require the application of any (oscillating or static) external magnetic fields. Our proposal instead employs spatial and time modulation of confining potential induced by electric gates, which, in turn leads to oscillating Rashba-type spin-orbit coupling. Moving electron back and forth in such a variable Rashba field allows for the realization of spin rotations around two different axes separately without using an external magnetic field. The results are supported by realistic three-dimensional time-dependent Poisson-Schrödinger calculations for systems and material parameters corresponding to experimentally accessible structures.
Effects of phonon scattering on the magneto-conductance in single and double quantum wires
NASA Astrophysics Data System (ADS)
Huang, D.; Lyo, S. K.
2003-03-01
We present an exact numerical formalism for the solution of the Boltzmann equation dominated by elastic (e.g., interface-roughness) and phonon scattering in a quasi-one-dimensional system. The result is employed to study the temperature-dependent conductance of a single and tunnel-coupled double quantum wells (DQWs) as a function of a perpendicular magnetic field. According to recent studies, the zero-temperature conductance is enhanced dramatically as a function of the field when the Fermi level lies inside the anticrossing gap of the DQWs. [S. K. Lyo, J. Phys.-Condens. Matter 8, L703 (1996), D. Huang and S. K. Lyo, ibid, 12, 3383 (2000), S. V. Korepov and M. A. Liberman, Phys. Rev. B 60, 13770 (1999)] Our results show that phonon scattering modifies the conductance and its enhancement significantly at temperatures corresponding to the gap energy or the sublevel separation or higher.
NASA Astrophysics Data System (ADS)
Bandyopadhyay, S.; Pramanik, S.; Cahay, M.
2004-01-01
We derive the eigenenergies and spin-dependent eigenstates of electrons in a quantum wire subjected to an external magnetic field. These are calculated in the presence of spin-orbit interactions arising from the Rashba (structural inversion asymmetry) and Dresselhaus (bulk inversion asymmetry) effects. We consider three cases: the external magnetic field is oriented (i) along the axis of the wire, (ii) perpendicular to the axis but parallel to the electric field associated with structural inversion asymmetry (Rashba effect), and (iii) perpendicular to the axis as well as this electric field. In all cases, the dispersions of the eigenenergies are non-parabolic and the subbands do not have a fixed spin quantization axis (meaning that the spin polarization of the electron is wavevector dependent). Except in the second case, the dispersion diagrams are also, in general, asymmetric about the energy axis.
NASA Astrophysics Data System (ADS)
Barseghyan, M. G.; Duque, C. A.; Niculescu, E. C.; Radu, A.
2014-02-01
We study the laser field effects on the intersubband optical absorption and the refractive index changes in a GaAs/AlGaAs quantum wire with equilateral triangle cross section. The wire is under the action of a laser beam which is assumed to be non-resonant with the semiconductor structure and linearly polarized perpendicularly to the triangle side. In the effective mass approximation and for a finite potential barrier we calculate the subband states by using a finite element method. Linear, non linear and total absorption coefficients and refractive index changes are calculated as functions of the laser field for the allowed intersubband transitions. Two polarizations of the pump radiation, parallel and perpendicular to the laser field direction, are discussed.
NASA Astrophysics Data System (ADS)
Azzini, Stefano; Grassani, Davide; Galli, Matteo; Gerace, Dario; Patrini, Maddalena; Liscidini, Marco; Velha, Philippe; Bajoni, Daniele
2013-07-01
We report on four-wave mixing in coupled photonic crystal nano-cavities on a silicon-on-insulator platform. Three photonic wire cavities are side-coupled to obtain three modes equally separated in energy. The structure is designed to be self-filtering, and we show that the pump is rejected by almost two orders of magnitude. We study both the stimulated and the spontaneous four-wave mixing processes: owing to the small modal volume, we find that signal and idler photons are generated with a hundred-fold increase in efficiency as compared to silicon micro-ring resonators.
Thermopower and thermal conductance through parallel coupled quantum dots
NASA Astrophysics Data System (ADS)
Franco, R.; Silva-Valencia, J.; Figueira, M. S.
2008-04-01
We study the thermoelectric transport properties through two parallel coupled, gate-defined quantum dots (QDs), in the framework of the X-boson treatment for the impurity Anderson model. We compute the thermopower S, the thermal conductance κ, the electrical conductance G, and the product of the thermoelectric figure of merit and the temperature ZT, as function of the dot energy. We concentrate the calculations on ZT, that is, a measure of the usefulness of materials or devices as thermopower generators or cooling systems. If the coupling between the QDs is weak, ZT is greater than 1 when T ≃Δ (Δ is the mixing width between the QD and the leads) but when the system is coupled, the second dot can tune the temperature region where ZT >1. This result increases the possibilities of practical application of the system in mesoscopic cooling process.
NASA Technical Reports Server (NTRS)
Gunapala, Sarath D. (Inventor); Bandara, Sumith V. (Inventor); Liu, John K. (Inventor)
2006-01-01
Devices and techniques for coupling radiation to intraband quantum-well semiconductor sensors that are insensitive to the wavelength of the coupled radiation. At least one reflective surface is implemented in the quantum-well region to direct incident radiation towards the quantum-well layers.
Quantum Field Theories Coupled to Supergravity: AdS/CFT and Local Couplings
NASA Astrophysics Data System (ADS)
Große, Johannes
2007-11-01
This article is based on my PhD thesis and covers the following topics: Holographic meson spectra in a dilaton flow background, the mixed Coulomb-Higgs branch in terms of instantons on D7 branes, and a dual description of heavy-light mesons. Moreover, in a second part the conformal anomaly of four dimensional supersymmetric quantum field theories coupled to classical N=1 supergravity is explored in a superfield formulation. The complete basis for the anomaly and consistency conditions, which arise from cohomological considerations, are given. Possible implications for an extension of Zamolodchikov's c-theorem to four dimensional supersymmetric quantum field theories are discussed.
Evolution of quantum correlations in the open quantum systems consisting of two coupled oscillators
NASA Astrophysics Data System (ADS)
Abbasnezhad, Farkhondeh; Mehrabankar, Somayeh; Afshar, Davood; Jafarpour, Mojtaba
2017-04-01
The open quantum systems consisting of coupled and uncoupled asymmetric oscillators are considered with an initial quantum-dot trapped-ion coherent state. The quantum correlations between spatial modes of this trapped ion are examined to find their dependence on the temperature, asymmetric parameter, dissipation coefficient and the magnetic field. It is observed that the discord of the initial state is an increasing function of the asymmetric parameter and the magnetic field. Moreover, in the case of two uncoupled modes, entanglement and discord are decreasing functions of temperature and the dissipation coefficient. However, as the temperature and dissipation coefficient increase, the discord fades out faster. In the case of two coupled modes, as the temperature and dissipation coefficient increase, the sudden death of the entanglement and fade out of the discord happen sooner; moreover, as the magnetic field increases, the entanglement sudden death and the discord fade out time occur sooner. Also, with the increase in the asymmetric parameter, the entanglement sudden death is postponed. In addition, in the asymmetric system, appreciable discord can be created in the temperature range 0-10 K, while appreciable entanglement can be created in the temperature range 0-5 mK. Finally, it is observed that non-monotonic evolution of quantum correlations is due to coupling of modes.
Laterally Coupled Quantum-Dot Distributed-Feedback Lasers
NASA Technical Reports Server (NTRS)
Qui, Yueming; Gogna, Pawan; Muller, Richard; Maker, paul; Wilson, Daniel; Stintz, Andreas; Lester, Luke
2003-01-01
InAs quantum-dot lasers that feature distributed feedback and lateral evanescent- wave coupling have been demonstrated in operation at a wavelength of 1.3 m. These lasers are prototypes of optical-communication oscillators that are required to be capable of stable single-frequency, single-spatial-mode operation. A laser of this type (see figure) includes an active layer that comprises multiple stacks of InAs quantum dots embedded within InGaAs quantum wells. Distributed feedback is provided by gratings formed on both sides of a ridge by electron lithography and reactive-ion etching on the surfaces of an AlGaAs/GaAs waveguide. The lateral evanescent-wave coupling between the gratings and the wave propagating in the waveguide is strong enough to ensure operation at a single frequency, and the waveguide is thick enough to sustain a stable single spatial mode. In tests, the lasers were found to emit continuous-wave radiation at temperatures up to about 90 C. Side modes were found to be suppressed by more than 30 dB.
Coupling single quantum dots to plasmonic nanocones: optical properties.
Meixner, Alfred J; Jäger, Regina; Jäger, Sebastian; Bräuer, Annika; Scherzinger, Kerstin; Fulmes, Julia; Krockhaus, Sven zur Oven; Gollmer, Dominik A; Kern, Dieter P; Fleischer, Monika
2015-01-01
Coupling a single quantum emitter, such as a fluorescent molecule or a quantum dot (QD), to a plasmonic nanostructure is an important issue in nano-optics and nano-spectroscopy, relevant for a wide range of applications, including tip-enhanced near-field optical microscopy, plasmon enhanced molecular sensing and spectroscopy, and nanophotonic amplifiers or nanolasers, to mention only a few. While the field enhancement of a sharp nanoantenna increasing the excitation rate of a very closely positioned single molecule or QD has been well investigated, the detailed physical mechanisms involved in the emission of a photon from such a system are, by far, less investigated. In one of our ongoing research projects, we try to address these issues by constructing and spectroscopically analysing geometrically simple hybrid heterostructures consisting of sharp gold cones with single quantum dots attached to the very tip apex. An important goal of this work is to tune the longitudinal plasmon resonance by adjusting the cones' geometry to the emission maximum of the core-shell CdSe/ZnS QDs at nominally 650 nm. Luminescence spectra of the bare cones, pure QDs and hybrid systems were distinguished successfully. In the next steps we will further investigate, experimentally and theoretically, the optical properties of the coupled systems in more detail, such as the fluorescence spectra, blinking statistics, and the current results on the fluorescence lifetimes, and compare them with uncoupled QDs to obtain a clearer picture of the radiative and non-radiative processes.
Quantum Dot Cellular Automata: Computing with Coupled Quantum-Dot Molecules
NASA Astrophysics Data System (ADS)
Porod, Wolfgang
1998-05-01
We have recently proposed a scheme of using coupled quantum dots to realize digital computing elements.(C. S. Lent, P. D. Tougaw, W. Porod, and G. H. Bernstein, Nanotechnology 4, 49 (1993); C. S. Lent, P. D. Tougaw, and W. Porod, Applied Physics Letters 62, 714 (1993).) Our scheme was inspired by recent work on nanometer-scale lithography in semiconductors which has permitted the construction of quantum dots which may be viewed as artificial atoms; furthermore, the principle of dot-dot coupling has also been demonstrated, thus realizing artificial semiconductor molecules. This talk will review the work of the Notre Dame group on the theory and modeling of cellular arrays of coupled quantum-dot molecules, which we refer to as quantum-dot cellular automata (QCA). We consider inhomogeneous arrays of quantum-dot molecules, where each molecule forms the basic unit in a cellular automaton-type array architecture. These cells (molecules) consists of four or five quantum dots in close enough proximity to enable electron tunneling between dots. Coulomb repulsion between electrons in the cell results in a bistable ground state whose configuration is determined by the configuration of neighboring cells. The electrons tend to occupy antipodal sites in one of two ground-state configurations which may be used to encode binary information. We have demonstrated that Boolean logic gates can be constructed, and simple design rules permit the fabrication of any logic function. The basic principle of QCA operation was demonstrated in recent experiments.(A. O. Orlov, I. Amlani, G. H. Bernstein, C. S. Lent, and G. L. Snider, Science 277, 928, (1997).)
Exciton dynamics in a site-controlled quantum dot coupled to a photonic crystal cavity
Jarlov, C. Lyasota, A.; Ferrier, L.; Gallo, P.; Dwir, B.; Rudra, A.; Kapon, E.
2015-11-09
Exciton and cavity mode (CM) dynamics in site-controlled pyramidal quantum dots (QDs), integrated with linear photonic crystal membrane cavities, are investigated for a range of temperatures and photo-excitation power levels. The absence of spurious multi-excitonic effects, normally observed in similar structures based on self-assembled QDs, permits the observation of effects intrinsic to two-level systems embedded in a solid state matrix and interacting with optical cavity modes. The coupled exciton and CM dynamics follow the same trend, indicating that the CM is fed only by the exciton transition. The Purcell reduction of the QD and CM decay times is reproduced well by a theoretical model that includes exciton linewidth broadening and temperature dependent non-radiative processes, from which we extract a Purcell factor of 17 ± 5. For excitation powers above QD saturation, we show the influence of quantum wire barrier states at short delay time, and demonstrate the absence of multiexcitonic background emission.
Exciton dynamics in a site-controlled quantum dot coupled to a photonic crystal cavity
NASA Astrophysics Data System (ADS)
Jarlov, C.; Lyasota, A.; Ferrier, L.; Gallo, P.; Dwir, B.; Rudra, A.; Kapon, E.
2015-11-01
Exciton and cavity mode (CM) dynamics in site-controlled pyramidal quantum dots (QDs), integrated with linear photonic crystal membrane cavities, are investigated for a range of temperatures and photo-excitation power levels. The absence of spurious multi-excitonic effects, normally observed in similar structures based on self-assembled QDs, permits the observation of effects intrinsic to two-level systems embedded in a solid state matrix and interacting with optical cavity modes. The coupled exciton and CM dynamics follow the same trend, indicating that the CM is fed only by the exciton transition. The Purcell reduction of the QD and CM decay times is reproduced well by a theoretical model that includes exciton linewidth broadening and temperature dependent non-radiative processes, from which we extract a Purcell factor of 17 ± 5. For excitation powers above QD saturation, we show the influence of quantum wire barrier states at short delay time, and demonstrate the absence of multiexcitonic background emission.
NASA Astrophysics Data System (ADS)
Skone, Jonathan H.
Development of quantum mechanical methods for the calculation of proton tunneling splittings and proton-coupled electron transfer vibronic couplings is presented in this thesis. The fundamental physical principles underlying proton transfer in the electronically adiabatic and nonadiabatic limits are illustrated by applying the quantum mechanical methods we developed to chemical systems exemplary of the electronically adiabatic and nonadiabatic proton-tunneling regimes. Overall, this thesis emphasizes the need for quantum chemical methods that avoid the adiabatic separation of the quantum proton and electron, are computationally tractable, and treat all quantum particles three-dimensionally. The nuclear-electronic orbital nonorthogonal configuration interaction (NEO-NOCI) approach is presented for calculating proton tunneling splittings and vibronic couplings. The NEO approach is a molecular orbital based method that avoids the Born-Oppenheimer separation of the select protons and electrons, thereby making methods developed within this scheme, such as NEO-NOCI, applicable to electronically nonadiabatic proton transfer. In the two-state NEO-NOCI approach, the ground and excited state delocalized nuclear-electronic wavefunctions are expressed as linear combinations of two nonorthogonal localized nuclear-electronic wavefunctions obtained at the NEO-Hartree-Fock level. The advantages of the NEO-NOCI approach are the removal of the adiabatic separation between the electrons and the quantum nuclei, the computational efficiency, the potential for systematic improvement by enhancing the basis sets and number of configurations, and the applicability to a broad range of chemical systems. The tunneling splitting is determined by the energy difference between these two delocalized vibronic states. The proton tunneling splittings calculated with the NEO-NOCI approach for the [He-H-He]+ model system with a range of fixed He-He distances are shown to be in excellent agreement with
NASA Astrophysics Data System (ADS)
Benali, A.; Traversa, F. L.; Albareda, G.; Aghoutane, M.; Oriols, X.
2013-04-01
Progress in high-frequency transistors is based on reducing electron transit time, either by scaling their lengths or by introducing materials with higher electron mobility. For gate-all-around quantum-wire transistors with lateral dimensions similar or smaller than their length, a careful analysis of the displacement current reveals that a time shorter than the transit time controls their high-frequency performance. Monte Carlo simulations of such transistors with a self-consistent solution of the 3D Poisson equation clearly show an improvement of the intrinsic cut-off frequency when their lateral areas are reduced, without length scaling.
Quantum phase transition of electron-hole liquid in coupled quantum wells
NASA Astrophysics Data System (ADS)
Babichenko, V. S.; Polishchuk, I. Ya.
2016-10-01
Many-component electron-hole plasma is considered in the coupled quantum wells. The electrons are assumed to be localized in one quantum well (QW) while the holes are localized in the other QW. It is found that the homogeneous charge distribution within the QWs is unstable if the carrier density is sufficiently small. The instability results in the breakdown of the homogeneous charge distribution into two coexisting phases—a low-density phase and a high-density phase, which is electron-hole liquid. In turn, the homogeneous state of the electron-hole liquid is stable if the distance between the quantum wells ℓ is sufficiently small. However, as the distance ℓ increases and reaches a certain critical value ℓcr, the plasmon spectrum of the electron-hole liquid becomes unstable. Hereupon, a quantum phase transition occurs, resulting in the appearance of charge-density waves of finite amplitude in both quantum wells. Strong mass renormalization and the strong Z -factor renormalization are found for the electron-hole liquid as the quantum phase transition occurs.
Quantum-limited amplification and entanglement in coupled nonlinear resonators.
Eichler, C; Salathe, Y; Mlynek, J; Schmidt, S; Wallraff, A
2014-09-12
We demonstrate a coupled cavity realization of a Bose-Hubbard dimer to achieve quantum-limited amplification and to generate frequency entangled microwave fields with squeezing parameters well below -12 dB. In contrast to previous implementations of parametric amplifiers, our dimer can be operated both as a degenerate and as a nondegenerate amplifier. The large measured gain-bandwidth product of more than 250 MHz for the nondegenerate operation and the saturation at input photon numbers as high as 2000 per μs are both expected to be improvable even further, while maintaining wide frequency tunability of about 2 GHz. Featuring flexible control over all relevant system parameters, the presented Bose-Hubbard dimer based on lumped element circuits has significant potential as an elementary cell in nonlinear cavity arrays for quantum simulations.
Suppression of arbitrary internal coupling in a quantum register
NASA Astrophysics Data System (ADS)
Stollsteimer, Marcus; Mahler, Günter
2001-11-01
For the implementation of a quantum computer it is necessary to exercise complete control over the Hamiltonian of the used physical system. For nuclear magnetic resonance quantum computing the effectively acting Hamiltonian can be manipulated via pulse sequences. Here we examine a register consisting of N selectively addressable spins with pairwise coupling between each spin pair. We show that complete decoupling of the spins is possible, independent of the particular form of the spin-spin interaction. The proposed method based on orthogonal arrays is efficient in the sense that the effort regarding time and amount of pulses increases only polynomially with the size N of the register. However, the effect of external control errors in terms of inaccurate control pulses eventually limits the achievable precision.
Quantum dynamics of impurities coupled to a Fermi sea
NASA Astrophysics Data System (ADS)
Parish, Meera M.; Levinsen, Jesper
2016-11-01
We consider the dynamics of an impurity atom immersed in an ideal Fermi gas at zero temperature. We focus on the coherent quantum evolution of the impurity following a quench to strong impurity-fermion interactions, where the interactions are assumed to be short range like in cold-atom experiments. To approximately model the many-body time evolution, we use a truncated basis method, where at most two particle-hole excitations of the Fermi sea are included. When the system is initially noninteracting, we show that our method exactly captures the short-time dynamics following the quench, and we find that the overlap between initial and final states displays a universal nonanalytic dependence on time in this limit. We further demonstrate how our method can be used to compute the impurity spectral function, as well as describe many-body phenomena involving coupled impurity spin states, such as Rabi oscillations in a medium or highly engineered quantum quenches.
Quantum simulations of strongly coupled quark-gluon plasma
NASA Astrophysics Data System (ADS)
Filinov, V. S.; Ivanov, Yu. B.; Bonitz, M.; Levashov, P. R.; Fortov, V. E.
2011-12-01
A strongly coupled quark-gluon plasma (QGP) of heavy constituent quasiparticles is studied by a path-integral Monte-Carlo method, which improves the corresponding classical simulations by extending them to the quantum regime. It is shown that this method is able to reproduce the lattice equation of state and also yields valuable insight into the internal structure of the QGP. The results indicate that the QGP reveals liquid-like rather than gas-like properties. At temperatures just above the critical one it was found that bound quark-antiquark states still survive. These states are bound by effective stringlike forces. Quantum effects turned out to be of prime importance in these simulations.
Quantum simulations of strongly coupled quark-gluon plasma
NASA Astrophysics Data System (ADS)
Filinov, V. S.; Ivanov, Yu. B.; Bonitz, M.; Levashov, P. R.; Fortov, V. E.
2011-09-01
A strongly coupled quark-gluon plasma (QGP) of heavy constituent quasiparticles is studied by a path-integral Monte-Carlo method. This approach is a quantum generalization of the model developed by B.A. Gelman, E.V. Shuryak, and I. Zahed. It is shown that this method is able to reproduce the QCD lattice equation of state and also yields valuable insight into the internal structure of the QGP. The results indicate that the QGP reveals liquid-like rather than gas-like properties. At temperatures just above the critical one it was found that bound quark-antiquark states still survive. These states are bound by effective string-like forces and turn out to be colorless. At the temperature as large as twice the critical one no bound states are observed. Quantum effects turned out to be of prime importance in these simulations.
Exchange interactions in coupled quantum dots observed through polarized photoluminescence
NASA Astrophysics Data System (ADS)
Wijesundara, Kushal C.; Garrido, Mauricio; Ramanathan, Swati; Stinaff, Eric; Bracker, Allan; Gammon, Dan
2009-03-01
Identification and manipulation of the exchange interaction between different spin configurations may be useful for implementing quantum logic operations. Coupled quantum dots offer the possibility of controlling the exchange interaction by continuously tuning between direct and indirect excitonic configurations. The effect of the anisotropic part of the exchange energy was clearly identified from polarization dependent photoluminescence (PL) results arising from the direct and indirect configurations of the neutral exciton. We also observe direct experimental evidence of the isotropic exchange interaction via PL measurements from positive trion configurations. The isotropic exchange interactions observed to be an order of magnitude larger than the anisotropic splitting as expected. High resolution measurements of this charged exciton configurations are expected to reveal additional insight into the details of the exchange interaction.
Spin-Orbit Coupling in Quantum Dot Cell Arrays
NASA Astrophysics Data System (ADS)
Fernando, Rojas; Francisco, Mireles; Ernesto, Cota; Ulloa, Sergio E.
2002-03-01
We investigate the role of spin-orbit interaction on the energy spectra, charge and spin configurations of a planar semiconductor quantum dot cell array in a square geometry. The quantum dot array is assumed to be formed by electrical gate confinement, for instance, of a two dimensional electron gas on a semiconductor heterojunction. Hence, it is expected that while tunneling between neighboring dots, the hopping electron will couple its spin with its orbital degree of freedom, due to the interfacial electric fields defining the structure. The spin-orbit (Rashba-like) coupling effect is modelled in a tight-binding formalism with nearest-neighbor spin-dependent interactions. An extended Hubbard Hamiltonian is used to describe the electrons in each cell (with an excess of two electrons per cell), taking into account intra- and inter-cell Coulomb interactions, as well as intra-cell tunneling. We present results for the energy spectrum as a function of the relevant parameters in the cell: tunneling amplitude, spin-orbit coupling t_SO, and dot size imperfections. We find that a number of spin-degeneracies are broken with increasing t_SO. The charge- and spin-polarizations in the cell in the presence of a driver, as well as the interplay between cells will be discussed for different system parameters.
Charging of a quantum dot coupled to Luttinger-liquid leads
NASA Astrophysics Data System (ADS)
Wächter, P.; Meden, V.; Schönhammer, K.
2007-09-01
Luttinger-liquid behavior of one-dimensional correlated electron systems is characterized by power-law scaling of a variety of physical observables with exponents determined by a single interaction dependent parameter K . We suggest a setup to study Luttinger-liquid behavior in quantum wires which allows us to determine K from two independent measurements: transport through a quantum dot embedded in the wire and the charge on the dot. Consistency of the two K ’s for a single probe would provide strong experimental evidence for the Luttinger-liquid paradigm.
Radiative coupling of quantum dots in photonic crystal structures
NASA Astrophysics Data System (ADS)
Minkov, Momchil; Savona, Vincenzo
2013-03-01
We derive a general formalism to model the polariton states resulting from the radiation-matter interaction between an arbitrary number of excitonic transitions in semiconductor quantum dots and photon modes in a photonic crystal structure in which the quantum dots are embedded. The Maxwell equations, including the linear nonlocal susceptibility of the exciton transitions in the quantum dots, are cast into an eigenvalue problem, which can be applied to any structure whose photon modes can be computed with reliable accuracy, and in addition naturally allows for disorder effects to be taken into account. We compute realistic photon modes using Bloch-mode expansion. As example systems, we study typical InGaAs quantum dots in a GaAs photonic crystal structures—an Ln cavity or a W1 waveguide. For a single dot, we reproduce known analytical results, while for the two-dot case we study the radiative excitation transfer mechanism and characterize its strength, the dependence on the detuning between quantum dot and photon modes, and the dependence on interdot distance. We find in particular that the interdot radiative coupling strength can reach 100μeV in a short cavity, and its decay with distance in longer cavities and waveguides is determined by the group velocity of the exchanged photons and their radiative lifetime. We also show that, for an Ln cavity of increasing length, the radiative excitation transfer mechanism is subject to a crossover from a regime where a single photon mode is dominating, to a multimode regime—occurring around n = 150 for the system under study.
NASA Astrophysics Data System (ADS)
Raza, Syed; Sirota, Alexander; Teo, Jeffrey
Weyl and Dirac semimetals in three dimensions have semi-robust massless electronic structures. We mimic these gapless systems using an array of coupled Dirac wires, and analytically study the gapping effect of many-body interactions. The Dirac wires are arranged in a way so that the charge conserving model exhibits an antiferromagnetic time reversal symmetry as well as a p2mg wallpaper group symmetry, which contains twofold rotations, reflections and glide planes. The gapless electrons can aquire a mass upon symmetry breaking dimerizations, or more interestingly, symmetry preserving many-body interactions. This involves the introduction of a topological insulating texture in the bulk supported by layers of gapped symmetric interacting surfaces of topological insulators. The resulting massive system is a three dimensional geometric topological state.
NASA Astrophysics Data System (ADS)
Masood, W.; Mirza, Arshad M.; Nargis, Shahida
2010-08-01
The coupled Shukla-Varma (SV) and convective cell mode is revisited in classical and quantum dusty magnetoplasmas. It is shown that the inclusion of electron thermal effects modifies the original coupled SV and convective cell mode. It is also discussed how the quantum effects can be incorporated in the coupled SV and convective cell mode.
Heat transport along a chain of coupled quantum harmonic oscillators
NASA Astrophysics Data System (ADS)
de Oliveira, Mário J.
2017-04-01
I study the heat transport properties of a chain of coupled quantum harmonic oscillators in contact at its ends with two heat reservoirs at distinct temperatures. My approach is based on the use of an evolution equation for the density operator which is a canonical quantization of the classical Fokker-Planck-Kramers equation. I set up the evolution equation for the covariances and obtain the stationary covariances at the stationary states from which I determine the thermal conductance in closed form when the interparticle interaction is small. The conductance is finite in the thermodynamic limit implying an infinite thermal conductivity.
Moskal, S.; Bednarek, S.; Adamowski, J.
2007-09-15
A two-electron system confined in two coupled semiconductor quantum dots is investigated as a candidate for performing quantum logic operations with spin qubits. We study different processes of swapping the electron spins by a controlled switching on and off of the exchange interaction. The resulting spin swap corresponds to an elementary operation in quantum-information processing. We perform direct simulations of the time evolution of the two-electron system. Our results show that, in order to obtain the full interchange of spins, the exchange interaction should change smoothly in time. The presence of jumps and spikes in the time characteristics of the confinement potential leads to a considerable increase of the spin-swap time. We propose several mechanisms to modify the exchange interaction by changing the confinement potential profile and discuss their advantages and disadvantages.
Wang, Fudong; Tang, Rui; Kao, Jeff L-F; Dingman, Sean D; Buhro, William E
2009-04-08
Tri-n-octylphosphine oxide (TOPO) is the most commonly used solvent for the synthesis of colloidal nanocrystals. Here we show that the use of different batches of commercially obtained TOPO solvent introduces significant variability into the outcomes of CdSe quantum-wire syntheses. This irreproducibility is attributed to varying amounts of phosphorus-containing impurities in the different TOPO batches. We employ (31)P NMR to identify 10 of the common TOPO impurities. Their beneficial, harmful, or negligible effects on quantum-wire growth are determined. The impurity di-n-octylphosphinic acid (DOPA) is found to be the important beneficial TOPO impurity for the reproducible growth of high-quality CdSe quantum wires. DOPA is shown to beneficially modify precursor reactivity through ligand substitution. The other significant TOPO impurities are ranked according to their abilities to similarly influence precursor reactivity. The results are likely of general relevance to most nanocrystal syntheses conducted in TOPO.
Barnes, George L.; Kellman, Michael E.
2013-12-07
Simulations are performed of a small quantum system interacting with a quantum environment. The system consists of various initial states of two harmonic oscillators coupled to give normal modes. The environment is “designed” by its level pattern to have a thermodynamic temperature. A random coupling causes the system and environment to become entangled in the course of time evolution. The approach to a Boltzmann distribution is observed, and effective fitted temperatures close to the designed temperature are obtained. All initial pure states of the system are driven to equilibrium at very similar rates, with quick loss of memory of the initial state. The time evolution of the von Neumann entropy is calculated as a measure of equilibration and of quantum coherence. It is pointed out using spatial density distribution plots that quantum interference is eliminated only with maximal entropy, which corresponds thermally to infinite temperature. Implications of our results for the notion of “classicalizing” behavior in the approach to thermal equilibrium are briefly considered.
Barnes, George L; Kellman, Michael E
2013-12-07
Simulations are performed of a small quantum system interacting with a quantum environment. The system consists of various initial states of two harmonic oscillators coupled to give normal modes. The environment is "designed" by its level pattern to have a thermodynamic temperature. A random coupling causes the system and environment to become entangled in the course of time evolution. The approach to a Boltzmann distribution is observed, and effective fitted temperatures close to the designed temperature are obtained. All initial pure states of the system are driven to equilibrium at very similar rates, with quick loss of memory of the initial state. The time evolution of the von Neumann entropy is calculated as a measure of equilibration and of quantum coherence. It is pointed out using spatial density distribution plots that quantum interference is eliminated only with maximal entropy, which corresponds thermally to infinite temperature. Implications of our results for the notion of "classicalizing" behavior in the approach to thermal equilibrium are briefly considered.
NASA Astrophysics Data System (ADS)
Barnes, George L.; Kellman, Michael E.
2013-12-01
Simulations are performed of a small quantum system interacting with a quantum environment. The system consists of various initial states of two harmonic oscillators coupled to give normal modes. The environment is "designed" by its level pattern to have a thermodynamic temperature. A random coupling causes the system and environment to become entangled in the course of time evolution. The approach to a Boltzmann distribution is observed, and effective fitted temperatures close to the designed temperature are obtained. All initial pure states of the system are driven to equilibrium at very similar rates, with quick loss of memory of the initial state. The time evolution of the von Neumann entropy is calculated as a measure of equilibration and of quantum coherence. It is pointed out using spatial density distribution plots that quantum interference is eliminated only with maximal entropy, which corresponds thermally to infinite temperature. Implications of our results for the notion of "classicalizing" behavior in the approach to thermal equilibrium are briefly considered.
Coupled ridge waveguide distributed feedback quantum cascade laser arrays
Liu, Ying-Hui; Zhang, Jin-Chuan Yan, Fang-Liang; Liu, Feng-Qi Zhuo, Ning; Wang, Li-Jun; Liu, Jun-Qi; Wang, Zhan-Guo
2015-04-06
A coupled ridge waveguide quantum cascade laser (QCL) array consisting of fifteen elements with parallel integration was presented. In-phase fundamental mode operation in each element is secured by both the index-guided nature of the ridge and delicate loss management by properly designed geometries of the ridges and interspaces. Single-lobe lateral far-field with a nearly diffraction limited beam pattern was obtained. By incorporating a one-dimensional buried distributed feedback grating, the in-phase-operating coupled ridge waveguide QCL design provides an efficient solution to obtaining high output power and stable single longitudinal mode emission. The simplicity of this structure and fabrication process makes this approach attractive to many practical applications.
Quantum entanglement in three accelerating qubits coupled to scalar fields
NASA Astrophysics Data System (ADS)
Dai, Yue; Shen, Zhejun; Shi, Yu
2016-07-01
We consider quantum entanglement of three accelerating qubits, each of which is locally coupled with a real scalar field, without causal influence among the qubits or among the fields. The initial states are assumed to be the GHZ and W states, which are the two representative three-partite entangled states. For each initial state, we study how various kinds of entanglement depend on the accelerations of the three qubits. All kinds of entanglement eventually suddenly die if at least two of three qubits have large enough accelerations. This result implies the eventual sudden death of all kinds of entanglement among three particles coupled with scalar fields when they are sufficiently close to the horizon of a black hole.
A plasmonic dipole optical antenna coupled quantum dot infrared photodetector
NASA Astrophysics Data System (ADS)
Mojaverian, Neda; Gu, Guiru; Lu, Xuejun
2015-12-01
In this paper, we report a full-wavelength plasmonic dipole optical antenna coupled quantum dot infrared photodetector (QDIP). The plasmonic dipole optical antenna can effectively modify the EM wave distribution and convert free-space propagation infrared light to localized surface plasmonic resonance (SPR) within the nanometer (nm) gap region of the full-wavelength dipole antenna. The plasmonic dipole optical antenna coupled QDIP shows incident-angle-dependent photocurrent enhancement. The angular dependence follows the far-field pattern of a full-wavelength dipole antenna. The directivity of the plasmonic dipole optical antenna is measured to be 1.8 dB, which agrees well with the antenna simulation. To our best knowledge, this is the first report of the antenna far-field and directivity measurement. The agreement of the detection pattern and the directivity with antenna theory confirms functions of an optical antenna are similar to that of a RF antenna.
Momentum coupling in non-Markovian quantum Brownian motion
NASA Astrophysics Data System (ADS)
Ferialdi, Luca; Smirne, Andrea
2017-07-01
We consider a model of non-Markovian quantum Brownian motion that consists of a harmonic oscillator bilinearly coupled to a thermal bath, both via its position and via its momentum operators. We derive the master equation for such a model, and we solve the equations of motion for a generic Gaussian system state. We then investigate the resulting evolution of the first and second moments for both an Ohmic and a super-Ohmic spectral density. In particular, we show that, irrespective of the specific form of the spectral density, the coupling with the momentum enhances the dissipation experienced by the system, accelerating its relaxation to the equilibrium as well as modifying the asymptotic state of the dynamics. Eventually, we characterize explicitly the non-Markovianity of the evolution using a general criterion which relies on the positivity of the master equation coefficients.
NASA Astrophysics Data System (ADS)
Smith, L. W.; Al-Taie, H.; Sfigakis, F.; See, P.; Lesage, A. A. J.; Xu, B.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
2014-07-01
The properties of conductance in one-dimensional (1D) quantum wires are statistically investigated using an array of 256 lithographically identical split gates, fabricated on a GaAs/AlGaAs heterostructure. All the split gates are measured during a single cooldown under the same conditions. Electron many-body effects give rise to an anomalous feature in the conductance of a one-dimensional quantum wire, known as the "0.7 structure" (or "0.7 anomaly"). To handle the large data set, a method of automatically estimating the conductance value of the 0.7 structure is developed. Large differences are observed in the strength and value of the 0.7 structure [from 0.63 to 0.84×(2e2/h)], despite the constant temperature and identical device design. Variations in the 1D potential profile are quantified by estimating the curvature of the barrier in the direction of electron transport, following a saddle-point model. The 0.7 structure appears to be highly sensitive to the specific confining potential within individual devices.
Quantum emitters coupled to circular nanoantennas for high-brightness quantum light sources
NASA Astrophysics Data System (ADS)
Abudayyeh, Hamza A.; Rapaport, Ronen
2017-09-01
Engineering the directionality and emission rate of quantum light sources is essential in the development of modern quantum applications. In this work we use numerical calculations to optimise the brightness of a broadband quantum emitter positioned in a hybrid metal-dielectric circular periodic nanoantenna. The optimised structure features a photon collection efficiency of 74 % (82 % ) and a photon flux enhancement of over 10 (6) into a numerical aperture of 0.22 (0.50), respectively, corresponding to a direct coupling into two types of multi-mode fibres. To enhance the emission rate, we present a new circular nanoantenna design where a quantum emitter is attached to a silver nanocone at the centre of the antenna. After optimisation, we find a collection efficiency of 61 % (78 % ) into a numerical aperture of 0.22 (0.50), giving a brightness enhancement of 1000 (600) for an unpolarised emitter. The enhancements in both structures are broadband due to the low-quality factor of the device and are therefore ideal for room-temperature sources. This type of a scalable design can be utilised towards on-chip, high-brightness quantum light sources operating at room temperature.
Calculation of exchange interaction for modified Gaussian coupled quantum dots
NASA Astrophysics Data System (ADS)
Khordad, R.
2017-08-01
A system of two laterally coupled quantum dots with modified Gaussian potential has been considered. Each quantum dot has an electron under electric and magnetic field. The quantum dots have been considered as hydrogen-like atoms. The physical picture has translated into the Heisenberg spin Hamiltonian. The Schrödinger equation using finite element method has been numerically solved. The exchange energy factor has been calculated as a functions of electric field, magnetic field, and the separation distance between the centers of the dots ( d). According to the results, it is found that there is the transition from anti-ferromagnetic to ferromagnetic for constant electric field. Also, the transition occurs from ferromagnetic to anti-ferromagnetic for constant magnetic field (B>1 T). With decreasing the distance between the centers of the dots and increasing magnetic field, the transition occurs from anti-ferromagnetic to ferromagnetic. It is found that a switching of exchange energy factor is presented without canceling the interactions of the electric and magnetic fields on the system.
Calculation of exchange interaction for modified Gaussian coupled quantum dots
NASA Astrophysics Data System (ADS)
Khordad, R.
2017-03-01
A system of two laterally coupled quantum dots with modified Gaussian potential has been considered. Each quantum dot has an electron under electric and magnetic field. The quantum dots have been considered as hydrogen-like atoms. The physical picture has translated into the Heisenberg spin Hamiltonian. The Schrödinger equation using finite element method has been numerically solved. The exchange energy factor has been calculated as a functions of electric field, magnetic field, and the separation distance between the centers of the dots (d). According to the results, it is found that there is the transition from anti-ferromagnetic to ferromagnetic for constant electric field. Also, the transition occurs from ferromagnetic to anti-ferromagnetic for constant magnetic field (B>1 T). With decreasing the distance between the centers of the dots and increasing magnetic field, the transition occurs from anti-ferromagnetic to ferromagnetic. It is found that a switching of exchange energy factor is presented without canceling the interactions of the electric and magnetic fields on the system.
Quantum plasmon resonances and coupling of small nanoparticles
NASA Astrophysics Data System (ADS)
Mario, Zapata-Herrera; Jefferson, Florez; Angela, Camacho
2013-03-01
In this work, we propose to extend a theoretical quantum approach to describe the behavior of the optical response as a function of both size and shape of small metal nanoparticles. By using classical models as well as quantum approaches we also want to study the nanoparticle's permittivity in the whole range of nanometers in order to define the different regimes at the nanoscale. In particular, we are interested in examining size and shape effects on the enhancement field factor and the absorption spectra for comparing with possible experiments. We study the role played by Localized Surface Plasmon Resonance in the coupling of small metal nanoparticles pairs by varying the distance between them by using an analogy between molecular electronic states and plasmonic excitations as a function of particle size and shape. We pay special atention on tunnelling and multipolar effects in order to predict the regime of dimer formation. The main interest in understanding the plasmon resonances of small nanoparticles lies in the applications in biology, catalysis and quantum optics.
Molecular nanomagnets with switchable coupling for quantum simulation
Chiesa, Alessandro; Whitehead, George F. S.; Carretta, Stefano; ...
2014-12-11
Molecular nanomagnets are attractive candidate qubits because of their wide inter- and intra-molecular tunability. Uniform magnetic pulses could be exploited to implement one- and two-qubit gates in presence of a properly engineered pattern of interactions, but the synthesis of suitable and potentially scalable supramolecular complexes has proven a very hard task. Indeed, no quantum algorithms have ever been implemented, not even a proof-of-principle two-qubit gate. In this paper we show that the magnetic couplings in two supramolecular {Cr7Ni}-Ni-{Cr7Ni} assemblies can be chemically engineered to fit the above requisites for conditional gates with no need of local control. Microscopic parameters aremore » determined by a recently developed many-body ab-initio approach and used to simulate quantum gates. We find that these systems are optimal for proof-of-principle two-qubit experiments and can be exploited as building blocks of scalable architectures for quantum simulation.« less
Color quantum simulations of strongly coupled quark-gluon plasma
NASA Astrophysics Data System (ADS)
Filinov, Vladimir; Fortov, Vladimir; Bonitz, Mishael; Ivanov, Yurii; Levashov, Pavel
2012-02-01
We propose stochastic simulation of thermodynamics and kinetic properties for quark-gluon plasma (QGP) in semi-classical approximation in the wide region of temperature, density and quasi-particles masses. In grand canonical ensemble for finite and zero baryon chemical potential we use the direct quantum path integral Monte Carlo method (PIMC) developed for finite temperature within Feynman formulation of quantum mechanics to do calculations of internal energy, pressure and pair correlation functions. The QGP quasi-particles representing dressed quarks, antiquarks and gluons interact via color quantum Kelbg pseudopotential rigorously derived for Coulomb particles. This method has been successfully applied to strongly coupled electrodynamic plasmas (EMP). A strongly correlated behavior of the QGP is expected to show up in long-ranged spatial correlations of quarks and gluons which, in fact, may give rise to liquid-like and, possibly, solid-like structures. We have done already the first calculation of the QGP equation of state, spatial and color pair distribution functions, diffusion coefficients and shear viscosity.
Wang, Fudong; Tang, Rui; Buhro, William E
2008-10-01
Tri-n-octylphosphine oxide (TOPO) is a commonly used solvent for nanocrystal synthesis. Commercial TOPO samples contain varying amounts of phosphorus-containing impurities, some of which significantly influence nanocrystal growth. Consequently, nanocrystal syntheses often give irreproducible results with different batches of TOPO solvent. In this study, we identify TOPO impurities by (31)P NMR, and correlate their presence with the outcomes of CdSe nanocrystal syntheses. We subsequently add the active impurity species, one by one, to purified TOPO to confirm their influence on nanocrystal syntheses. In this manner, di-n-octylphosphine oxide (DOPO) is shown to assist CdSe quantum-dot growth; di-n-octylphosphinic acid (DOPA) and mono-n-octylphosphinic acid (MOPA) are shown to assist CdSe quantum-rod growth, and DOPA is shown to assist CdSe quantum-wire growth. (The TOPO impurity n-octylphosphonic acid, OPA, has been previously shown to assist quantum-rod growth.) The beneficial impurities are prepared on multigram scales and can be added to recrystallized TOPO to provide reproducible synthetic results.
Dynamical Recurrence and the Quantum Control of Coupled Oscillators
NASA Astrophysics Data System (ADS)
Genoni, Marco G.; Serafini, Alessio; Kim, M. S.; Burgarth, Daniel
2012-04-01
Controllability—the possibility of performing any target dynamics by applying a set of available operations—is a fundamental requirement for the practical use of any physical system. For finite-dimensional systems, such as spin systems, precise criteria to establish controllability, such as the so-called rank criterion, are well known. However, most physical systems require a description in terms of an infinite-dimensional Hilbert space whose controllability properties are poorly understood. Here, we investigate infinite-dimensional bosonic quantum systems—encompassing quantum light, ensembles of bosonic atoms, motional degrees of freedom of ions, and nanomechanical oscillators—governed by quadratic Hamiltonians (such that their evolution is analogous to coupled harmonic oscillators). After having highlighted the intimate connection between controllability and recurrence in the Hilbert space, we prove that, for coupled oscillators, a simple extra condition has to be fulfilled to extend the rank criterion to infinite-dimensional quadratic systems. Further, we present a useful application of our finding, by proving indirect controllability of a chain of harmonic oscillators.
NASA Astrophysics Data System (ADS)
Khoa, Doan Quoc; Phuong, Le Thi Thu; Hoi, Bui Dinh
2017-03-01
A quantum kinetic equation for electrons interacting with confined phonons is used to investigate the nonlinear absorption of an intense electromagnetic wave by electrons in cylindrical GaAs/AlAs quantum wires. The analytic expression for absorption coefficient is calculated for three models of confined optical phonons: the dielectric continuum (DC), hydrodynamic continuum (HC), and Huang-Zhu (HZ) models. The absorption coefficient depends on the square of the electromagnetic wave amplitude. The electrophonon resonance and optically detected electrophonon resonance (ODEPR) are observed through the absorption spectrum. The full width at half maximum (the line-width) of the ODEPR peaks is obtained by a computational method. The line-width is found to increase with increasing temperature and decrease with increasing the quantum wire radius. In particular, numerical results show that the DC and HZ models lead to a similar behaviour of electron - confined phonon interaction whereas the HC model results in a quite different one, especially at small quantum wire radius. For large quantum wire radii, above mentioned phonon models have equivalent contributions to the ODEPR line-width.
NASA Astrophysics Data System (ADS)
Vernek, Edson; Ruiz-Tijerina, David; da Silva, Luis D.; Egues, José Carlos
2015-09-01
Quantum dot attached to topological wires has become an interesting setup to study Majorana bound state in condensed matter[1]. One of the major advantage of using a quantum dot for this purpose is that it provides a suitable manner to study the interplay between Majorana bound states and the Kondo effect. Recently we have shown that a non-interacting quantum dot side-connected to a 1D topological superconductor and to metallic normal leads can sustain a Majorana mode even when the dot is empty. This is due to the Majorana bound state of the wire leaking into the quantum dot. Now we investigate the system for the case in which the quantum dot is interacting[3]. We explore the signatures of a Majorana zero-mode leaking into the quantum dot, using a recursive Green's function approach. We then study the Kondo regime using numerical renormalization group calculations. In this regime, we show that a "0.5" contribution to the conductance appears in system due to the presence of the Majorana mode, and that it persists for a wide range of the dot parameters. In the particle-hole symmetric point, in which the Kondo effect is more robust, the total conductance reaches 3e^2/2h, clearly indicating the coexistence of a Majorana mode and the Kondo resonance in the dot. However, the Kondo effect is suppressed by a gate voltage that detunes the dot from its particle-hole symmetric point as well as by a Zeeman field. The Majorana mode, on the other hand, is almost insensitive to both of them. We show that the zero-bias conductance as a function of the magnetic field follows a well-known universal curve. This can be observed experimentally, and we propose that this universality followed by a persistent conductance of 0.5,e^2/h are evidence for the presence of Majorana-Kondo physics. This work is supported by the Brazilians agencies FAPESP, CNPq and FAPEMIG. [1] A. Y. Kitaev, Ann.Phys. {bf 303}, 2 (2003). [2] E. Vernek, P.H. Penteado, A. C. Seridonio, J. C. Egues, Phys. Rev. B {bf
NASA Astrophysics Data System (ADS)
Ge, Rong-Chun; Hughes, Stephen
2015-11-01
We study the quantum dynamics of two quantum dots (QDs) or artificial atoms coupled through the fundamental localized plasmon of a gold nanorod resonator. We derive an intuitive and efficient time-local master equation, in which the effect of the metal nanorod is taken into consideration self-consistently using a quasinormal mode (QNM) expansion technique of the photon Green function. Our efficient QNM technique offers an alternative and more powerful approach over the standard Jaynes-Cummings model, where the radiative decay, nonradiative decay, and spectral reshaping effect of the electromagnetic environment is rigorously included in a clear and transparent way. We also show how one can use our approach to compliment the approximate Jaynes-Cummings model in certain spatial regimes where it is deemed to be valid. We then present a study of the quantum dynamics and photoluminescence spectra of the two plasmon-coupled QDs. We first explore the non-Markovian regime, which is found to be important only on the ultrashort time scale of the plasmon mode which is about 40 fs. For the field free evolution case of excited QDs near the nanorod, we demonstrate how spatially separated QDs can be effectively coupled through the plasmon resonance and we show how frequencies away from the plasmon resonance can be more effective for coherently coupling the QDs. Despite the strong inherent dissipation of gold nanoresonators, we show that qubit entanglements as large as 0.7 can be achieved from an initially separate state, which has been limited to less than 0.5 in previous work for weakly coupled reservoirs. We also study the superradiance and subradiance decay dynamics of the QD pair. Finally, we investigate the rich quantum dynamics of QDs that are incoherently pumped, and study the polarization dependent behavior of the emitted photoluminescence spectrum where a double-resonance structure is observed due to the strong photon exchange interactions. Our general quantum plasmonics
THE COUPLING AND MUTUAL IMPEDANCE BETWEEN BALANCED WIRE-ARM CONICAL LOG-SPIRAL ANTENNAS
CONICAL ANTENNAS, *COUPLED ANTENNAS, * HELICAL ANTENNAS, ANTENNA COMPONENTS, ANTENNA RADIATION PATTERNS, COUPLINGS, DESIGN, ELECTRIC CURRENTS...ELECTRIC POTENTIAL, ELECTRICAL IMPEDANCE, MEASUREMENT, POLARIZATION, PROPAGATION, ROTATION, SPIRAL ANTENNAS, THEORY
Özcan, Mehmet Musa; Al Juhaimi, Fahad Y
2012-04-01
Two honey samples are taken from two parts of the same honeycomb: one that contacts to the surface of the wire and the other taken from the surface that does not contact the wires. Heavy metal contents of these two samples were determined by inductively coupled plasma atomic emission spectrometry). The Mo, Cd, Cr, Fe, Mn, Ni and Zn contents of the honey in contact with wire is higher when compared to the other. Especially, Fe and Zn contents of honey in contact with wire is much higher than the non-contact one. These values are, respectively, 190.21 and 112.76 ppm. Besides, Ni content of honey in contact with wire is approximately 50% higher.
Conductance noise of submicron wires in the regime of quantum Hall effect
NASA Astrophysics Data System (ADS)
Wróbel, J.; Jaroszyń Ski, J.; Dietl, T.; Regiń Ski, K.; Bugajski, M.
1998-12-01
A detailed study of low-temperature magnetoconductance in between quantized Hall plateaux is presented. The data are obtained for disordered two-terminal submicron wires defined in GaAs/AlGaAs heterostructures modulation-doped by Si. Slow time evolution of conductance G is observed on the high-field side of the quantized plateaux - for filling factors ν<3 and ν<2. This surprising noise is attributed to glassy dynamics of localized electrons in the wire centre, and to the corresponding time dependence of the impurity-assisted tunnelling probability between the current carrying regions.
NASA Astrophysics Data System (ADS)
Sau, Jay D.; Tewari, Sumanta; Lutchyn, Roman M.; Stanescu, Tudor D.; Das Sarma, S.
2010-12-01
We show that an ordinary semiconducting thin film with spin-orbit coupling can, under appropriate circumstances, be in a quantum topologically ordered state supporting exotic Majorana excitations which follow non-Abelian statistics. The key to the quantum topological order is the coexistence of spin-orbit coupling with proximity-induced s -wave superconductivity and an externally induced Zeeman coupling of the spins. For the Zeeman coupling below a critical value, the system is a nontopological (proximity-induced) s -wave superconductor. However, for a range of Zeeman coupling above the critical value, the lowest energy excited state inside a vortex is a zero-energy Majorana fermion state. The system, thus, has entered into a non-Abelian s -wave superconducting state via a topological quantum phase transition (TQPT) tuned by the Zeeman coupling. In the topological phase, since the time-reversal symmetry is explicitly broken by the Zeeman term in the Hamiltonian, the edge of the film constitutes a chiral Majorana wire. Just like the s -wave superconductivity, the Zeeman coupling can also be proximity induced in the film by an adjacent magnetic insulator. We show this by an explicit model tight-binding calculation for both types of proximity effects in the heterostructure geometry. Here we show that the same TQPT can be accessed by varying the interface transparency between the film and the superconductor. For the transparency below (above) a critical value, the system is a topological (regular) s -wave superconductor. In the one-dimensional version of the same structure and for the Zeeman coupling above the critical value, there are localized Majorana zero-energy modes at the two ends of a semiconducting quantum nanowire. In this case, the Zeeman coupling can be induced more easily by an external magnetic field parallel to the wire, obviating the need for a magnetic insulator. We show that, despite the fact that the superconducting pair potential in the nanowire is
Quantum physics inspired optical effects in evanescently coupled waveguides
NASA Astrophysics Data System (ADS)
Thompson, Clinton Edward
The tight-binding model that has been used for many years in condensed matter physics, due to its analytic and numerical tractability, has recently been used to describe light propagating through an array of evanescently coupled waveguides. This dissertation presents analytic and numerical simulation results of light propagating in a waveguide array. The first result presented is that photonic transport can be achieved in an array where the propagation constant is linearly increasing across the array. For an input at the center waveguide, the breathing modes of the system are observed, while for a phase displaced, asymmetric input, phase-controlled photonic transport is predicted. For an array with a waveguide-dependent, parity-symmetric coupling constant, the wave packet dynamics are predicted to be tunable. In addition to modifying the propagation constant, the coupling between waveguides can also be modified, and the quantum correlations are sensitive to the form of the tunneling function. In addition to modifying the waveguide array parameters in a structured manner, they can be randomized as to mimic the insertion of impurities during the fabrication process. When the refractive indices are randomized and real, the amount of light that localizes to the initial waveguide is found to be dependent on the initial waveguide when the waveguide coupling is non-uniform. In addition, when the variance of the refractive indices is small, light localizes in the initial waveguide as well as the parity-symmetric waveguide. In addition to real valued disorder, complex valued disorder can be introduced into the array through the imaginary component of the refractive index. It is shown that the two-particle correlation function is qualitatively similar to the case when the waveguide coupling is real and random, as both cases preserve the symmetry of the eigenvalues. Lastly, different input fields have been used to investigate the quantum statistical aspects of Anderson
Coherent coupling of two quantum dots embedded in an Aharonov-Bohm interferometer.
Holleitner, A W; Decker, C R; Qin, H; Eberl, K; Blick, R H
2001-12-17
We define two laterally gated small quantum dots with less than 15 electrons in an Aharonov-Bohm geometry in which the coupling between the two dots can be changed. We measure Aharonov-Bohm oscillations for weakly coupled quantum dots. In an intermediate coupling regime we study molecular states of the double dot and extract the magnetic field dependence of the coherently coupled states.
Quantum Computation by Optically Coupled Steady Atoms/Quantum-Dots Inside a Quantum Cavity
NASA Technical Reports Server (NTRS)
Pradhan, P.; Wang, K. L.; Roychowdhury, V. P.; Anantram, M. P.; Mor, T.; Saini, Subhash (Technical Monitor)
1999-01-01
We present a model for quantum computation using $n$ steady 3-level atoms kept inside a quantum cavity, or using $n$ quantum-dots (QDs) kept inside a quantum cavity. In this model one external laser is pointed towards all the atoms/QDs, and $n$ pairs of electrodes are addressing the atoms/QDs, so that each atom is addressed by one pair. The energy levels of each atom/QD are controlled by an external Stark field given to the atom/QD by its external pair of electrodes. Transition between two energy levels of an individual atom/ QD are controlled by the voltage on its electrodes, and by the external laser. Interactions between two atoms/ QDs are performed with the additional help of the cavity mode (using on-resonance condition). Laser frequency, cavity frequency, and energy levels are far off-resonance most of the time, and they are brought to the resonance (using the Stark effect) only at the time of operations. Steps for a controlled-NOT gate between any two atoms/QDs have been described for this model. Our model demands some challenging technological efforts, such as manufacturing single-electron QDs inside a cavity. However, it promises big advantages over other existing models which are currently implemented, and might enable a much easier scale-up, to compute with many more qubits.
Quantum Computation by Optically Coupled Steady Atoms/Quantum-Dots Inside a Quantum Cavity
NASA Technical Reports Server (NTRS)
Pradhan, P.; Wang, K. L.; Roychowdhury, V. P.; Anantram, M. P.; Mor, T.; Saini, Subhash (Technical Monitor)
1999-01-01
We present a model for quantum computation using $n$ steady 3-level atoms kept inside a quantum cavity, or using $n$ quantum-dots (QDs) kept inside a quantum cavity. In this model one external laser is pointed towards all the atoms/QDs, and $n$ pairs of electrodes are addressing the atoms/QDs, so that each atom is addressed by one pair. The energy levels of each atom/QD are controlled by an external Stark field given to the atom/QD by its external pair of electrodes. Transition between two energy levels of an individual atom/ QD are controlled by the voltage on its electrodes, and by the external laser. Interactions between two atoms/ QDs are performed with the additional help of the cavity mode (using on-resonance condition). Laser frequency, cavity frequency, and energy levels are far off-resonance most of the time, and they are brought to the resonance (using the Stark effect) only at the time of operations. Steps for a controlled-NOT gate between any two atoms/QDs have been described for this model. Our model demands some challenging technological efforts, such as manufacturing single-electron QDs inside a cavity. However, it promises big advantages over other existing models which are currently implemented, and might enable a much easier scale-up, to compute with many more qubits.
Nanophotonic structures for coupling to quantum emitters in the visible
NASA Astrophysics Data System (ADS)
Choy, Jennifer Tze-Heng
This thesis is about the design, fabrication, and characterization of nanophotonic elements in the visible that can enhance light-matter interaction for single quantum emitters. We focus on two material systems: single photon sources based on the nitrogen-vacancy (NV) center in diamond with improved spontaneous emission rates and collection efficiencies, and passive TiO 2 devices that comprise a potentially broadband (from the visible to the infrared), low loss photonics platform and that are suitable for probing and manipulating single colloidal quantum dots. We first discuss the requirements for using color center emission in bulk diamond crystals for potential applications in quantum information processing, and provide examples of using nanowire structures and planar resonators made in diamond for engineering the NV center's pump and collection efficiencies, and spontaneous emission rates, respectively. We also describe the integration of diamond with plasmonic structures. We have designed and implemented diamond-silver apertures for broadband enhancements of the spontaneous emission rates of NV centers. We show that shallow-implanted NV centers in diamond nanoposts provide a good system for controlling the NV center spontaneous emission rates, allowing for quenched emission with long lifetimes in the bare case, and enhanced emission with fast decay rates (corresponding to a Purcell factor of around 6) when coated with silver. We add plasmonic gratings around the diamond-silver apertures to improve the collection efficiency of the system, and observe over two-fold improvement in collection. We demonstrate the fabrication of chip-scale linear optical elements such as waveguides and racetrack resonators in low-loss TiO2 thin films. The fabricated waveguides operate over a wide bandwidth with propagation losses from 30 dB/cm in the visible to 4 dB/cm in the IR, while racetrack resonators can critically couple to waveguides and have quality factors as high as
NASA Astrophysics Data System (ADS)
Han, Yu; Li, Qiang; Chang, Shih-Pang; Hsu, Wen-Da; Lau, Kei May
2016-06-01
We report InGaAs quasi-quantum wires embedded in planar InP nanowires grown on (001) silicon emitting in the 1550 nm communication band. An array of highly ordered InP nanowire with semi-rhombic cross-section was obtained in pre-defined silicon V-grooves through selective-area hetero-epitaxy. The 8% lattice mismatch between InP and Si was accommodated by an ultra-thin stacking disordered InP/GaAs nucleation layer. X-ray diffraction and transmission electron microscope characterizations suggest excellent crystalline quality of the nanowires. By exploiting the morphological evolution of the InP and a self-limiting growth process in the V-grooves, we grew embedded InGaAs quantum-wells and quasi-quantum-wires with tunable shape and position. Room temperature analysis reveals substantially improved photoluminescence in the quasi-quantum wires as compared to the quantum-well reference, due to the reduced intrusion defects and enhanced quantum confinement. These results show great promise for integration of III-V based long wavelength nanowire lasers on the well-established (001) Si platform.
NASA Astrophysics Data System (ADS)
Morrison, Paul J.
This dissertation reports the novel synthesis of PbS quantum plates (QPs) via a reaction executed at low temperatures. The PbS QPs are grown in a template-assisted mechanism, with the first excitonic peak observed in the visible region of light at ~620 nm (2.0 eV), at an energy radically different than the band gap of bulk PbS (~3000 nm, 0.42eV). The large energy shift is due to the extremely thin dimension of these nanoplates, which is determined to be ~1 nm in thickness. Thickness measurements are made in a powder X-ray diffractometer (XRD) at low angles (between 2-15º 2θ). These measurements show a series of reflections in highly lamellar structures and give a spacing easily calculated using a modified version of the Bragg diffraction equation and applied to a straight line. During experimentation, a discrepancy was noticed while observing the lamellar template under various conditions. The TEM consistently showed an expanded d spacing of lamellar templates that does not fit with data from the XRD. The high vacuum atmosphere of the electron microscope induces a consistent expansion of template systems, as shown here for the first time, and proven from multiple previously published results. An improvement on CdSe quantum wire (QW) growth using alternate synthetic procedures and shell growth avenues is also reported. Also presented here are the underlying fundamentals for a new shell growth opportunity with a previously overlooked material: MgSe. Quantum wires made from CdSe are uniquely accommodating to MgSe as a shell material due to its intrinsic properties: much closer lattice parameters versus CdSe's current widely used shell (CdS), a highly insulating band gap (4.0 eV), and the ability to be grown in many of the common semiconductor crystal structures (rock salt, zinc blende, and wurtzite).
Jiao, Haisong; Pu, Tao; Zheng, Jilin; Xiang, Peng; Fang, Tao
2017-05-15
The physical-layer security of a quantum-noise randomized cipher (QNRC) system is, for the first time, quantitatively evaluated with secrecy capacity employed as the performance metric. Considering quantum noise as a channel advantage for legitimate parties over eavesdroppers, the specific wire-tap models for both channels of the key and data are built with channel outputs yielded by quantum heterodyne measurement; the general expressions of secrecy capacities for both channels are derived, where the matching codes are proved to be uniformly distributed. The maximal achievable secrecy rate of the system is proposed, under which secrecy of both the key and data is guaranteed. The influences of various system parameters on secrecy capacities are assessed in detail. The results indicate that QNRC combined with proper channel codes is a promising framework of secure communication for long distance with high speed, which can be orders of magnitude higher than the perfect secrecy rates of other encryption systems. Even if the eavesdropper intercepts more signal power than the legitimate receiver, secure communication (up to Gb/s) can still be achievable. Moreover, the secrecy of running key is found to be the main constraint to the systemic maximal secrecy rate.
Liu, Zhe; Jiang, Liwei; Zheng, Yisong
2016-07-13
By means of a numerical diagonalization approach, we calculate the electronic structure of a three-dimensional topological insulator (3DTI) quantum wire (QW) in the presence of a magnetic field. The QW can be viewed as a 3DTI film with lateral surfaces, when its rectangular cross section has a large aspect ratio. Our calculation indicates that nonchiral edge states emerge because of the confined states at the lateral surfaces. These states completely cover the valence band region among the Landau levels, which reasonably account for the absence of the [Formula: see text] quantum Hall effect in the relevant experimental works. In an ultrathin 3DTI film, inversion between the electron-type and hole-type bands occurs, which leads to the so-called pseudo-spin Hall effect. In a 3DTI QW with a square cross section, a tilting magnetic field can establish well-defined Landau levels in all four surfaces. In such a case, the quantum Hall edge states are localized at the square corners, characterized by the linearly crossing one-dimensional band profile. And they can be shifted between the adjacent corners by simply rotating the magnetic field.
Reprint of : Nanomagnet coupled to quantum spin Hall edge: An adiabatic quantum motor
NASA Astrophysics Data System (ADS)
Arrachea, Liliana; von Oppen, Felix
2016-08-01
The precessing magnetization of a magnetic islands coupled to a quantum spin Hall edge pumps charge along the edge. Conversely, a bias voltage applied to the edge makes the magnetization precess. We point out that this device realizes an adiabatic quantum motor and discuss the efficiency of its operation based on a scattering matrix approach akin to Landauer-Büttiker theory. Scattering theory provides a microscopic derivation of the Landau-Lifshitz-Gilbert equation for the magnetization dynamics of the device, including spin-transfer torque, Gilbert damping, and Langevin torque. We find that the device can be viewed as a Thouless motor, attaining unit efficiency when the chemical potential of the edge states falls into the magnetization-induced gap. For more general parameters, we characterize the device by means of a figure of merit analogous to the ZT value in thermoelectrics.
Nanomagnet coupled to quantum spin Hall edge: An adiabatic quantum motor
NASA Astrophysics Data System (ADS)
Arrachea, Liliana; von Oppen, Felix
2015-11-01
The precessing magnetization of a magnetic islands coupled to a quantum spin Hall edge pumps charge along the edge. Conversely, a bias voltage applied to the edge makes the magnetization precess. We point out that this device realizes an adiabatic quantum motor and discuss the efficiency of its operation based on a scattering matrix approach akin to Landauer-Büttiker theory. Scattering theory provides a microscopic derivation of the Landau-Lifshitz-Gilbert equation for the magnetization dynamics of the device, including spin-transfer torque, Gilbert damping, and Langevin torque. We find that the device can be viewed as a Thouless motor, attaining unit efficiency when the chemical potential of the edge states falls into the magnetization-induced gap. For more general parameters, we characterize the device by means of a figure of merit analogous to the ZT value in thermoelectrics.
Uchida, Takafumi Arita, Masashi; Takahashi, Yasuo; Fujiwara, Akira
2015-02-28
Tunability of capacitive coupling in the Si double-quantum-dot system is discussed by changing the number of electrons in quantum dots (QDs), in which the QDs are fabricated using pattern-dependent oxidation (PADOX) of a Si nanowire and multi-fine-gate structure. A single QD formed by PADOX is divided into multiple QDs by additional oxidation through the gap between the fine gates. When the number of electrons occupying the QDs is large, the coupling capacitance increases gradually and almost monotonically with the number of electrons. This phenomenon is attributed to the gradual growth in the effective QD size due to the increase in the number of electrons in the QDs. On the other hand, when the number of electrons changes in the few-electron regime, the coupling capacitance irregularly changes. This irregularity can be observed even up to 40 electrons. This behavior is attributable the rough structure of Si nano-dots made by PADOX. This roughness is thought to induce complicated change in the electron wave function when an electron is added to or subtracted from a QD.
A fiber-coupled quantum-dot on a photonic tip
Cadeddu, Davide; Teissier, Jean; Braakman, Floris R.; Warburton, Richard J.; Poggio, Martino; Munsch, Mathieu; Gregersen, Niels; Stepanov, Petr; Gérard, Jean-Michel; Claudon, Julien
2016-01-04
We present the experimental realization of a quantum fiber-pigtail. The device consists of a semiconductor quantum-dot embedded into a conical photonic wire that is directly connected to the core of a fiber-pigtail. We demonstrate a photon collection efficiency at the output of the fiber of 5.8% and suggest realistic improvements for the implementation of a useful device in the context of quantum information. We also discuss potential applications in scanning probe microscopy. The approach is generic and transferable to other materials including diamond and silicon.
Effective interaction and condensation of dipolaritons in coupled quantum wells
NASA Astrophysics Data System (ADS)
Byrnes, Tim; Kolmakov, German V.; Kezerashvili, Roman Ya.; Yamamoto, Yoshihisa
2014-09-01
Dipolaritons are a three-way superposition of a photon, a direct exciton, and an indirect exciton that are formed in coupled quantum well microcavities. As is the case with exciton-polaritons, dipolaritons have a self-interaction due to direct and exchange effects of the underlying electrons and holes. Here we present a theoretical description of dipolaritons and derive simple formulas for their basic parameters. In particular, we derive the effective dipolariton-dipolariton interaction taking into account exchange effects between the excitons. We obtain a simple relation to describe the effective interaction at low densities. We find that dipolaritons should condense under suitable conditions, described by a dissipative Gross-Pitaevskii equation. While the parameters for condensation are promising, we find that the level of tunability of the interactions is limited.
Enhancement of electron mobility in asymmetric coupled quantum well structures
Das, S.; Nayak, R. K.; Sahu, T. Panda, A. K.
2014-02-21
We study the low temperature multisubband electron mobility in a structurally asymmetric GaAs/Al{sub x}Ga{sub 1-x}As delta doped double quantum well. We calculate the subband energy levels and wave functions through selfconsistent solution of the coupled Schrodinger equation and Poisson's equation. We consider ionized impurity scattering, interface roughness scattering, and alloy disorder scattering to calculate the electron mobility. The screening of the scattering potentials is obtained by using static dielectric response function formalism within the random phase approximation. We analyze, for the first time, the effect of asymmetric structure parameters on the enhancement of multisubband electron mobility through intersubband interactions. We show that the asymmetric variation of well width, doping concentration, and spacer width considerably influences the interplay of scattering mechanisms on mobility. Our results of asymmetry induced enhancement of electron mobility can be utilized for low temperature device applications.
Non-minimally coupled varying constants quantum cosmologies
Balcerzak, Adam
2015-04-01
We consider gravity theory with varying speed of light and varying gravitational constant. Both constants are represented by non-minimally coupled scalar fields. We examine the cosmological evolution in the near curvature singularity regime. We find that at the curvature singularity the speed of light goes to infinity while the gravitational constant vanishes. This corresponds to the Newton's Mechanics limit represented by one of the vertex of the Bronshtein-Zelmanov-Okun cube [1,2]. The cosmological evolution includes both the pre-big-bang and post-big-bang phases separated by the curvature singularity. We also investigate the quantum counterpart of the considered theory and find the probability of transition of the universe from the collapsing pre-big-bang phase to the expanding post-big-bang phase.
Nanoshell-mediated robust entanglement between coupled quantum dots
NASA Astrophysics Data System (ADS)
Hakami, Jabir; Zubairy, M. Suhail
2016-02-01
The exact entanglement dynamics in a hybrid structure consisting of two quantum dots (QDs) in the proximity of a metal nanoshell is investigated. Nanoshells can enhance the local density of states, leading to a strong-coupling regime where the excitation energy can coherently be transferred between the QDs and the nanoshell in the form of Rabi oscillations. The long-lived entangled states can be created deterministically by optimizing the shell thickness as well as the ratio of the distances between the QDs and the surface of the shell. The loss of the system is greatly reduced even when the QDs are ultraclose to the shell, which signifies a slow decay rate of the coherence information and longtime entanglement preservation. Our protocol allows for an on-demand, fast, and almost perfect entanglement even at strong carrier-phonon interaction where other systems fail.
Spin effects in coupled quantum dots under ac electric fields
NASA Astrophysics Data System (ADS)
Meza-Montes, Lilia; Hernandez, Arezky H.; Ulloa, Sergio E.
2007-03-01
Spin control has recently attracted attention for applications in spin-based devices. Different effects and applied fields have been suggested to accomplish the goal. We explore the time evolution of electronic spin in coupled quantum dots under harmonic electric fields. Using the Floquet formalism, we obtain the time dependent wave function in terms of the Floquet states and the quasi-energy spectrum for a single electron in double InSb dots. The spatial part of the wave function includes the SIA and BIA spin-orbit effects. The spectral force is analyzed at anti-crossings of the quasi-energy bands as a function of the field strength. The resulting dynamical symmetries and the way they reflect in the time evolution of the spin clouds will be discussed.
Three-terminal energy harvester with coupled quantum dots
NASA Astrophysics Data System (ADS)
Thierschmann, Holger; Sánchez, Rafael; Sothmann, Björn; Arnold, Fabian; Heyn, Christian; Hansen, Wolfgang; Buhmann, Hartmut; Molenkamp, Laurens W.
2015-10-01
Rectification of thermal fluctuations in mesoscopic conductors is the key idea behind recent attempts to build nanoscale thermoelectric energy harvesters to convert heat into useful electric power. So far, most concepts have made use of the Seebeck effect in a two-terminal geometry, where heat and charge are both carried by the same particles. Here, we experimentally demonstrate the working principle of a new kind of energy harvester, proposed recently, using two capacitively coupled quantum dots. We show that, due to the novel three-terminal design of our device, which spatially separates the heat reservoir from the conductor circuit, the directions of charge and heat flow become decoupled. This enables us to manipulate the direction of the generated charge current by means of external gate voltages while leaving the direction of heat flow unaffected. Our results pave the way for a new generation of multi-terminal nanoscale heat engines.
Thermodynamics of information exchange between two coupled quantum dots
NASA Astrophysics Data System (ADS)
Kutvonen, Aki; Sagawa, Takahiro; Ala-Nissila, Tapio
2016-03-01
We propose a setup based on two coupled quantum dots where thermodynamics of a measurement can be quantitatively characterized. The information obtained in the measurement can be utilized by performing feedback in a manner apparently breaking the second law of thermodynamics. In this way the setup can be operated as a Maxwell's demon, where both the measurement and feedback are performed separately by controlling an external parameter. This is analogous to the case of the original Szilard engine. Since the setup contains both the microscopic demon and the engine itself, the operation of the whole measurement-feedback cycle can be explained in detail at the level of single realizations. In addition, we derive integral fluctuation relations for both the bare and coarse-grained entropy productions in the setup.
Selective protected state preparation of coupled dissipative quantum emitters
Plankensteiner, D.; Ostermann, L.; Ritsch, H.; Genes, C.
2015-01-01
Inherent binary or collective interactions in ensembles of quantum emitters induce a spread in the energy and lifetime of their eigenstates. While this typically causes fast decay and dephasing, in many cases certain special entangled collective states with minimal decay can be found, which possess ideal properties for spectroscopy, precision measurements or information storage. We show that for a specific choice of laser frequency, power and geometry or a suitable configuration of control fields one can efficiently prepare these states. We demonstrate this by studying preparation schemes for strongly subradiant entangled states of a chain of dipole-dipole coupled emitters. The prepared state fidelity and its entanglement depth is further improved via spatial excitation phase engineering or tailored magnetic fields. PMID:26549501
Free Energies of Quantum Particles: The Coupled-Perturbed Quantum Umbrella Sampling Method.
Glover, William J; Casey, Jennifer R; Schwartz, Benjamin J
2014-10-14
We introduce a new simulation method called Coupled-Perturbed Quantum Umbrella Sampling that extends the classical umbrella sampling approach to reaction coordinates involving quantum mechanical degrees of freedom. The central idea in our method is to solve coupled-perturbed equations to find the response of the quantum system's wave function along a reaction coordinate of interest. This allows for propagation of the system's dynamics under the influence of a quantum biasing umbrella potential and provides a method to rigorously undo the effects of the bias to compute equilibrium ensemble averages. In this way, one can drag electrons into regions of high free energy where they would otherwise not go, thus enabling chemistry by fiat. We demonstrate the applicability of our method for two condensed-phase systems of interest. First, we consider the interaction of a hydrated electron with an aqueous sodium cation, and we calculate a potential of mean force that shows that an e(-):Na(+) contact pair is the thermodynamically favored product starting from either a neutral sodium atom or the separate cation and electron species. Second, we present the first determination of a hydrated electron's free-energy profile relative to an air/water interface. For the particular model parameters used, we find that the hydrated electron is more thermodynamically stable in the bulk rather than at the interface. Our analysis suggests that the primary driving force keeping the electron away from the interface is the long-range electron-solvent polarization interaction rather than the short-range details of the chosen pseudopotential.
Silicon quantum computation based on magnetic dipolar coupling
NASA Astrophysics Data System (ADS)
de Sousa, Rogerio; Delgado, J. D.; Das Sarma, S.
2004-03-01
A dipolar gate alternative to the Kane quantum computer is proposed where the qubits are electron spins of shallow group V donors in silicon [1]. An optimal qubit separation of the order of 250 Åallows implementation of two-qubit gates using the spin-spin dipolar interaction with residual exchange treated as gate error amenable to quantum error correction. Such a scheme does not have the stringent requirements on donor positioning characteristic of all silicon exchange-based implementations [B. Koiller et al., Phys. Rev. Lett. 88, 027903 (2002)]. In addition it allows quality factors of 10^4 with simplified lithography. We show that bismuth donors lead to faster gates than any other group V element. Contrary to common speculation, such a scheme is scalable with no overhead in gating time even though it is based on long-range dipolar inter-qubit coupling.This work is supported by ARDA, LPS, US-ONR, and NSF. [1] Rogerio de Sousa, J. D. Delgado, and S. Das Sarma, LANL preprint cond-mat/0311403.
Strong coupling and polariton lasing in Te based microcavities embedding (Cd,Zn)Te quantum wells
Rousset, J.-G. Piętka, B.; Król, M.; Mirek, R.; Lekenta, K.; Szczytko, J.; Borysiuk, J.; Suffczyński, J.; Kazimierczuk, T.; Goryca, M.; Smoleński, T.; Kossacki, P.; Nawrocki, M.; Pacuski, W.
2015-11-16
We report on properties of an optical microcavity based on (Cd,Zn,Mg)Te layers and embedding (Cd,Zn)Te quantum wells. The key point of the structure design is the lattice matching of the whole structure to MgTe, which eliminates the internal strain and allows one to embed an arbitrary number of unstrained quantum wells in the microcavity. We evidence the strong light-matter coupling regime already for the structure containing a single quantum well. Embedding four unstrained quantum wells results in further enhancement of the exciton-photon coupling and the polariton lasing in the strong coupling regime.
Quantum control of a spin qubit coupled to a photonic crystal cavity
NASA Astrophysics Data System (ADS)
Carter, Samuel G.; Sweeney, Timothy M.; Kim, Mijin; Kim, Chul Soo; Solenov, Dmitry; Economou, Sophia E.; Reinecke, Thomas L.; Yang, Lily; Bracker, Allan S.; Gammon, Daniel
2013-04-01
A key ingredient for a quantum network is an interface between stationary quantum bits and photons, which act as flying qubits for interactions and communication. Photonic crystal architectures are promising platforms for enhancing the coupling of light to solid-state qubits. Quantum dots can be integrated into a photonic crystal, with optical transitions coupling to photons and spin states forming a long-lived quantum memory. Many researchers have now succeeded in coupling these emitters to photonic crystal cavities, but there have been no demonstrations of a functional spin qubit and quantum gates in this environment. Here, we have developed a coupled cavity-quantum dot system in which the dot is controllably charged with a single electron. We perform the initialization, rotation and measurement of a single electron spin qubit using laser pulses, and find that the cavity can significantly improve these processes.
He Xiaoling; Luo Junyan; Yang Chuiping; Li Sheng; Han Siyuan
2010-08-15
We propose a way for realizing a two-qubit controlled phase gate with superconducting quantum interference devices (SQUIDs) coupled to a superconducting resonator. In this proposal, the two lowest levels of each SQUID serve as the logical states and two intermediate levels of each SQUID are used for the gate realization. We show that neither adjustment of SQUID level spacings during the gate operation nor uniformity in SQUID parameters is required by this proposal. In addition, this proposal does not require the adiabatic passage or a second-order detuning and thus the gate is much faster.
NASA Astrophysics Data System (ADS)
Khordad, R.; Bahramiyan, H.
2017-03-01
We have investigated the influence of electron-phonon (e-p) interaction and hydrogenic donor impurity simultaneously on energy difference, binding energy, the linear, nonlinear and total refractive index changes and absorption coefficients of a hexagonal-shaped quantum wire. For this goal, we have used finite-element method (FEM), a compact density matrix approach and an iterative procedure. It is deduced that energy difference and binding energy decrease by changing the impurity position with and without e-p interaction. The dipole matrix elements have complex behaviours in the presence of impurity with and without e-p interaction. The refractive index changes and absorption coefficients increase and shift towards lower energies by enhancing a 1 with central impurity. In the presence of central impurity, the absorption coefficients and refractive index changes enhance and shift toward higher energies when e-p interaction is considered.
Agnihotri, Pratik; Bandyopadhyay, Supriyo
2012-05-30
Using ensemble Monte Carlo simulation, we have studied hot carrier spin dynamics and spin noise in a multi-subband GaAs quantum wire in the presence of a randomly varying Rashba spin-orbit interaction. The random variation reduces the carrier ensemble's spin dephasing time due to the D'yakonov-Perel' mechanism, but otherwise makes no qualitative difference to the temporal spin relaxation characteristics. However, it makes a qualitative difference to the spatial spin relaxation characteristics which change from monotonic and smooth to non-monotonic and chaotic because of a complex interplay between carriers in different subbands. As far as spin fluctuation and spin noise are concerned, the random variation has no major effect except that the low-frequency noise power spectral density increases slightly when the magnitude of the Rashba spin-orbit interaction field is varied randomly while holding the direction constant.
NASA Astrophysics Data System (ADS)
Radu, A.; Duque, C. A.
2015-08-01
The conduction subband structure of a triangular cross-section GaAs/AlGaAs quantum well wire under intense laser field is theoretically investigated by taking into account a finite confining potential. The calculation of the subband energy levels is based on a two-dimensional finite element method within the effective mass approximation. It is shown that a transversally polarized laser field non-uniformly shifts the subband energy levels and could be used for tuning the intersubband transitions and altering the related optical susceptibilities. We found that the non-resonant laser field allows the magnification and the red- or blueshift of the third-order non-linear susceptibility peaks for particular polarizations of the pump light and proper laser parameter values. The effects of the laser dressing field on the intersubband third harmonic generation and quadratic electro-optical process are discussed.
NASA Astrophysics Data System (ADS)
Moraga, Luis; Henriquez, Ricardo; Solis, Basilio
2015-08-01
We calculate the electrical conductivity of a metallic sample under the effects of distributed impurities and a random distribution of grain boundaries by means of a quantum mechanical procedure based on Kubo formula. Grain boundaries are represented either by a one-dimensional regular array of Dirac delta potentials (Mayadas and Shatzkes model) or by its three-dimensional extension (Szczyrbowski and Schmalzbauer model). We give formulas expressing the conductivity of bulk samples, thin films and thin wires of rectangular cross-sections in the case when the samples are bounded by perfectly flat surfaces. We find that, even in the absence of surface roughness, the conductivity in thin samples is reduced from its bulk value. If there are too many grain boundaries per unit length, or their scattering strength is high enough, there is a critical value Rc of the reflectivity R of an individual boundary such that the electrical conductivity vanishes for R >Rc. Also, the conductivity of thin wires shows a stepwise dependence on R. The effect of weak random variations in the strength or separation of the grain boundaries is computed by means of the method of correlation length. Finally, the resistivity of nanometric polycrystalline tungsten films reported in Choi et al. J. Appl. Phys. (2014) 115 104308 is tentatively analyzed by means of the present formalism.
Prasankumar, Rohit P; Taylor, Antoinette J
2009-01-01
Ultrafast density-dependent optical spectroscopic measurements on a quantum dots-in-a-well heterostructure reveal several distinctive phenomena, most notably a strong coupling between the quantum well population and light absorption at the quantum dot excited state.
Atomic quantum dots coupled to a reservoir of a superfluid Bose-Einstein condensate.
Recati, A; Fedichev, P O; Zwerger, W; von Delft, J; Zoller, P
2005-02-04
We study the dynamics of an atomic quantum dot, i.e., a single atom in a tight optical trap which is coupled to a superfluid reservoir via laser transitions. Quantum interference between the collisional interactions and the laser induced coupling results in a tunable dot-bath coupling, allowing an essentially complete decoupling from the environment. Quantum dots embedded in a 1D Luttinger liquid of cold bosonic atoms realize a spin-boson model with Ohmic coupling, which exhibits a dissipative phase transition and allows us to directly measure atomic Luttinger parameters.
Double quantum dot Cooper-pair splitter at finite couplings
NASA Astrophysics Data System (ADS)
Hussein, Robert; Jaurigue, Lina; Governale, Michele; Braggio, Alessandro
2016-12-01
We consider the subgap physics of a hybrid double-quantum dot Cooper-pair splitter with large single-level spacings, in the presence of tunneling between the dots and finite Coulomb intra- and interdot Coulomb repulsion. In the limit of a large superconducting gap, we treat the coupling of the dots to the superconductor exactly. We employ a generalized master-equation method, which easily yields currents, noise, and cross-correlators. In particular, for finite inter- and intradot Coulomb interaction, we investigate how the transport properties are determined by the interplay between local and nonlocal tunneling processes between the superconductor and the dots. We examine the effect of interdot tunneling on the particle-hole symmetry of the currents with and without spin-orbit interaction. We show that spin-orbit interaction in combination with finite Coulomb energy opens the possibility to control the nonlocal entanglement and its symmetry (singlet/triplet). We demonstrate that the generation of nonlocal entanglement can be achieved even without any direct nonlocal coupling to the superconducting lead.
Controlled Photon Switch Assisted by Coupled Quantum Dots
Luo, Ming-Xing; Ma, Song-Ya; Chen, Xiu-Bo; Wang, Xiaojun
2015-01-01
Quantum switch is a primitive element in quantum network communication. In contrast to previous switch schemes on one degree of freedom (DOF) of quantum systems, we consider controlled switches of photon system with two DOFs. These controlled photon switches are constructed by exploring the optical selection rules derived from the quantum-dot spins in one-sided optical microcavities. Several double controlled-NOT gate on different joint systems are greatly simplified with an auxiliary DOF of the controlling photon. The photon switches show that two DOFs of photons can be independently transmitted in quantum networks. This result reduces the quantum resources for quantum network communication. PMID:26095049
Analog quantum simulation of the Rabi model in the ultra-strong coupling regime.
Braumüller, Jochen; Marthaler, Michael; Schneider, Andre; Stehli, Alexander; Rotzinger, Hannes; Weides, Martin; Ustinov, Alexey V
2017-10-03
The quantum Rabi model describes the fundamental mechanism of light-matter interaction. It consists of a two-level atom or qubit coupled to a quantized harmonic mode via a transversal interaction. In the weak coupling regime, it reduces to the well-known Jaynes-Cummings model by applying a rotating wave approximation. The rotating wave approximation breaks down in the ultra-strong coupling regime, where the effective coupling strength g is comparable to the energy ω of the bosonic mode, and remarkable features in the system dynamics are revealed. Here we demonstrate an analog quantum simulation of an effective quantum Rabi model in the ultra-strong coupling regime, achieving a relative coupling ratio of g/ω ~ 0.6. The quantum hardware of the simulator is a superconducting circuit embedded in a cQED setup. We observe fast and periodic quantum state collapses and revivals of the initial qubit state, being the most distinct signature of the synthesized model.An analog quantum simulation scheme has been explored with a quantum hardware based on a superconducting circuit. Here the authors investigate the time evolution of the quantum Rabi model at ultra-strong coupling conditions, which is synthesized by slowing down the system dynamics in an effective frame.
Thermoelectrics in Coulomb-coupled quantum dots: Cotunneling and energy-dependent lead couplings
NASA Astrophysics Data System (ADS)
Walldorf, Nicklas; Jauho, Antti-Pekka; Kaasbjerg, Kristen
2017-09-01
We study thermoelectric effects in Coulomb-coupled quantum-dot (CCQD) systems beyond lowest-order tunneling processes and the often applied wide-band approximation. To this end, we present a master-equation (ME) approach based on a perturbative T -matrix calculation of the charge and heat tunneling rates and transport currents. Applying the method to transport through a noninteracting single-level QD, we demonstrate excellent agreement with the Landauer-Büttiker theory when higher-order (cotunneling) processes are included in the ME. Next, we study the effect of cotunneling and energy-dependent lead couplings on the heat currents in a system of two CCQDs. We find that cotunneling processes (i) can dominate the off-resonant heat currents at low temperature and bias compared to the interdot interaction, and (ii) give rise to a pronounced reduction of the cooling power achievable with the recently demonstrated Maxwell's demon cooling mechanism. Furthermore, we demonstrate that the cooling power can be boosted significantly by carefully engineering the energy dependence of the lead couplings to filter out undesired transport processes. Our findings emphasize the importance of higher-order cotunneling processes as well as engineered energy-dependent lead couplings in the optimization of the thermoelectric performance of CCQD systems.
Real Space Renormalization of Majorana Fermions in Quantum Nano-Wire Superconductors
NASA Astrophysics Data System (ADS)
Jafari, R.; Langari, A.; Akbari, Alireza; Kim, Ki-Seok
2017-02-01
We develop the real space quantum renormalization group (QRG) approach for majorana fermions. As an example we focus on the Kitaev chain to investigate the topological quantum phase transition (TQPT) in the one-dimensional spinless p-wave superconductor. Studying the behaviour of local compressibility and ground-state fidelity, show that the TQPT is signalled by the maximum of local compressibility at the quantum critical point tuned by the chemical potential. Moreover, a sudden drop of the ground-state fidelity and the divergence of fidelity susceptibility at the topological quantum critical point are used as proper indicators for the TQPT, which signals the appearance of Majorana fermions. Finally, we present the scaling analysis of ground-state fidelity near the critical point that manifests the universal information about the TQPT, which reveals two different scaling behaviors as we approach the critical point and thermodynamic limit.
Wang, Z H; Zheng, Q; Wang, Xiaoguang; Li, Yong
2016-03-02
We study the energy-level crossing behavior in a two-dimensional quantum well with the Rashba and Dresselhaus spin-orbit couplings (SOCs). By mapping the SOC Hamiltonian onto an anisotropic Rabi model, we obtain the approximate ground state and its quantum Fisher information (QFI) via performing a unitary transformation. We find that the energy-level crossing can occur in the quantum well system within the available parameters rather than in cavity and circuit quantum eletrodynamics systems. Furthermore, the influence of two kinds of SOCs on the QFI is investigated and an intuitive explanation from the viewpoint of the stationary perturbation theory is given.
Wang, Z. H.; Zheng, Q.; Wang, Xiaoguang; Li, Yong
2016-01-01
We study the energy-level crossing behavior in a two-dimensional quantum well with the Rashba and Dresselhaus spin-orbit couplings (SOCs). By mapping the SOC Hamiltonian onto an anisotropic Rabi model, we obtain the approximate ground state and its quantum Fisher information (QFI) via performing a unitary transformation. We find that the energy-level crossing can occur in the quantum well system within the available parameters rather than in cavity and circuit quantum eletrodynamics systems. Furthermore, the influence of two kinds of SOCs on the QFI is investigated and an intuitive explanation from the viewpoint of the stationary perturbation theory is given. PMID:26931762
Comparison of coherently coupled multi-cavity and quantum dot embedded single cavity systems.
Kocaman, Serdar; Sayan, Gönül Turhan
2016-12-12
Temporal group delays originating from the optical analogue to electromagnetically induced transparency (EIT) are compared in two systems. Similar transmission characteristics are observed between a coherently coupled high-Q multi-cavity array and a single quantum dot (QD) embedded cavity in the weak coupling regime. However, theoretically generated group delay values for the multi-cavity case are around two times higher. Both configurations allow direct scalability for chip-scale optical pulse trapping and coupled-cavity quantum electrodynamics (QED).
Direct photonic coupling of a semiconductor quantum dot and a trapped ion.
Meyer, H M; Stockill, R; Steiner, M; Le Gall, C; Matthiesen, C; Clarke, E; Ludwig, A; Reichel, J; Atatüre, M; Köhl, M
2015-03-27
Coupling individual quantum systems lies at the heart of building scalable quantum networks. Here, we report the first direct photonic coupling between a semiconductor quantum dot and a trapped ion and we demonstrate that single photons generated by a quantum dot controllably change the internal state of a Yb^{+} ion. We ameliorate the effect of the 60-fold mismatch of the radiative linewidths with coherent photon generation and a high-finesse fiber-based optical cavity enhancing the coupling between the single photon and the ion. The transfer of information presented here via the classical correlations between the σ_{z} projection of the quantum-dot spin and the internal state of the ion provides a promising step towards quantum-state transfer in a hybrid photonic network.
NASA Astrophysics Data System (ADS)
Izrael, A.; Marzin, J. Y.; Sermage, B.; Birotheau, L.; Robein, D.; Azoulay, R.; Benchimol, J. L.; Henry, L.; Thierry-Mieg, V.; Ladan, F. R.; Taylor, L.
1991-11-01
We present the fabrication process of narrow quantum wires in both In1-xGaxAs/InP and GaAs/Ga1-xAlxAs systems based upon e-beam lithography, lift-off, reactive ion etching and MOCVD overgrowth. The carrier lifetime, deduced from low temperature time-resolved photoluminescence, remains of the order of one nanosecond in both systems, showing the good quality of the overgrown interfaces. An efficient capture of the carriers created in the barrier material into the wires is demonstrated. One-dimension quantum confinement effects are observed for the lowest lateral sizes: in In1-xGaxAs/InP wires (width down to 15 nm), quantum shifts of the low temperature cw photoluminescence peak (up to 30 meV) are observed while in GaAs/Ga1-xAlxAs wires (width down to 20 nm), we report shifts up to 10 meV of the emission peak.
Transport through an impurity tunnel coupled to a Si/SiGe quantum dot
Foote, Ryan H. Ward, Daniel R.; Thorgrimsson, Brandur; Savage, D. E.; Friesen, Mark; Coppersmith, S. N.; Eriksson, M. A.; Prance, J. R.; Gamble, John King; Nielsen, Erik; Saraiva, A. L.
2015-09-07
Achieving controllable coupling of dopants in silicon is crucial for operating donor-based qubit devices, but it is difficult because of the small size of donor-bound electron wavefunctions. Here, we report the characterization of a quantum dot coupled to a localized electronic state and present evidence of controllable coupling between the quantum dot and the localized state. A set of measurements of transport through the device enable the determination that the most likely location of the localized state is consistent with a location in the quantum well near the edge of the quantum dot. Our results are consistent with a gate-voltage controllable tunnel coupling, which is an important building block for hybrid donor and gate-defined quantum dot devices.
Quantum-coherent coupling of a mechanical oscillator to an optical cavity mode.
Verhagen, E; Deléglise, S; Weis, S; Schliesser, A; Kippenberg, T J
2012-02-01
Optical laser fields have been widely used to achieve quantum control over the motional and internal degrees of freedom of atoms and ions, molecules and atomic gases. A route to controlling the quantum states of macroscopic mechanical oscillators in a similar fashion is to exploit the parametric coupling between optical and mechanical degrees of freedom through radiation pressure in suitably engineered optical cavities. If the optomechanical coupling is 'quantum coherent'--that is, if the coherent coupling rate exceeds both the optical and the mechanical decoherence rate--quantum states are transferred from the optical field to the mechanical oscillator and vice versa. This transfer allows control of the mechanical oscillator state using the wide range of available quantum optical techniques. So far, however, quantum-coherent coupling of micromechanical oscillators has only been achieved using microwave fields at millikelvin temperatures. Optical experiments have not attained this regime owing to the large mechanical decoherence rates and the difficulty of overcoming optical dissipation. Here we achieve quantum-coherent coupling between optical photons and a micromechanical oscillator. Simultaneously, coupling to the cold photon bath cools the mechanical oscillator to an average occupancy of 1.7 ± 0.1 motional quanta. Excitation with weak classical light pulses reveals the exchange of energy between the optical light field and the micromechanical oscillator in the time domain at the level of less than one quantum on average. This optomechanical system establishes an efficient quantum interface between mechanical oscillators and optical photons, which can provide decoherence-free transport of quantum states through optical fibres. Our results offer a route towards the use of mechanical oscillators as quantum transducers or in microwave-to-optical quantum links.
Yang, Xiao-Jie Kiba, Takayuki; Yamamura, Takafumi; Takayama, Junichi; Subagyo, Agus; Sueoka, Kazuhisa; Murayama, Akihiro
2014-01-06
We investigate the electron-spin injection dynamics via tunneling from an In{sub 0.1}Ga{sub 0.9}As quantum well (QW) to In{sub 0.5}Ga{sub 0.5}As quantum dots (QDs) in coupled QW-QDs nanostructures. These coupled nanostructures demonstrate ultrafast (5 to 20 ps) spin injection into the QDs. The degree of spin polarization up to 45% is obtained in the QDs after the injection, essentially depending on the injection time. The spin injection and conservation are enhanced with thinner barriers due to the stronger electronic coupling between the QW and QDs.
NASA Astrophysics Data System (ADS)
Stepanyan, A.; Yeranosyan, M.; Vardanyan, L.; Asatryan, A.; Kirakosyan, A.; Vartanian, A.
2017-08-01
The hot-electron energy-loss rate via the acoustic-phonons in an embedded semiconductor quantum wire of circular cross section in the presence of external electric field has been investigated using deformation potential theory. Dimensional confinement effect on modifying acoustic-phonon modes are taken into account. The energy-loss rate as a function of electric field strength, electron density and electron temperature is obtained. Our calculations show that the electric field applied perpendicularly to the wire axis can be used as an important tool for the control of the energy-loss processes in nanowires.
Mokkapati, Sudha; Saxena, Dhruv; Tan, Hark Hoe; Jagadish, Chennupati
2013-12-09
The optimal geometries for reducing the radiative recombination lifetime and thus enhancing the quantum efficiency of III-V semiconductor nanowires by coupling them to plasmonic nanoparticles are established. The quantum efficiency enhancement factor due to coupling to plasmonic nanoparticles reduces as the initial quality of the nanowire increases. Significant quantum efficiency enhancement is observed for semiconductors only within about 15 nm from the nanoparticle. It is also identified that the modes responsible for resonant enhancement in the quantum efficiency of an emitter in the nanowire are geometric resonances of surface plasmon polariton modes supported at the nanowire/nanoparticle interface.
Cascade of quantum phase transitions in tunnel-coupled edge states.
Yang, I; Kang, W; Baldwin, K W; Pfeiffer, L N; West, K W
2004-02-06
We report on the cascade of quantum phase transitions exhibited by tunnel-coupled edge states across a quantum Hall line junction. We identify a series of quantum critical points between successive strong and weak tunneling regimes in the zero-bias conductance. Scaling analysis shows that the conductance near the critical magnetic fields B(c) is a function of a single scaling argument /B-B(c)/T(-kappa), where the exponent kappa=0.42. This puzzling resemblance to a quantum Hall-insulator transition points to the importance of interedge correlation between the coupled edge states.
Spin-state transfer in laterally coupled quantum-dot chains with disorders
NASA Astrophysics Data System (ADS)
Yang, Song; Bayat, Abolfazl; Bose, Sougato
2010-08-01
Quantum dot arrays are a promising medium for transferring quantum information between two distant points without resorting to mobile qubits. Here we study the two most common disorders, namely hyperfine interaction and exchange coupling fluctuations, in quantum dot arrays and their effects on quantum communication through these chains. Our results show that the hyperfine interaction is more destructive than the exchange coupling fluctuations. The average optimal time for communication is not affected by any disorder in the system and our simulations show that antiferromagnetic chains are much more resistive than the ferromagnetic ones against both kind of disorders. Even when time modulation of a coupling and optimal control is employed to improve the transmission, the antiferromagnetic chain performs much better. We have assumed the quasistatic approximation for hyperfine interaction and time-dependent fluctuations in the exchange couplings. Particularly for studying exchange coupling fluctuations we have considered the static disorder, white noise, and 1/f noise.
NASA Astrophysics Data System (ADS)
Cinti, Maria Nerina; Pani, Roberto; Pellegrini, Rosanna; Bennati, Paolo; Orlandi, Chiara; Fabbri, Andrea; Ridolfi, Stefano; Scafè, Raffaele
2013-10-01
High quantum efficiency semiconductor photodetectors have recently drawn the attention of the scientific community for their potential in the realization of a new class of scintillation imagers with very high energy and spatial resolution performance. However, this goal does not seem within easy reach, due to various technological issues such as, for example, the difficulty to scale the characteristics of a single detector to an imager with suitable dimensions. Lately a definite technical improvement in increasing quantum efficiency up to 42% for position sensitive photomultipliers was achieved. The aim of this work is thus to test this new technological progress and to study the possible implications in imaging applications. Four Hamamatsu PMTs were tested: two multi anode photomultipliers, one with a bialkali (27% quantum efficiency) and the other one with a super-bialkali photocathode (38% quantum efficiency), and two 1×1 in. PMTs, both equipped with an ultra bialkali photocathode (42% quantum efficiency). In particular one of the ultra bialkali PMT has also an increased efficiency of first dynode charge collection. The results were compared with the ones obtained with a reference PMT (Hamamatsu R6231), mainly used in spectroscopy. The PMTs were coupled to LaBr3(Ce), NaI(Tl) and LSO(Ce) continuous scintillation crystals. The tests were done using two independent electronic chains: one dedicated for spectroscopic application and a second one, using a multi wire 64 channel readout, for imaging applications. The super-bialkali MA-PMTs have shown high energy resolution, both with spectroscopic and imaging setup, highlighting the appropriateness of these devices for the development of imaging devices with high spectroscopic performance.
Experimental Verification of the Very Strong Coupling Regime in a GaAs Quantum Well Microcavity
NASA Astrophysics Data System (ADS)
Brodbeck, S.; De Liberato, S.; Amthor, M.; Klaas, M.; Kamp, M.; Worschech, L.; Schneider, C.; Höfling, S.
2017-07-01
The dipole coupling strength g between cavity photons and quantum well excitons determines the regime of light matter coupling in quantum well microcavities. In the strong coupling regime, a reversible energy transfer between exciton and cavity photon takes place, which leads to the formation of hybrid polaritonic resonances. If the coupling is further increased, a hybridization of different single exciton states emerges, which is referred to as the very strong coupling regime. In semiconductor quantum wells such a regime is predicted to manifest as a photon-mediated electron-hole coupling leading to different excitonic wave functions for the two polaritonic branches when the ratio of the coupling strength to exciton binding energy g /EB approaches unity. Here, we verify experimentally the existence of this regime in magneto-optical measurements on a microcavity characterized by g /EB≈0.64 , showing that the average electron-hole separation of the upper polariton is significantly increased compared to the bare quantum well exciton Bohr radius. This yields a diamagnetic shift around 0 detuning that exceeds the shift of the lower polariton by 1 order of magnitude and the bare quantum well exciton diamagnetic shift by a factor of 2. The lower polariton exhibits a diamagnetic shift smaller than expected from the coupling of a rigid exciton to the cavity mode, which suggests more tightly bound electron-hole pairs than in the bare quantum well.
NASA Astrophysics Data System (ADS)
Topalović, D. B.; Arsoski, V. V.; Pavlović, S.; Čukarić, N. A.; Tadić, M. Ž.; Peeters, F. M.
2016-01-01
We use the Galerkin approach and the finite-element method to numerically solve the effective-mass Schrödinger equation. The accuracy of the solution is explored as it varies with the range of the numerical domain. The model potentials are those of interdiffused semiconductor quantum wells and axially symmetric quantum wires. Also, the model of a linear harmonic oscillator is considered for comparison reasons. It is demonstrated that the absolute error of the electron ground state energy level exhibits a minimum at a certain domain range, which is thus considered to be optimal. This range is found to depend on the number of mesh nodes N approximately as α0 logeα1(α2N), where the values of the constants α0, α1, and α2 are determined by fitting the numerical data. And the optimal range is found to be a weak function of the diffusion length. Moreover, it was demonstrated that a domain range adaptation to the optimal value leads to substantial improvement of accuracy of the solution of the Schrödinger equation. Supported by the Ministry of Education, Science, and Technological Development of Serbia and the Flemish fund for Scientific Research (FWO Vlaanderen)
Quantum Monte Carlo calculations of magnetic couplings in cuprates
NASA Astrophysics Data System (ADS)
Foyevtsova, Kateryna; Krogel, Jaron; Kim, Jeongnim; Reboredo, Fernando
2014-03-01
Spin excitations are generally believed to play a fundamental role in the mechanism of high temperature superconductivity in cuprates. However, accurate description of the cuprates' magnetic properties and, in particular, calculation of spin exchange couplings have been a long-standing challenge to the electronic structure theory. While the quantum-mechanically more rigorous cluster methods suffer from finite-size effects, the density functional theory approach, on the other hand, is ambiguous due to a rich variety of approximations to the exchange-correlation functional available which often give very different numbers for the spin exchange constants. For example, in some cuprates the theoretically predicted values of the nearest-neighbor superexchange range from 1 eV (local density approximation) to 0.05 eV (periodic unrestricted Hartree Fock) [C. de Graaf et al, PRB 63 014404 (2000)]. We compute spin exchange constants with the fixed-node diffusion Monte Carlo method (FN-DMC). In one-dimensional cuprates, we find that the FN-DMC computed nearest-neighbor spin superexchange is in an excellent agreement with experiment. This both demonstrates that FN-DMC is capable of describing properly the magnetism of strongly correlated oxides as well as positions this technique as the method of choice for theoretical parameterization of spin models. Research supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.
Realizing ferromagnetic coupling in diluted magnetic semiconductor quantum dots.
Yan, Wensheng; Liu, Qinghua; Wang, Chao; Yang, Xiaoyu; Yao, Tao; He, Jingfu; Sun, Zhihu; Pan, Zhiyun; Hu, Fengchun; Wu, Ziyu; Xie, Zhi; Wei, Shiqiang
2014-01-22
Manipulating the ferromagnetic interactions in diluted magnetic semiconductor quantum dots (DMSQDs) is a central theme to the development of next-generation spin-based information technologies, but this remains a great challenge because of the intrinsic antiferromagnetic coupling between impurity ions therein. Here, we propose an effective approach capable of activating ferromagnetic exchange in ZnO-based DMSQDs, by virtue of a core/shell structure that engineers the energy level of the magnetic impurity 3d levels relative to the band edge. This idea has been successfully applied to Zn(0.96)Co(0.04)O DMSQDs covered by a shell of ZnS or Ag2S. First-principles calculations further indicate that covering a ZnS shell around the Co-doped ZnO core drives a transition of antiferromagnetic-to-ferromagnetic interaction, which occurs within an effective depth of 1.2 nm underneath the surface in the core. This design opens up new possibility for effective manipulation of exchange interactions in doped oxide nanostructures for future spintronics applications.
Quantum corrections to the cosmological evolution of conformally coupled fields
Cembranos, Jose A.R.; Olive, Keith A.; Peloso, Marco; Uzan, Jean-Philippe E-mail: olive@physics.umn.edu E-mail: uzan@iap.fr
2009-07-01
Because the source term for the equations of motion of a conformally coupled scalar field, such as the dilaton, is given by the trace of the matter energy momentum tensor, it is commonly assumed to vanish during the radiation dominated epoch in the early universe. As a consequence, such fields are generally frozen in the early universe. Here we compute the finite temperature radiative correction to the source term and discuss its consequences on the evolution of such fields in the early universe. We discuss in particular, the case of scalar tensor theories of gravity which have general relativity as an attractor solution. We show that, in some cases, the universe can experience an early phase of contraction, followed by a non-singular bounce, and standard expansion. This can have interesting consequences for the abundance of thermal relics; for instance, it can provide a solution to the gravitino problem. We conclude by discussing the possible consequences of the quantum corrections to the evolution of the dilaton.
Interatomic Coulombic decay in two coupled quantum wells
NASA Astrophysics Data System (ADS)
Goldzak, Tamar; Gantz, Liron; Gilary, Ido; Bahir, Gad; Moiseyev, Nimrod
2015-04-01
Interatomic Coulombic decay (ICD) is a relaxation process induced by electronic correlation. In this work we study the ICD process in a two coupled quantum wells (QWs) nanostructure. We study a simple one-dimensional effective potential using experimental parameters of the semiconductor QW layers, i.e., using the single-band effective-mass approximation. In our calculations we consider the discontinuity of the effective mass of the electron in each of the QW layers. We control the ICD lifetime by changing the distance between the two wells. The expected overall trend is a decrease of ICD lifetime with a decrease in the distance between the wells. We show that the distance can be tuned such that the emitted ICD electron is trapped in a metastable state in the con-tinuum, i.e., a one-electron resonance state. This causes the lifetime of the ICD to be an order of magnitude smaller even at very long distances, and improves the efficiency of the ICD process. For the ICD to be the dominant decay mechanism it must prevail over all other possible competitive decay processes. We have found that the lifetime of the ICD is on the time scale of picoseconds. Therefore, based on our results we can design an experiment that will observe the ICD phenomenon in QWs nanostructure. This work can lead to a design of a wavelength-sensitive detector which is efficient even at low intensities.
Exciton-plasmon coupling of a single quantum dot and a metal nanowire
NASA Astrophysics Data System (ADS)
Wei, Hong
2016-11-01
The interactions between surface plasmons in metal nanostructures and excitons in quantum emitters lead to many interesting phenomena that are strongly dependent on the quantum yield of surface plasmons. The experimental measurement of this quantum yield is hindered due to the difficulty in distinguishing all the possible exciton recombination channels. By utilizing the propagation of surface plasmons, we experimentally measured the decay rates of all exciton recombination channels, and thus obtained the quantum yield of single surface plasmons generated by a quantum dot coupled with a silver nanowire.
Bandyopadhyay, S.; Das, B.; Miller, A.E.; Eastman, J.A.
1994-05-01
The idea of using the spin of an electron to encode binary bit information was proposed by Feynman in the 1960s. In this paper the authors show that an array of quantum dots, each containing a single electron, can realize a quantum coupled computer architecture where each electron acts as a bistable logic device and its spin encodes its logic state. Such an architecture has many advantages such as (1) the absence of physical interconnects between devices, (b) ultrafast switching times (spin flip times) of {approximately}1 picosecond for individual devices, (c) extremely high bit density approaching 10 Terabits/cm{sup 2}, (4) non-volatile memory, (5) robustness and possible room temperature operation with very high noise margin and reliability, (6) a very low power delay product for switching between logic levels, and (7) a very small power dissipation of a few tens of nanowatts per switching event. In spite of the above advantages, the technology also has some serious drawbacks in that the fan-out of individual logic devices may be small, wiring crossover is very problematic and the devices themselves have no inherent gain so that isolation between input and output is a problem. The authors discuss these problems, and where possible, offer plausible solutions.
Distance and coupling dependence of entanglement in the presence of a quantum field
NASA Astrophysics Data System (ADS)
Hsiang, J.-T.; Hu, B. L.
2015-12-01
We study the entanglement between two coupled detectors, the internal degrees of freedom of which are modeled by harmonic oscillators, interacting with a common quantum field, paying special attention to two less studied yet important features: finite separation and direct coupling. Distance dependence is essential in quantum teleportation and relativistic quantum information considerations. The presence of a quantum field as the environment accords an indirect interaction between the two oscillators at finite separation of a non-Markovian nature which competes with the direct coupling between them. The interplay between these two factors results in a rich variety of interesting entanglement behaviors at late times. We show that the entanglement behavior reported in prior work assuming no separation between the detectors can at best be a transient effect at very short times and claims that such behaviors represent late-time entanglement are misplaced. Entanglement between the detectors with direct coupling enters in the consideration of macroscopic quantum phenomena and other frontline issues. We find that with direct coupling entanglement between the two detectors can sustain over a finite distance, in contrast to the no direct coupling case reported before, where entanglement cannot survive at a separation more than a few inverse high-frequency cutoff scales. This work provides a functional platform for systematic investigations into the entanglement behavior of continuous variable quantum systems, such as used in quantum electro- and optomechanics.
Anomalous diamagnetic shifts in InP-GaP lateral quantum-wires.
Shin, Y H; Choi, B K; Kim, Yongmin; Song, J D; Nakamura, D; Matsuda, Y H; Takeyama, S
2015-11-02
Linearly polarized photoluminescence (PL) measurements were carried out on InP-GaP lateral nanowires grown using a lateral composition modulation method in pulsed magnetic fields up to ∼ 50 T. In these structures, the energy band alignment becomes type-I and type-II in In-rich wire and Ga-rich barrier regions, respectively. It is revealed that the polarization of the type-I PL is oriented along the [11̄0] crystal direction, whereas that of the type-II PL is along the [110] direction in the absence of magnetic field. These two different PL peaks exhibit anomalous energy shifts with respect to the direction of the magnetic field due to the variation of the confined energy in the exciton center of mass potential.
Quantum phases of Yang-Mills matrix model coupled to fundamental fermions
NASA Astrophysics Data System (ADS)
Pandey, Mahul; Vaidya, Sachindeo
2017-02-01
By investigating the SU(2) Yang-Mills matrix model coupled to fundamental fermions in the adiabatic limit, we demonstrate quantum critical behaviour at special corners of the gauge field configuration space. The quantum scalar potential for the gauge field induced by the fermions diverges at the corners and is intimately related to points of enhanced degeneracy of the fermionic Hamiltonian. This in turn leads to superselection sectors in the Hilbert space of the gauge field, the ground states in different sectors being orthogonal to each other. As a consequence of our analysis, we show that 2-color quantum chromodynamics coupled to two Weyl fermions has three quantum phases. When coupled to a massless Dirac fermion, the number of quantum phases is four. One of these phases is the color-spin locked phase.
Spin-orbit coupled molecular quantum magnetism realized in inorganic solid.
Park, Sang-Youn; Do, S-H; Choi, K-Y; Kang, J-H; Jang, Dongjin; Schmidt, B; Brando, Manuel; Kim, B-H; Kim, D-H; Butch, N P; Lee, Seongsu; Park, J-H; Ji, Sungdae
2016-09-21
Molecular quantum magnetism involving an isolated spin state is of particular interest due to the characteristic quantum phenomena underlying spin qubits or molecular spintronics for quantum information devices, as demonstrated in magnetic metal-organic molecular systems, the so-called molecular magnets. Here we report the molecular quantum magnetism realized in an inorganic solid Ba3Yb2Zn5O11 with spin-orbit coupled pseudospin-½ Yb(3+) ions. The magnetization represents the magnetic quantum values of an isolated Yb4 tetrahedron with a total (pseudo)spin 0, 1 and 2. Inelastic neutron scattering results reveal that a large Dzyaloshinsky-Moriya interaction originating from strong spin-orbit coupling of Yb 4f is a key ingredient to explain magnetic excitations of the molecular magnet states. The Dzyaloshinsky-Moriya interaction allows a non-adiabatic quantum transition between avoided crossing energy levels, and also results in unexpected magnetic behaviours in conventional molecular magnets.
Spin–orbit coupled molecular quantum magnetism realized in inorganic solid
Park, Sang-Youn; Do, S.-H.; Choi, K.-Y.; Kang, J.-H.; Jang, Dongjin; Schmidt, B.; Brando, Manuel; Kim, B.-H.; Kim, D.-H.; Butch, N. P.; Lee, Seongsu; Park, J.-H.; Ji, Sungdae
2016-01-01
Molecular quantum magnetism involving an isolated spin state is of particular interest due to the characteristic quantum phenomena underlying spin qubits or molecular spintronics for quantum information devices, as demonstrated in magnetic metal–organic molecular systems, the so-called molecular magnets. Here we report the molecular quantum magnetism realized in an inorganic solid Ba3Yb2Zn5O11 with spin–orbit coupled pseudospin-½ Yb3+ ions. The magnetization represents the magnetic quantum values of an isolated Yb4 tetrahedron with a total (pseudo)spin 0, 1 and 2. Inelastic neutron scattering results reveal that a large Dzyaloshinsky–Moriya interaction originating from strong spin–orbit coupling of Yb 4f is a key ingredient to explain magnetic excitations of the molecular magnet states. The Dzyaloshinsky–Moriya interaction allows a non-adiabatic quantum transition between avoided crossing energy levels, and also results in unexpected magnetic behaviours in conventional molecular magnets. PMID:27650796
Hybrid annealing: Coupling a quantum simulator to a classical computer
NASA Astrophysics Data System (ADS)
Graß, Tobias; Lewenstein, Maciej
2017-05-01
Finding the global minimum in a rugged potential landscape is a computationally hard task, often equivalent to relevant optimization problems. Annealing strategies, either classical or quantum, explore the configuration space by evolving the system under the influence of thermal or quantum fluctuations. The thermal annealing dynamics can rapidly freeze the system into a low-energy configuration, and it can be simulated well on a classical computer, but it easily gets stuck in local minima. Quantum annealing, on the other hand, can be guaranteed to find the true ground state and can be implemented in modern quantum simulators; however, quantum adiabatic schemes become prohibitively slow in the presence of quasidegeneracies. Here, we propose a strategy which combines ideas from simulated annealing and quantum annealing. In such a hybrid algorithm, the outcome of a quantum simulator is processed on a classical device. While the quantum simulator explores the configuration space by repeatedly applying quantum fluctuations and performing projective measurements, the classical computer evaluates each configuration and enforces a lowering of the energy. We have simulated this algorithm for small instances of the random energy model, showing that it potentially outperforms both simulated thermal annealing and adiabatic quantum annealing. It becomes most efficient for problems involving many quasidegenerate ground states.
NASA Astrophysics Data System (ADS)
Bhowmik, Dhrubajyoti; Saha, Apu Kr; Dutta, Paramartha; Nandi, Supratim
2017-08-01
Quantum-dot Cellular Automata (QCA) is one of the most substitutes developing nanotechnologies for electronic circuits, as a result of lower force utilization, higher speed and smaller size in correlation with CMOS innovation. The essential devices, a Quantum-dot cell can be utilized to logic gates and wires. As it is the key building block on nanotechnology circuits. By applying simple gates, the hardware requirements for a QCA circuit can be decreased and circuits can be less complex as far as level, delay and cell check. This article exhibits an unobtrusive methodology for actualizing novel upgraded simple and universal gates, which can be connected to outline numerous variations of complex QCA circuits. Proposed gates are straightforward in structure and capable as far as implementing any digital circuits. The main aim is to build all basic and universal gates in a simple circuit with and without crossbar-wire. Simulation results and physical relations affirm its handiness in actualizing each advanced circuit.
Gupta, Shilpi; Waks, Edo
2013-12-02
We demonstrate spontaneous emission rate enhancement and saturable absorption of cadmium selenide colloidal quantum dots coupled to a nanobeam photonic crystal cavity. We perform time-resolved lifetime measurements and observe an average enhancement of 4.6 for the spontaneous emission rate of quantum dots located at the cavity as compared to those located on an unpatterned surface. We also demonstrate that the cavity linewidth narrows with increasing pump intensity due to quantum dot saturable absorption.
Single quantum dot coupled to a scanning optical antenna: a tunable superemitter.
Farahani, J N; Pohl, D W; Eisler, H-J; Hecht, B
2005-07-01
The interaction of a single quantum dot with a bowtie antenna is demonstrated for visible light. The antenna is generated at the apex of a Si3N4 atomic force microscopy tip by focused ion beam milling. When scanned over the quantum dot, its photoluminescence is enhanced while its excited-state lifetime is decreased. Our observations demonstrate that the relaxation channels of a single quantum emitter can be controlled by coupling to an efficiently radiating metallic nanoantenna.
A novel framework of classical and quantum prisoner’s dilemma games on coupled networks
NASA Astrophysics Data System (ADS)
Deng, Xinyang; Zhang, Qi; Deng, Yong; Wang, Zhen
2016-03-01
Evolutionary games on multilayer networks are attracting growing interest. While among previous studies, the role of quantum games in such a infrastructure is still virgin and may become a fascinating issue across a myriad of research realms. To mimick two kinds of different interactive environments and mechanisms, in this paper a new framework of classical and quantum prisoner’s dilemma games on two-layer coupled networks is considered. Within the proposed model, the impact of coupling factor of networks and entanglement degree in quantum games on the evolutionary process has been studied. Simulation results show that the entanglement has no impact on the evolution of the classical prisoner’s dilemma, while the rise of the coupling factor obviously impedes cooperation in this game, and the evolution of quantum prisoner’s dilemma is greatly impacted by the combined effect of entanglement and coupling.
Deterministic Coupling of Quantum Emitters in 2D Materials to Plasmonic Nanocavity Arrays.
Tran, Toan Trong; Wang, Danqing; Xu, Zai-Quan; Yang, Ankun; Toth, Milos; Odom, Teri W; Aharonovich, Igor
2017-04-12
Quantum emitters in two-dimensional materials are promising candidates for studies of light-matter interaction and next generation, integrated on-chip quantum nanophotonics. However, the realization of integrated nanophotonic systems requires the coupling of emitters to optical cavities and resonators. In this work, we demonstrate hybrid systems in which quantum emitters in 2D hexagonal boron nitride (hBN) are deterministically coupled to high-quality plasmonic nanocavity arrays. The plasmonic nanoparticle arrays offer a high-quality, low-loss cavity in the same spectral range as the quantum emitters in hBN. The coupled emitters exhibit enhanced emission rates and reduced fluorescence lifetimes, consistent with Purcell enhancement in the weak coupling regime. Our results provide the foundation for a versatile approach for achieving scalable, integrated hybrid systems based on low-loss plasmonic nanoparticle arrays and 2D materials.
A novel framework of classical and quantum prisoner's dilemma games on coupled networks.
Deng, Xinyang; Zhang, Qi; Deng, Yong; Wang, Zhen
2016-03-15
Evolutionary games on multilayer networks are attracting growing interest. While among previous studies, the role of quantum games in such a infrastructure is still virgin and may become a fascinating issue across a myriad of research realms. To mimick two kinds of different interactive environments and mechanisms, in this paper a new framework of classical and quantum prisoner's dilemma games on two-layer coupled networks is considered. Within the proposed model, the impact of coupling factor of networks and entanglement degree in quantum games on the evolutionary process has been studied. Simulation results show that the entanglement has no impact on the evolution of the classical prisoner's dilemma, while the rise of the coupling factor obviously impedes cooperation in this game, and the evolution of quantum prisoner's dilemma is greatly impacted by the combined effect of entanglement and coupling.
Rodrigues, Joao P.; Zaidi, Alia
2010-10-15
We derive a planar sector of the large N nonsupersymmetric background of the quantum mechanical Hamiltonian of two Hermitian matrices coupled via a Yang-Mills interaction, in terms of the density of eigenvalues of one of the matrices. This background satisfies an implicit nonlinear integral equation, with a perturbative small coupling expansion and a solvable large coupling solution, which is obtained. The energy of system and the expectation value of several correlators are obtained in this strong coupling limit. They are free of infrared divergences.
Quantum Hysteresis in Coupled Light-Matter Systems
NASA Astrophysics Data System (ADS)
Gómez-Ruiz, Fernando; Acevedo, Oscar; Quiroga, Luis; Rodríguez, Ferney; Johnson, Neil
2016-09-01
We investigate the non-equilibrium quantum dynamics of a canonical light-matter system, namely the Dicke model, when the light-matter interaction is ramped up and down through a cycle across the quantum phase transition. Our calculations reveal a rich set of dynamical behaviors determined by the cycle times, ranging from the slow, near adiabatic regime through to the fast, sudden quench regime. As the cycle time decreases, we uncover a crossover from an oscillatory exchange of quantum information between light and matter that approaches a reversible adiabatic process, to a dispersive regime that generates large values of light-matter entanglement. The phenomena uncovered in this work have implications in quantum control, quantum interferometry, as well as in quantum information theory.
Quantum and classical chaos in kicked coupled Jaynes-Cummings cavities
Hayward, A. L. C.; Greentree, Andrew D.
2010-06-15
We consider two Jaynes-Cummings cavities coupled periodically with a photon hopping term. The semiclassical phase space is chaotic, with regions of stability over some ranges of the parameters. The quantum case exhibits dynamic localization and dynamic tunneling between classically forbidden regions. We explore the correspondence between the classical and quantum phase space and propose an implementation in a circuit QED system.
Dynamic phase response and amplitude-phase coupling of self-assembled semiconductor quantum dots
NASA Astrophysics Data System (ADS)
Lingnau, Benjamin; Herzog, Bastian; Kolarczik, Mirco; Woggon, Ulrike; Lüdge, Kathy; Owschimikow, Nina
2017-06-01
The optical excitation of semiconductor gain media introduces both gain and refractive index changes, commonly referred to as amplitude-phase coupling. Quantum-confined structures with an energetically well separated carrier reservoir usually exhibit a decreased amplitude-phase coupling compared to bulk materials. However, its magnitude and definition is still controversially discussed. We investigate the fundamental processes influencing the amplitude-phase coupling in semiconductor quantum-dot media using a coupled-carrier rate-equation model. We are able to analyze the dependence on the electronic structure and suggest routes towards an optimization of the dynamic phase response of the gain material.
Optical absorption in semiconductor quantum dots coupling to dispersive phonons of infinite modes
NASA Astrophysics Data System (ADS)
Ding, Zhiwen; Wang, Qin; Zheng, Hang
2012-10-01
Optical absorption spectrum of semiconductor quantum dot is investigated by means of an analytical approach based on the Green's function for different forms of coupling strength in an unified method by using the standard model with valence and conduction band levels coupled to dispersive quantum phonons of infinite modes. The analytical expression of the optical absorption coefficient in semiconductor quantum dots is obtained and by this expression the line shape and the peak position of the absorption spectrum are procured. The relation between the properties of absorption spectrum and the forms of coupling strength is clarified, which can be referenced for choosing the proper form of the coupling strength or spectral density to control the features of absorption spectrum of quantum dot. The coupling and confinement induced energy shift and intensity decrease in the absorption spectrum are determined precisely for a wide range of parameters. The results show that the activation energy of the optical absorption is reduced by the effect of exciton-phonon coupling and photons with lower frequencies could also be absorbed in absorption process. With increase of the coupling constant, the line shape of optical absorption spectrum broadens and the peak position moves to lower photon energy with a rapid decrease in intensity at the same time. Both the coupling induced red shift and the confinement induced blue shift conduce to decrease in the intensity of absorption spectrum. Furthermore, this method may have application potential to other confined quantum systems.
High fidelity quantum state transfer in electromechanical systems with intermediate coupling
Zhou, Jian; Hu, Yong; Yin, Zhang-qi; Wang, Z. D.; Zhu, Shi-Liang; Xue, Zheng-Yuan
2014-01-01
Hybrid quantum systems usually consist of two or more subsystems, which may take the advantages of the different systems. Recently, the hybrid system consisting of circuit electromechanical subsystems have attracted great attention due to its advanced fabrication and scalable integrated photonic circuit techniques. Here, we propose a scheme for high fidelity quantum state transfer between a superconducting qubit and a nitrogen-vacancy center in diamond, which are coupled to a superconducting transmission-line resonator with coupling strength g1 and a nanomechanical resonator with coupling strength g2, respectively. Meanwhile, the two resonators are parametrically coupled with coupling strength J. The system dynamics, including the decoherence effects, is numerical investigated. It is found that both the small () and large () coupling regimes of this hybrid system can not support high fidelity quantum state transfer before significant technique advances. However, in the intermediate coupling regime (J ~ g1 ~ g2), in contrast to a conventional wisdom, high fidelity quantum information transfer can be implemented, providing a promising route towards high fidelity quantum state transfer in similar coupled resonators systems. PMID:25168206
High fidelity quantum state transfer in electromechanical systems with intermediate coupling.
Zhou, Jian; Hu, Yong; Yin, Zhang-qi; Wang, Z D; Zhu, Shi-Liang; Xue, Zheng-Yuan
2014-08-29
Hybrid quantum systems usually consist of two or more subsystems, which may take the advantages of the different systems. Recently, the hybrid system consisting of circuit electromechanical subsystems have attracted great attention due to its advanced fabrication and scalable integrated photonic circuit techniques. Here, we propose a scheme for high fidelity quantum state transfer between a superconducting qubit and a nitrogen-vacancy center in diamond, which are coupled to a superconducting transmission-line resonator with coupling strength g1 and a nanomechanical resonator with coupling strength g2, respectively. Meanwhile, the two resonators are parametrically coupled with coupling strength J. The system dynamics, including the decoherence effects, is numerical investigated. It is found that both the small (J<{g1,g2}) and large (J>{g1,g2}) coupling regimes of this hybrid system can not support high fidelity quantum state transfer before significant technique advances. However, in the intermediate coupling regime (J ~ g1 ~ g2), in contrast to a conventional wisdom, high fidelity quantum information transfer can be implemented, providing a promising route towards high fidelity quantum state transfer in similar coupled resonators systems.
NASA Astrophysics Data System (ADS)
Borzdov, Andrei V.; Borzdov, Vladimir M.; V'yurkov, Vladimir V.
2016-12-01
Ensemble Monte Carlo simulation of electron transport in GaAs/AlAs quantum wire transistor structure is performed. The response of electron drift velocity on the action of harmonic longitudinal electric field is calculated for several values of electric field strength amplitude and gate bias at 77 and 300 K. The periodical electric field has a 1 THz frequency. The nonlinear behaviour of electron drift velocity due to scattering processes is observed.
Quantum Photonic in Hybrid Cavity Systems with Strong Matter-Light Couplings
2015-08-24
AFRL-AFOSR-VA-TR-2015-0246 Quantum Photonic in Hybrid Cavity Systems with Strong Matter- Light Couplings Hui Deng UNIVERSITY OF MICHIGAN Final Report...TITLE AND SUBTITLE Quantum Photonic in Hybrid Cavity Systems with Strong Matter- Light Couplings 5a. CONTRACT NUMBER 5b. GRANT NUMBER FA9550-12-1-0256...photons was demonstrated in the designable microcavity structure for the first time, establishing a robust light -matter hybrid states with designable
Abdelkader, Elseddik M; Jelliss, Paul A; Buckner, Steven W
2015-06-15
In this study, metal-containing nanoparticles (NPs) were produced using electrical explosion of wires (EEW) in organic solvents. The explosion chamber was constructed from Teflon to withstand the shockwave, allow growth and reaction of the incipient NPs in various organic solvents containing dissolved ligands, and allow a constant flow of argon to maintain an inert environment. A survey of different transition d-block metals was conducted with metals from groups 4-8, affording metal carbide NPs, while metals from groups 9-12 gave elemental metallic NPs. Tungsten carbide phase WC1-x, which has not been previously isolated as a single-phase material, was exclusively formed during EEW. We used polymerization initiation by electron-rich metallic nanoparticles (PIERMEN) as a capping technique for the nascent NPs with an alkyl epoxide employed as the monomers. Transmission electron microscopy showed spherical particles with the metallic core embedded in a polymer matrix with predominantly smaller particles (<50 nm), but also a broad size distribution with some larger particles (>100 nm). Powder X-ray diffraction (PXRD) was used to confirm the identity of the metallic NPs. The capping agents were characterized using ATR-FTIR spectroscopy. No evidence is observed for the formation of crystalline oxides during EEW for any metals used. Differential scanning calorimetry/thermal gravimetric analysis was used to study the NP's behavior upon heating under an air flow up to 800 °C with the product oxides characterized by PXRD. The bifurcation between metal-carbide NPs and metal NPs correlates with the enthalpy of formation of the product carbides. We observed PIERMEN capping of elemental metal NPs only when the metal has negative standard electrode potentials (relative to a bis(biphenyl) chromium(I)/(0) reference electrode).
NASA Astrophysics Data System (ADS)
Filippone, Michele; Brouwer, Piet W.
2016-12-01
Tunneling between a point contact and a one-dimensional wire is usually described with the help of a tunneling Hamiltonian that contains a δ function in position space. Whereas the leading-order contribution to the tunneling current is independent of the way this δ function is regularized, higher-order corrections with respect to the tunneling amplitude are known to depend on the regularization. Instead of regularizing the δ function in the tunneling Hamiltonian, one may also obtain a finite tunneling current by invoking the ultraviolet cutoffs in a field-theoretic description of the electrons in the one-dimensional conductor, a procedure that is often used in the literature. For the latter case, we show that standard ultraviolet cutoffs lead to different results for the tunneling current in fermionic and bosonized formulations of the theory, when going beyond leading order in the tunneling amplitude. We show how to recover the standard fermionic result using the formalism of functional bosonization and revisit the tunneling current to leading order in the interacting case.
Diruthenium-polyyn-diyl-diruthenium wires: electronic coupling in the long distance regime.
Cao, Zhi; Xi, Bin; Jodoin, Diane S; Zhang, Lei; Cummings, Steven P; Gao, Yang; Tyler, Sarah F; Fanwick, Phillip E; Crutchley, Robert J; Ren, Tong
2014-08-27
Reported herein is a series of Ru2(Xap)4 capped polyyn-diyl compounds, where Xap is either 2-anilinopyridinate (ap) or its aniline substituted derivatives. Symmetric [Ru2(Xap)4](μ-C4k)[Ru2(Xap)4] (compounds 4ka (X = 3-isobutoxy) and 4kc (X = 3,5-dimethoxy) with k = 2, 3, 4, and 5) was obtained from the Glaser coupling reaction of Ru2(Xap)4(C2kH). Unsymmetric [Ru2(Xap)4](μ-C(4k+2))[Ru2(ap)4] (compounds 4k+2b with k = 2, 3, and 4) were prepared from the Glaser coupling reaction between Ru2(Xap)4(C(2k+2)H) and Ru2(ap)4(C2kH). X-ray diffraction study of compound 12c revealed both the sigmoidal topology of the polyyn-diyl bridge and the fine structural detail about the Ru2 cores. Cyclic and differential pulse voltammetric (CV and DPV) measurements and spectroelectrochemical studies revealed that (i) the reduced monoanions [Ru2-C2m-Ru2](-1) (m = 4-8) belong to the Robin-Day class II mixed valent ions and (ii) the electronic coupling between Ru2 termini depends on the length of the polyyn-diyl bridge with an attenuation constant (γ) between 0.12 and 0.15 Å(-1). In addition, spin-unrestricted DFT calculations provide insight about the nature of orbitals that mediate the long distance electronic coupling.
Quantum Transfer Energy and Nonlocal Correlation in a Dimer with Time-Dependent Coupling Effect
NASA Astrophysics Data System (ADS)
El-Shishtawy, Reda M.; Berrada, K.; C. Haddon, Robert; Al-Hadeethi, Yas F.; H. Al-Heniti, Saleh.; Raffah, Bahaaudin M.
2017-05-01
The presence of coherence phenomenon due to the interference of probability amplitude terms, is one of the most important features of quantum mechanics theory. Recent experiments show the presence of quantum processes whose coherence provided over suddenly large interval-time. In particular, photosynthetic mechanisms in light-harvesting complexes provide oscillatory behaviors in quantum mechanics due to quantum coherence. In this work, we investigate the coherent quantum transfer energy for a single-excitation and nonlocal correlation in a dimer system modelled by a two-level atom system with and without time-dependent coupling effect. We analyze and explore the required conditions that are feasible with real experimental realization for optimal transfer of quantum energy and generation of nonlocal quantum correlation. We show that the enhancement of the probability for a single-excitation transfer energy is greatly benefits from the combination of the energy detuning and time-dependent coupling effect. We investigate the presence of quantum correlations in the dimer using the entanglement of formation. We also find that the entanglement between the donor and acceptor is very sensitive to the physical parameters and it can be generated during the coherent energy transfer. On the other hand, we study the dynamical behavior of the quantum variance when performing a measurement on an observable of the density matrix operator. Finally, an interesting relationship between the transfer probability, entanglement and quantum variance is explored during the time evolution in terms of the physical parameters.
Quantum Transfer Energy and Nonlocal Correlation in a Dimer with Time-Dependent Coupling Effect
NASA Astrophysics Data System (ADS)
El-Shishtawy, Reda M.; Berrada, K.; Haddon, Robert C.; Al-Hadeethi, Yas F.; Al-Heniti, Saleh H.; Raffah, Bahaaudin M.
2017-02-01
The presence of coherence phenomenon due to the interference of probability amplitude terms, is one of the most important features of quantum mechanics theory. Recent experiments show the presence of quantum processes whose coherence provided over suddenly large interval-time. In particular, photosynthetic mechanisms in light-harvesting complexes provide oscillatory behaviors in quantum mechanics due to quantum coherence. In this work, we investigate the coherent quantum transfer energy for a single-excitation and nonlocal correlation in a dimer system modelled by a two-level atom system with and without time-dependent coupling effect. We analyze and explore the required conditions that are feasible with real experimental realization for optimal transfer of quantum energy and generation of nonlocal quantum correlation. We show that the enhancement of the probability for a single-excitation transfer energy is greatly benefits from the combination of the energy detuning and time-dependent coupling effect. We investigate the presence of quantum correlations in the dimer using the entanglement of formation. We also find that the entanglement between the donor and acceptor is very sensitive to the physical parameters and it can be generated during the coherent energy transfer. On the other hand, we study the dynamical behavior of the quantum variance when performing a measurement on an observable of the density matrix operator. Finally, an interesting relationship between the transfer probability, entanglement and quantum variance is explored during the time evolution in terms of the physical parameters.
Atomically thin cesium lead bromide perovskite quantum wires with high luminescence.
Huang, Hongwen; Liu, Mei; Li, Jing; Luo, Laihao; Zhao, Jiangtao; Luo, Zhenlin; Wang, Xiaoping; Ye, Zhizhen; He, Haiping; Zeng, Jie
2017-01-07
We report a room-temperature colloidal synthesis of few-unit-cell-thick CsPbBr3 QWs with lengths over a hundred nanometers. The surfactant-directed oriented attachment growth mechanism was proposed to explain the formation of such CsPbBr3 QWs. Owing to the strong quantum confinement effect, the photoluminescence (PL) emission peak of few-unit-cell-thick CsPbBr3 QWs blue-shifted to 430 nm. The ensemble PL quantum yield (PLQY) of the few-unit-cell-thick CsPbBr3 QWs increased to 21.13% through a simple heat-treatment process. The improvement of PLQY was ascribed to the reduction of the density of surface trap states and defect states induced by the heat-treatment process. Notably, the dependence of the bandgap on the diameter with different numbers of unit cells was presented for the first time in 1-D CsPbBr3 QWs on the basis of the produced few-unit-cell-thick CsPbBr3 QWs.
Faithful conditional quantum state transfer between weakly coupled qubits
Miková, M.; Straka, I.; Mičuda, M.; Krčmarský, V.; Dušek, M.; Ježek, M.; Fiurášek, J.; Filip, R.
2016-01-01
One of the strengths of quantum information theory is that it can treat quantum states without referring to their particular physical representation. In principle, quantum states can be therefore fully swapped between various quantum systems by their mutual interaction and this quantum state transfer is crucial for many quantum communication and information processing tasks. In practice, however, the achievable interaction time and strength are often limited by decoherence. Here we propose and experimentally demonstrate a procedure for faithful quantum state transfer between two weakly interacting qubits. Our scheme enables a probabilistic yet perfect unidirectional transfer of an arbitrary unknown state of a source qubit onto a target qubit prepared initially in a known state. The transfer is achieved by a combination of a suitable measurement of the source qubit and quantum filtering on the target qubit depending on the outcome of measurement on the source qubit. We experimentally verify feasibility and robustness of the transfer using a linear optical setup with qubits encoded into polarization states of single photons. PMID:27562544
Nature of the many-body excitations in a quantum wire: Theory and experiment
NASA Astrophysics Data System (ADS)
Tsyplyatyev, O.; Schofield, A. J.; Jin, Y.; Moreno, M.; Tan, W. K.; Anirban, A. S.; Ford, C. J. B.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.
2016-02-01
The natural excitations of an interacting one-dimensional system at low energy are the hydrodynamic modes of a Luttinger liquid, protected by the Lorentz invariance of the linear dispersion. We show that beyond low energies, where the quadratic dispersion reduces the symmetry to Galilean, the main character of the many-body excitations changes into a hierarchy: calculations of dynamic correlation functions for fermions (without spin) show that the spectral weights of the excitations are proportional to powers of R2/L2 , where R is a length-scale related to interactions and L is the system length. Thus only small numbers of excitations carry the principal spectral power in representative regions on the energy-momentum planes. We have analyzed the spectral function in detail and have shown that the first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalized single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power laws at the spectral edge. We have illustrated a crossover to a Luttinger liquid at low energy by calculating the local density of states through all energy scales: from linear to nonlinear, and to above the chemical potential energies. In order to test this model, we have carried out experiments to measure the momentum-resolved tunneling of electrons (fermions with spin) from/to a wire formed within a GaAs heterostructure. We observe a well-resolved spin-charge separation at low energy with appreciable interaction strength and only a parabolic dispersion of the first-level mode at higher energies. We find a structure resembling the second-level excitations, which dies away rapidly at high momentum in line with the theoretical predictions here.
Generation of spin-dependent coherent states in a quantum wire
NASA Astrophysics Data System (ADS)
Pawłowski, J.; Szumniak, P.; Bednarek, S.
2016-10-01
We propose an all-electrically controlled nanodevice—a gated semiconductor nanowire—capable of generating a coherent state of a single electron trapped in a harmonic oscillator or superposition of such coherent states—the Schrödinger cat state. In the proposed scheme, the electron in the ground state of the harmonic potential is driven by resonantly oscillating Rashba spin-orbit coupling. This allows for the creation of the Schrödinger cat state with superposition amplitudes depending on the initial electron spin state. Such a method can be used for initialization of a single-spin qubit defined in a coherent state. The harmonic confinement potential along the InSb nanowire and the modulation of the Rashba spin-orbit coupling is obtained by proper gating. The results are supported by realistic three-dimensional time-dependent self-consistent Poisson-Schrödinger calculations.
Spectral properties of a strongly coupled quantum-dot-metal-nanoparticle system
NASA Astrophysics Data System (ADS)
Hakami, Jabir; Wang, Ligang; Zubairy, M. Suhail
2014-05-01
We investigate the coherent control of the quantum optical properties of a quantum dot coupled to a metallic nanoparticle using a photon Green's function method, which is based on the exact quantization of the electromagnetic fields in a dissipative medium. The properties of the spontaneous emission spectra of such a system are studied in detail with and without involving the coherent field. The Rabi splitting effect in the spectrum emitted by the quantum dot under particular conditions is predicted for different sizes of the metal nanoparticles. We show that the spontaneous emission spectra of the transition coupled to surface plasmons may be further modified by adjusting the external coherent control on the adjacent transitions. Furthermore, the pronounced oscillatory behavior for the quantum-dot dynamics is demonstrated with the presence of the metal nanoparticle by the non-Markovian treatment. Our results may have potential applications in plasmonic-based quantum manipulation.
Leaci, Paola; Ortolan, Antonello
2007-12-15
We discuss limitations in precision measurements of a weak classical force coupled to quantum mechanical systems, the so-called standard quantum limit (SQL). Among the several contexts exploiting the measurement of classical signals, gravitational wave (GW) detection is of paramount importance. In this framework, we analyze the quantum limited sensitivity of a free test mass, a quantum mechanical harmonic oscillator, two harmonic oscillators with equal masses and different resonance frequencies, and finally two mechanical oscillators with different masses and resonating at the same frequency. The sensitivity analysis of the latter two cases illustrates the potentialities of back-action reduction and classical impedance matching schemes, respectively. By examining coupled quantum oscillators as detectors of classical signals, we found a viable path to approach the SQL for planned or operating GW detectors, such as DUAL and AURIGA.
Holonomic quantum computation in the ultrastrong-coupling regime of circuit QED
NASA Astrophysics Data System (ADS)
Wang, Yimin; Zhang, Jiang; Wu, Chunfeng; You, J. Q.; Romero, G.
2016-07-01
We present an experimentally feasible scheme to implement holonomic quantum computation in the ultrastrong-coupling regime of light-matter interaction. The large anharmonicity and the Z2 symmetry of the quantum Rabi model allow us to build an effective three-level Λ -structured artificial atom for quantum computation. The proposed physical implementation includes two gradiometric flux qubits and two microwave resonators where single-qubit gates are realized by a two-tone driving on one physical qubit, and a two-qubit gate is achieved with a time-dependent coupling between the field quadratures of both resonators. Our work paves the way for scalable holonomic quantum computation in ultrastrongly coupled systems.
Band structures of cylindrical AlN/GaN quantum dots with fully coupled piezoelectric models
NASA Astrophysics Data System (ADS)
Prabhakar, Sanjay; Melnik, Roderick
2010-08-01
We study the coupled electro-mechanical effects in the band structure calculations of low dimensional semiconductor nanostructures (LDSNs) such as AlN/GaN quantum dots. Some effects in these systems are essentially nonlinear. Strain, piezoelectric effects, eigenvalues and wave functions of a quantum dot have been used as tuning parameters for the optical response of LDSNs in photonics, band gap engineering and other applications. However, with a few noticeable exceptions, the influence of piezoelectric effects in the electron wave functions in Quantum Dots (QDs) studied with fully coupled models has been largely neglected in the literature. In this paper, by using the fully coupled model of electroelasticity, we analyze the piezoelectric effects into the band structure of cylindrical quantum dots. Results are reported for III-V type semiconductors with a major focus given to AlN/GaN based QD systems.
Out-of-equilibrium quantum dot coupled to a microwave cavity
NASA Astrophysics Data System (ADS)
Dmytruk, Olesia; Trif, Mircea; Mora, Christophe; Simon, Pascal
2016-02-01
We consider a superconducting microwave cavity capacitively coupled to both a quantum conductor and its electronic reservoirs. We analyze in detail how the measurements of the cavity microwave field, which are related to the electronic charge susceptibility, can be used to extract information on the transport properties of the quantum conductor. We show that the asymmetry of the capacitive couplings between the electronic reservoirs and the cavity plays a crucial role in relating optical measurements to transport properties. For asymmetric capacitive couplings, photonic measurements can be used to probe the finite low-frequency admittance of the quantum conductor, the real part of which is related to the differential conductance. In particular, when the quantum dot is far from resonance, the charge susceptibility is directly proportional to the admittance for a large range of frequencies and voltages. However, when the quantum conductor is near resonance, such a relation generally holds only at low frequency and for equal tunnel coupling or low voltage. Beyond this low-energy near-equilibrium regime, the charge susceptibility and thus the optical transmission offer new insights into the quantum conductors since the optical observables are not directly connected to transport quantities. For symmetric lead capacitive couplings, we show that the optical measurements can be used to reveal the Korringa-Shiba relation, connecting the reactive to the dissipative part of the susceptibility, at low frequency and low bias.
Coupled field induced conversion between destructive and constructive quantum interference
NASA Astrophysics Data System (ADS)
Jiang, Xiangqian; Sun, Xiudong
2016-12-01
We study the control of quantum interference in a four-level atom driven by three coherent fields forming a closed loop. The spontaneous emission spectrum shows two sets of peaks which are dramatically influenced by the fields. Due to destructive quantum interference, a dark line can be observed in the emission spectrum, and the condition of the dark line is given. We found that the conversion between destructive and constructive quantum interference can be achieved through controlling the Rabi frequency of the external fields.
Coupled field induced conversion between destructive and constructive quantum interference
Jiang, Xiangqian Sun, Xiudong
2016-12-15
We study the control of quantum interference in a four-level atom driven by three coherent fields forming a closed loop. The spontaneous emission spectrum shows two sets of peaks which are dramatically influenced by the fields. Due to destructive quantum interference, a dark line can be observed in the emission spectrum, and the condition of the dark line is given. We found that the conversion between destructive and constructive quantum interference can be achieved through controlling the Rabi frequency of the external fields.
NASA Astrophysics Data System (ADS)
Kubo, T.; Tokura, Y.; Tarucha, S.
2010-01-01
We theoretically investigate spin-dependent electron transport through an Aharonov-Bohm-Casher interferometer containing a laterally coupled double quantum dot. In particular, we numerically calculate the Aharonov-Bohm and Aharonov-Casher oscillations of the linear conductance in the Kondo regime. We show that the AC oscillation in the Kondo regime deviates from the sinusoidal form.
NASA Astrophysics Data System (ADS)
Alonso-Álvarez, D.; Thomas, T.; Führer, M.; Hylton, N. P.; Ekins-Daukes, N. J.; Lackner, D.; Philipps, S. P.; Bett, A. W.; Sodabanlu, H.; Fujii, H.; Watanabe, K.; Sugiyama, M.; Nasi, L.; Campanini, M.
2014-08-01
Quantum wires (QWRs) form naturally when growing strain balanced InGaAs/GaAsP multi-quantum wells (MQW) on GaAs [100] 6° misoriented substrates under the usual growth conditions. The presence of wires instead of wells could have several unexpected consequences for the performance of the MQW solar cells, both positive and negative, that need to be assessed to achieve high conversion efficiencies. In this letter, we study QWR properties from the point of view of their performance as solar cells by means of transmission electron microscopy, time resolved photoluminescence and external quantum efficiency (EQE) using polarised light. We find that these QWRs have longer lifetimes than nominally identical QWs grown on exact [100] GaAs substrates, of up to 1 μs, at any level of illumination. We attribute this effect to an asymmetric carrier escape from the nanostructures leading to a strong 1D-photo-charging, keeping electrons confined along the wire and holes in the barriers. In principle, these extended lifetimes could be exploited to enhance carrier collection and reduce dark current losses. Light absorption by these QWRs is 1.6 times weaker than QWs, as revealed by EQE measurements, which emphasises the need for more layers of nanostructures or the use light trapping techniques. Contrary to what we expected, QWR show very low absorption anisotropy, only 3.5%, which was the main drawback a priori of this nanostructure. We attribute this to a reduced lateral confinement inside the wires. These results encourage further study and optimization of QWRs for high efficiency solar cells.