Chemla, D.S.
1993-06-30
This article reviews recent investigations of nonlinear optical processes in semiconductors. Section II discusses theory of coherent wave mixing in semiconductors, with emphasis on resonant excitation with only one exciton state. Section III reviews recent experimental investigations of amplitude and phase of coherent wave-mixing resonant with quasi-2d excitons in GaAs quantum wells.
Kim, T.; Liu, B.; Smith, R.; Athanasiou, M.; Gong, Y.; Wang, T.
2014-04-21
A “coherent” nanocavity structure has been designed on two-dimensional well-ordered InGaN/GaN nanodisk arrays with an emission wavelength in the green spectral region, leading to a massive enhancement in resonance mode in the green spectra region. By means of a cost-effective nanosphere lithography technique, we have fabricated such a structure on an InGaN/GaN multiple quantum well epiwafer and have observed the “coherent” nanocavity effect, which leads to an enhanced spontaneous emission (SE) rate. The enhanced SE rate has been confirmed by time resolved photoluminescence measurements. Due to the coherent nanocavity effect, we have achieved a massive improvement in internal quantum efficiency with a factor of 88, compared with the as-grown sample, which could be significant to bridge the “green gap” in solid-state lighting.
Narrow dark polariton due to coupled coherence in a quantum well microcavity
NASA Astrophysics Data System (ADS)
Wang, Tao; Li, Cui Li; Zhang, Rui; Zhuo, Zhong Chang; Su, Xue Mei
2015-10-01
A scheme is proposed to obtain slow light in a coulped quantum wells microcavity with tunneling induced transparency between intersubband electronic transitions. Three prolaritons are created by intracavity Fano interference between fundamental mode photon and two quantum oscillators of coherent subband electronic excitations. A narrow middle dark polariton of the three can be produced, which can be used to suppress the line profiles of the transmission or reflection spectra for the incident light. This leads to slow propagation of the incident light in the microcavity. The semiconductor optical microcavity can be an alternative choice of quantum photoelectronic devices in nanoscale.
Optical pump-probe measurements of local nuclear spin coherence in semiconductor quantum wells.
Sanada, H; Kondo, Y; Matsuzaka, S; Morita, K; Hu, C Y; Ohno, Y; Ohno, H
2006-02-17
We demonstrate local manipulation and detection of nuclear spin coherence in semiconductor quantum wells by an optical pump-probe technique combined with pulse rf NMR. The Larmor precession of photoexcited electron spins is monitored by time-resolved Kerr rotation (TRKR) as a measure of nuclear magnetic field. Under the irradiation of resonant pulsed rf magnetic fields, Rabi oscillations of nuclear spins are traced by TRKR signals. The intrinsic coherence time evaluated by a spin-echo technique reveals the dependence on the orientation of the magnetic field with respect to the crystalline axis as expected by the nearest neighbor dipole-dipole interaction. PMID:16606048
Coherent Coupling of Excitons and Trions in a Photoexcited CdTe/CdMgTe Quantum Well
NASA Astrophysics Data System (ADS)
Moody, G.; Akimov, I. A.; Li, H.; Singh, R.; Yakovlev, D. R.; Karczewski, G.; Wiater, M.; Wojtowicz, T.; Bayer, M.; Cundiff, S. T.
2014-03-01
We present zero-, one-, and two-quantum two-dimensional coherent spectra of excitons and trions in a CdTe/(Cd,Mg)Te quantum well. The set of spectra provides a unique and comprehensive picture of the coherent nonlinear optical response. Distinct peaks in the spectra are manifestations of exciton-exciton and exciton-trion coherent coupling. Excellent agreement using density matrix calculations highlights the essential role of many-body effects on the coupling. Strong exciton-trion coherent interactions open up the possibility for novel conditional control schemes in coherent optoelectronics.
Coherent coupling of excitons and trions in a photoexcited CdTe/CdMgTe quantum well.
Moody, G; Akimov, I A; Li, H; Singh, R; Yakovlev, D R; Karczewski, G; Wiater, M; Wojtowicz, T; Bayer, M; Cundiff, S T
2014-03-01
We present zero-, one-, and two-quantum two-dimensional coherent spectra of excitons and trions in a CdTe/(Cd,Mg)Te quantum well. The set of spectra provides a unique and comprehensive picture of the coherent nonlinear optical response. Distinct peaks in the spectra are manifestations of exciton-exciton and exciton-trion coherent coupling. Excellent agreement using density matrix calculations highlights the essential role of many-body effects on the coupling. Strong exciton-trion coherent interactions open up the possibility for novel conditional control schemes in coherent optoelectronics. PMID:24655274
Chen, Yuan; Deng, Li; Chen, Aixi
2015-02-15
We investigate the nonlinear optical phenomena of the optical bistability and multistability via spontaneously generated coherence in an asymmetric double quantum well structure coupled by a weak probe field and a controlling field. It is shown that the threshold and hysteresis cycle of the optical bistability can be conveniently controlled only by adjusting the intensity of the SGC or the controlling field. Moreover, switching between optical bistability and multistability can be achieved. These studies may have practical significance for the preparation of optical bistable switching device.
Quantum correlation via quantum coherence
NASA Astrophysics Data System (ADS)
Yu, Chang-shui; Zhang, Yang; Zhao, Haiqing
2014-06-01
Quantum correlation includes quantum entanglement and quantum discord. Both entanglement and discord have a common necessary condition—quantum coherence or quantum superposition. In this paper, we attempt to give an alternative understanding of how quantum correlation is related to quantum coherence. We divide the coherence of a quantum state into several classes and find the complete coincidence between geometric (symmetric and asymmetric) quantum discords and some particular classes of quantum coherence. We propose a revised measure for total coherence and find that this measure can lead to a symmetric version of geometric quantum correlation, which is analytic for two qubits. In particular, this measure can also arrive at a monogamy equality on the distribution of quantum coherence. Finally, we also quantify a remaining type of quantum coherence and find that for two qubits, it is directly connected with quantum nonlocality.
Spin coherence of the two-dimensional electron gas in a GaAs quantum well
Larionov, A. V.
2015-01-15
The coherent spin dynamics of the quasi-two-dimensional electron gas in a GaAs quantum well is experimentally investigated using the time-resolved spin Kerr effect in an optical cryostat with a split coil inducing magnetic fields of up to 6 T at a temperature of about 2 K. The electron spin dephasing times and degree of anisotropy of the spin relaxation of electrons are measured in zero magnetic field at different electron densities. The dependence of the spin-orbit splitting on the electron-gas density is established. In the integral quantum-Hall-effect mode, the unsteady behavior of the spin dephasing time of 2D electrons of the lower Landau spin sublevel near the odd occupation factor ν = 3 is found. The experimentally observed unsteady behavior of the spin dephasing time can be explained in terms of new-type cyclotron modes that occur in a liquid spin texture.
Multipulse operation and optical detection of nuclear spin coherence in a GaAs/AlGaAs quantum well.
Kondo, Y; Ono, M; Matsuzaka, S; Morita, K; Sanada, H; Ohno, Y; Ohno, H
2008-11-14
We demonstrate manipulation of nuclear spin coherence in a GaAs/AlGaAs quantum well by optically detected nuclear magnetic resonance (NMR). A phase shift of the Larmor precession of photoexcited electron spins is detected to read out the hyperfine-coupled nuclear spin polarization. Multipulse NMR sequences are generated to control the population and examine the phase coherence in quadrupolar-split spin-3/2 75As nuclei. The phase coherence among the multilevel nuclear spin states is addressed by application of pulse sequences that are used in quantum gate operations. PMID:19113379
Nanosecond spin coherence of excitons bound to acceptors in a CdTe quantum well
NASA Astrophysics Data System (ADS)
Grinberg, P.; Bernardot, F.; Eble, B.; Karczewski, G.; Testelin, C.; Chamarro, M.
2016-03-01
We have studied the coherent spin dynamics of excitons bound to acceptors, A0X, immersed in a CdTe quantum well by using time resolved photo-induced Faraday rotation. We have also measured the time-resolved differential transmission in order to determine a A0X lifetime of 220 ps, which is independent of the applied magnetic field. We show that at low magnetic field, the spin of A0X is completely frozen during a time, ≅ 4.5 ns, at least twenty times longer than its lifetime. We compare the spin properties of A0X with the spin properties of other charged excitons systems, and we conclude that the hyperfine interaction of the photo-created electron spin with nuclear spins is very likely to be at the origin of the observed spin dephasing times.
Real-time contrast-enhanced holographic imaging using phase coherent photorefractive quantum wells.
Dongol, A; Thompson, J; Schmitzer, H; Tierney, D; Wagner, H P
2015-05-18
We demonstrate wide-field real-time and depth-resolved contrast enhanced holographic imaging (CEHI) using the all-optical phase coherent photorefractive effect in ZnSe quantum wells. Moving objects are imaged at large depth-of-field by the local enhancement of a static reference hologram. The high refresh rate of the holographic films enables direct-to-video monitoring of floating glass beads and of living Paramecium and Euglena cells moving in water. Depth resolution is achieved by tilting the incident laser beam with respect to the normal of the cuvette. This creates double images of the objects, which are analyzed geometrically and with Fresnel diffraction theory. A two-color CEHI set-up further enables the visualization of a concealed 95 µm thick wire behind a thin layer of chicken skin. PMID:26074534
NASA Astrophysics Data System (ADS)
Kabir, A.; Dongol, A.; Wang, X.; Wagner, H. P.
2010-12-01
We demonstrate two real-time optical coherence imaging acquisition modes using all-optical phase coherent photorefractive ZnSe quantum wells as dynamic holographic films. These films use the coherence of excitons for time-gating which provides depth information of an object according to the brightness profile of its holographic image. This quality allows depth-resolved imaging of moving particles with a resolution of a few micrometers in a single-shot three-dimensional mode. In a complementary contrast-enhanced mode moving particles are imaged by the local enhancement of a static reference hologram, enabling optical coherence imaging at a large depth-of-field.
Phase control of Goos-Hänchen shift via biexciton coherence in a multiple quantum well
NASA Astrophysics Data System (ADS)
Asadpour, Seyyed Hossein; Nasehi, Rajab; Soleimani, H. Rahimpour; Mahmoudi, M.
2015-09-01
The behavior of the Goos-Hänchen (GH) shifts of the reflected and transmitted probe and signal pulses through a cavity containing four-level GaAs/AlGaAs multiple quantum wells with 15 periods of 17.5 nm GaAs wells and 15-nm Al0.3Ga0.7As barriers is theoretically discussed. The biexciton coherence set up by two coupling fields can induce the destructive interference to control the absorption and gain properties of probe field under appropriate conditions. It is realized that for the specific values of the intensities and the relative phase of applied fields, the simultaneous negative or positive GH shift in the transmitted and reflected light beam can be obtained via amplification in a probe light. It is found that by adjusting the controllable parameters, the GH shifts can be switched between the large positive and negative values in the medium. Moreover, the effect of exciton spin relaxation on the GH shift has also been discussed. We find that the exciton spin relaxation can manipulate the behavior of GH shift in the reflected and transmitted probe beam through the cavity. We show that by controlling the incident angles of probe beam and under certain conditions, the GH shifts in the reflected and transmitted probe beams can become either negative or positive corresponding to the superluminal or subluminal light propagation. Our proposed model may supply a new prospect in technological applications for the light amplification in optical sensors working on quantum coherence impacts in solid-state systems.
NASA Astrophysics Data System (ADS)
Busch, Alexander Anthony
2003-10-01
This thesis reports a systematic study of near-band edge linear and nonlinear optical properties of doped and undoped semiconductor multiple quantum well samples, aimed at quantifying and separating the numerous contributions to the overall material response from photon excitation. Information obtained from both linear absorption and nonlinear, degenerate four-wave-mixing experiments is compared with elaborate numerical simulations. Accurate measures of 1S--2S binding energies and dephasing rates as a function of temperature from 5 to 40 K is established. The biexciton binding energy and dephasing rate over the temperature range 5 to 40 K is measured and, by comparison with theories reported in the literature, it is found that localization effects have a significant influence on the biexciton binding energy in 5 nm quantum wells. The first systematic attempt to quantitatively account for the continuum contribution to nonlinear response by fitting a series of spectra obtained at various input laser pulse detunings was conducted. Unique evidence for coherent beating between multi-exciton/free electron complexes in lightly doped material was also found.
NASA Astrophysics Data System (ADS)
Kabir, Amin
The phase coherent photorefractive (PCP) effect in different ZnSe quantum well structures and its dependence on various extrinsic and intrinsic parameters have been investigated using 90 fs laser pulse in a two-beam four-wave-mixing (FWM) configuration. At low excitation intensities the signal is dominated by the PCP effect (which is attributed to a long living electron grating formed in the QW due to coherent QW excitons) and pulse overlap (PO) effect while at high excitation intensities it is governed by chi(3) FWM processes and the PO effect. With increasing excitation intensity the signal dip at pulse overlap (tau ≈ 0 ) which is characteristic for the destructive interference between the PO and PCP effect shifts to positive delay times tau > 0. The higher PCP diffraction efficiency value of ˜1.5 x10-3 in QW B (Zn0.92Mg0.08Se/ZnSe) as compared to the value of ˜3.5 x 10-4 in QW A (Zn0.94Mg 0.06Se/ZnSe) at 55 K is attributed to an increased Mg concentration in the barrier of QW B leading to a higher captured equilibrium electron density ne. Repetition rate dependent measurements on QW B show a drop of the diffraction efficiency for repetition times larger than 1.25 mus which is attributed to the reduction of the electron grating amplitude due to thermally activated electron tunneling. FWM experiments on two 10 nm ZnSe QWs with different barrier thicknesses of 20 (QW1) and 50 nm (QW2) between the QW and substrate show a redshift of the exciton line and an increased exciton dephasing rate due to increasing E-field induced tilt of the QW structure indicating an increased density of captured electrons ne. At temperatures below 35 K and laser excitation close to the exciton energy the creation of trions significantly compensates the formation of the spatially modulated electron density grating. At lower excitation energies increasing space-charge-fields significantly tilt the QW which reduces the trion binding energy leading to an enhanced thermal ionization of
Tsuchiya, Takuma
2013-12-04
We have investigated the possibility that the coherence length of spatially oscillating electron-spin polarization is improved in dilute magnetic semiconductors. In usual nonmagnetic quantum wells, the spin polarization of the electrons injected from a ferromagnetic source electrode oscillates spatially because of the spin precession due to spin-orbit effective magnetic fields, i.e., the Rashba and Dresselhaus fields. However, the polarization is damped within an oscillation period by the D’yakonov-Perel’ spin relaxation. In paramagnetic dilute magnetic semiconductors, impurity spin polarization is induced under the electron-spin polarization, and this impurity polarization influences the electron-spin precession and possibly improves the spatial electron-spin coherence. The validity of this effect is demonstrated by a numerical simulation for a CdMnTe quantum well.
Relaxation and coherent oscillations in the spin dynamics of II-VI diluted magnetic quantum wells
NASA Astrophysics Data System (ADS)
Ungar, F.; Cygorek, M.; Tamborenea, P. I.; Axt, V. M.
2015-10-01
We study theoretically the ultrafast spin dynamics of II-VI diluted magnetic quantum wells in the presence of spin-orbit interaction. We extend a recent study where it was shown that the spin-orbit interaction and the exchange sd coupling in bulk and quantum wells can compete resulting in qualitatively new dynamics when they act simultaneously. We concentrate on Hg1-x-yMnxCdyTe quantum wells, which have a highly tunable Rashba spin-orbit coupling. Our calculations use a recently developed formalism which incorporates electronic correlations originating from the exchange sd-coupling. We find that the dependence of electronic spin oscillations on the excess energy changes qualitatively depending on whether or not the spin-orbit interaction dominates or is of comparable strength with the sd interaction.
Carrier Dynamics and Application of the Phase Coherent Photorefractive Effect in ZnSe Quantum Wells
NASA Astrophysics Data System (ADS)
Dongol, Amit
The intensity dependent diffraction efficiency of a phase coherent photorefractive (PCP) ZnSe quantum well (QW) is investigated at 80 K in a two-beam four-wave mixing (FWM) configuration using 100 fs laser pulses with a repetition rate of 80 MHz. The observed diffraction efficiencies of the first and second-order diffracted beam are on the order of 10-3 and 10-5, respectively, revealing nearly no intensity dependence. The first-order diffraction is caused by the PCP effect where the probe-pulse is diffracted due to a long-living incoherent electron density grating in the QW. The second-order diffraction is created by a combination of diffraction processes. For negative probe-pulse delay, the exciton polarization is diffracted at the electron grating twice by a cascade effect. For positive delay, the diffracted signal is modified by the destructive interference with a chi(5) generated signal due to a dynamical screening effect. Model calculations of the signal traces based on the optical Bloch equations considering inhomogeneous broadening of exciton energies are in good agreement with the experimental data. To study the carrier dynamics responsible for the occurrence of the PCP effect, threebeam FWM experiments are carried out. The non-collinear wave-vectors k1 , k2 and k3 at central wavelength of 441 nm (~2.81 eV) were resonantly tuned to the heavy-hole exciton transition energy at 20 K. In the FWM experiment the time coincident strong pump pulses k1 and k2 create both an exciton density grating in the QW and an electron-hole pair grating in the GaAs while the delayed weak pulse k3 simultaneously probes the exciton lifetime as well as the electron grating capture time. The model calculations are in good agreement with the experimental results also providing information about the transfer delay of electrons arriving from the substrate to the QW. For negative probe-pulse delay we still observe a diffracted signal due to the long living electron density grating in
Highly coherent long cavity GaAs/AlGaAs single-quantum-well lasers
Larsson, A. ); Andrekson, P.A.; Jonsson, B.; Lindstrom, C. )
1989-09-01
The authors report on measurements of the spectral properties of ridge waveguide graded index separate confinement heterostructure single-quantum-well GaAs/AlGaAs lasers. Long cavity lasers (800{mu}m) exhibit remarkably pure single-longitudinal-mode spectra under continuous operation in spite of the short cavity mode spacing. At an output power of 5 mW, the sidemode suppression exceeds 24 dB and the linewidth is 1.5 MHz. This is among the narrowest linewidths reported for solitary AlGaAs lasers. The linewidth-power product is 6.4 MHz mW. Measurements of the linewidth-power product as a function of cavity length L gives an L/sup -2/ dependence in agreement with theory for lasers with small internal loss. No significant deviation from this dependence was observed for lasers short enough to operate at the second quantized state. The results are also used to deduce the linewidth enhancement factor {alpha} at the gain peak wavelength and its dependence on the excitation level. The sublinear gain-carrier density relation in the single quantum well results in an increase in a with increasing carrier density (decreasing cavity length) in contrast to conventional double heterostructure lasers and multiple-quantum-well lasers. In addition, a decrease in {alpha} was observed for lasers operating at the second quantized state due to recovery of the differential gain.
NASA Astrophysics Data System (ADS)
Ullah, S.; Gusev, G. M.; Bakarov, A. K.; Hernandez, F. G. G.
2016-06-01
We investigated the spin coherence of high-mobility two-dimensional electron gases confined in multilayer GaAs quantum wells. The dynamics of the spin polarization was optically studied using pump-probe techniques: time-resolved Kerr rotation and resonant spin amplification. For double and triple quantum wells doped beyond the metal-to-insulator transition, the spin-orbit interaction was tailored by the sample parameters of structural symmetry (Rashba constant), width, and electron density (Dresselhaus linear and cubic constants) which allow us to attain long dephasing times in the nanoseconds range. The determination of the scales, namely, transport scattering time, single-electron scattering time, electron-electron scattering time, and spin polarization decay time further supports the possibility of using n-doped multilayer systems for developing spintronic devices.
Determination of Spin-Orbit Coefficients and Phase Coherence Times in InGaAs/InAlAs Quantum Wells
NASA Astrophysics Data System (ADS)
Koga, Takaaki; Faniel, Sebastien; Matsuura, Toru; Mineshige, Shunsuke; Sekine, Yoshiaki; Sugiyama, H.
2011-12-01
We report the determination of the intrinsic spin-orbit interaction (SOI) parameters and phase coherence times for In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum wells (QWs) from the analysis of the weak antilocalization (WAL) measurements at dilution temperatures. We find that the Dresselhaus SOI is mostly negligible in this system and that the intrinsic parameters for the Rashba effect, aSO≡α/
NASA Technical Reports Server (NTRS)
Liu, Ansheng; Ning, Cun-Zheng
1999-01-01
Terahertz optical gain due to intersubband transitions in optically-pumped semiconductor quantum wells (QW's) is calculated nonperturbatively. We solve the pump- field-induced nonequilibrium distribution function for each subband of the QW system from a set of rate equations that include both intrasubband and intersubband relaxation processes. The gain arising from population inversion and stimulated Raman processes is calculated in a unified manner. We show that the coherent pump and signal wave interactions contribute significantly to the THz gain. Because of the optical Stark effect and pump-induced population redistribution, optical gain saturation at larger pump intensities is predicted.
Multiple quantum coherence spectroscopy.
Mathew, Nathan A; Yurs, Lena A; Block, Stephen B; Pakoulev, Andrei V; Kornau, Kathryn M; Wright, John C
2009-08-20
Multiple quantum coherences provide a powerful approach for studies of complex systems because increasing the number of quantum states in a quantum mechanical superposition state increases the selectivity of a spectroscopic measurement. We show that frequency domain multiple quantum coherence multidimensional spectroscopy can create these superposition states using different frequency excitation pulses. The superposition state is created using two excitation frequencies to excite the symmetric and asymmetric stretch modes in a rhodium dicarbonyl chelate and the dynamic Stark effect to climb the vibrational ladders involving different overtone and combination band states. A monochromator resolves the free induction decay of different coherences comprising the superposition state. The three spectral dimensions provide the selectivity required to observe 19 different spectral features associated with fully coherent nonlinear processes involving up to 11 interactions with the excitation fields. The different features act as spectroscopic probes of the diagonal and off-diagonal parts of the molecular potential energy hypersurface. This approach can be considered as a coherent pump-probe spectroscopy where the pump is a series of excitation pulses that prepares a multiple quantum coherence and the probe is another series of pulses that creates the output coherence. PMID:19507812
Converting Coherence to Quantum Correlations
NASA Astrophysics Data System (ADS)
Ma, Jiajun; Yadin, Benjamin; Girolami, Davide; Vedral, Vlatko; Gu, Mile
2016-04-01
Recent results in quantum information theory characterize quantum coherence in the context of resource theories. Here, we study the relation between quantum coherence and quantum discord, a kind of quantum correlation which appears even in nonentangled states. We prove that the creation of quantum discord with multipartite incoherent operations is bounded by the amount of quantum coherence consumed in its subsystems during the process. We show how the interplay between quantum coherence consumption and creation of quantum discord works in the preparation of multipartite quantum correlated states and in the model of deterministic quantum computation with one qubit.
NASA Astrophysics Data System (ADS)
Baraff, G. A.
1998-11-01
We study the effect of an imaginary potential and (separately) of a finite coherence length on the transmission, reflection, and capture fractions for a thermal distribution of carriers incident on a single quantum well. The formalism used is closely related to one used by Kuhn and Mahler for the same purpose. Closed-form expressions are obtained for the three transport fractions resulting from a single incident beam. Three independent fitting parameters are used in this formalism, namely, the size of the imaginary potential, the extent it penetrates into the barriers adjacent to the well, and the phase-coherence length. This last is a length scale associated with a correlation function that appears when the phase of the wave function is treated as a stochastic variable. We show that the parameters can be chosen so that the transport fractions agree with those calculated from first principles, and show how a shortening of the coherence length, e.g., by electron-electron interactions that have been left out of the first-principles calculation, destroys the resonant behavior of these fractions predicted by Brum and Bastard [Phys. Rev. B 33, 1420 (1986)].
Hole spin coherence in coupled GaAs/AlAs quantum wells
NASA Astrophysics Data System (ADS)
Gradl, Christian; Kempf, Michael; Holler, Johannes; Schuh, Dieter; Bougeard, Dominique; Schueller, Christian; Korn, Tobias
Due to its p-like character, the valence band in GaAs-based heterostructures offers rich and complex spin-dependent phenomena. Especially for some low-symmetry growth directions, a strong anisotropy of the hole g factor with respect to the in-plane magnetic field direction is theoretically predicted. Therefore, we perform time-resolved Kerr rotation measurements on an undoped [113]-grown double quantum well (QW) structure to resolve the spin dynamics of hole ensembles at low temperatures. Our gated system consists of two QWs with different well widths, which we use for the spatial separation of the optically excited electron-hole pairs. Thus, we are able to create hole ensembles with spin lifetimes of several hundreds of picoseconds in the broader QW without any doping. This allows the observation of a strong hole g factor anisotropy by varying the magnetic field direction in the QW plane. The experimental g factor values are in very good agreement with theoretical predictions. Furthermore, we observe an unexpected additional non-precessing component in the Kerr signal for certain in-plane magnetic field directions. This might have its origin in a precession axis that is tilted relative to the magnetic field due to the crystal structure of this low-symmetry growth direction. Financial support by the DFG via SFB 689 is gratefully acknowledged.
NASA Astrophysics Data System (ADS)
Zhu, Zhonghu; Chen, Ai-Xi; Bai, Yanfeng; Yang, Wen-Xing; Lee, Ray-Kuang
2014-05-01
In this paper, we analyze theoretically the optical steady behavior in GaAs quantum well structure which interacts with a single elliptically polarized field (EPF) and a π-polarized probe field. Due to the existence of the robust nonradiative coherence, we demonstrate that the controllable optical steady behavior including multi-stability (OM) and optical bistability (OB) can be obtained. More interestingly, our numerical results also illustrate that tuning the phase difference between two components of polarized electric field of the EPF can realize the conversion between OB and OM. Our results illustrate the potential to utilize the optical phase for developing the new all-optical switching devices, as well as a guidance in the design for possible experimental implementations.
Quantum coherence and correlations in quantum system
Xi, Zhengjun; Li, Yongming; Fan, Heng
2015-01-01
Criteria of measure quantifying quantum coherence, a unique property of quantum system, are proposed recently. In this paper, we first give an uncertainty-like expression relating the coherence and the entropy of quantum system. This finding allows us to discuss the relations between the entanglement and the coherence. Further, we discuss in detail the relations among the coherence, the discord and the deficit in the bipartite quantum system. We show that, the one-way quantum deficit is equal to the sum between quantum discord and the relative entropy of coherence of measured subsystem. PMID:26094795
Total quantum coherence and its applications
NASA Astrophysics Data System (ADS)
Yu, Chang-shui; Yang, Si-ren; Guo, Bao-qing
2016-06-01
Quantum coherence is the most fundamental feature of quantum mechanics. The usual understanding of it depends on the choice of the basis, that is, the coherence of the same quantum state is different within different reference framework. To reveal all the potential coherence, we present the total quantum coherence measures in terms of two different methods. One is optimizing maximal basis-dependent coherence with all potential bases considered and the other is quantifying the distance between the state and the incoherent state set. Interestingly, the coherence measures based on relative entropy and l_2 norm have the same form in the two different methods. In particular, we show that the measures based on the non-contractive l_2 norm are also a good measure different from the basis-dependent coherence. In addition, we show that all the measures are analytically calculable and have all the good properties. The experimental schemes for the detection of these coherence measures are also proposed by multiple copies of quantum states instead of reconstructing the full density matrix. By studying one type of quantum probing schemes, we find that both the normalized trace in the scheme of deterministic quantum computation with one qubit and the overlap of two states in quantum overlap measurement schemes can be well described by the change of total coherence of the probing qubit. Hence the nontrivial probing always leads to the change of the total coherence.
NASA Astrophysics Data System (ADS)
Walecki, Wojciech J.; Fittinghoff, David N.; Smirl, Arthur L.
1997-03-01
Four wave mixing (FWM) techniques using ultrashort pulses have proven to be extremely powerful tools for studying coherent processes and excitonic effects in semiconductors and multiple quantum wells (MQWs). Complete characterization of the emitted electromagnetic field requires the measurement of the phase, the polarization, and the amplitude. Failure to measure any one of these will result in a loss of essential information about the optical interactions. Present techniques for measuring the phase and the polarization state, however, are insensitive, and labor intensive. Here, we demonstrate that spectral interferometry can be used to completely characterize the FWM emission from MQWs. This method, involving only a linear measurement at a single fixed time delay, is simpler and more sensitive than previous techniques, which require measurements for various orientations of waveplates and for various time delays and which require a cross correlation with a reference pulse. We demonstrate the power of this technique by investigating the temporal dynamics of the FWM signal emitted from GaAs/AlGaAs MQWs as a function of the excitation fluence, time delay between the two incident pulses, and orientation of the input polarizations.
Quantum coherence in multipartite systems
NASA Astrophysics Data System (ADS)
Yao, Yao; Xiao, Xing; Ge, Li; Sun, C. P.
2015-08-01
Within the unified framework of exploiting the relative entropy as a distance measure of quantum correlations, we make explicit the hierarchical structure of quantum coherence, quantum discord, and quantum entanglement in multipartite systems. On this basis, we define a basis-independent measure of quantum coherence and prove that it is exactly equivalent to quantum discord. Furthermore, since the original relative entropy of coherence is a basis-dependent quantity, we investigate the local and nonlocal unitary creation of quantum coherence, focusing on the two-qubit unitary gates. Intriguingly, our results demonstrate that nonlocal unitary gates do not necessarily outperform the local unitary gates. Finally, the additivity relationship of quantum coherence in tripartite systems is discussed in detail, where the strong subadditivity of von Neumann entropy plays an essential role.
Bagaev, V. S.; Davletov, E. T.; Krivobok, V. S. Nikolaev, S. N.; Novikov, A. V.; Onishchenko, E. E.; Pruchkina, A. A.; Skorikov, M. L.
2015-12-15
The measured stationary and time-resolved photoluminescence is used to study the properties of the exciton gas in a second-order 5-nm-thick Si{sub 0.905}Ge{sub 0.095}/Si quantum well. It is shown that, despite the presence of an electron barrier in the Si{sub 0.905}Ge{sub 0.095} layer, a spatially indirect biexciton is the most favorable energy state of the electron–hole system at low temperatures. This biexciton is characterized by a lifetime of 1100 ns and a binding energy of 2.0–2.5 meV and consists of two holes localized in the SiGe layer and two electrons mainly localized in silicon. The formation of biexcitons is shown to cause low-temperature (5 K) luminescence spectra over a wide excitation density range and to suppress the formation of an exciton gas, in which quantum statistics effects are significant. The Bose statistics can only be experimentally observed for a biexciton gas at a temperature of 1 K or below because of the high degree of degeneracy of biexciton states (28) and a comparatively large effective mass (about 1.3m{sub e}). The heat energy at such temperatures is much lower than the measured energy of localization at potential fluctuations (about 1 meV). This feature leads to biexciton localization and fundamentally limits the possibility of observation of quantum coherence in the biexciton gas.
Measuring Quantum Coherence with Entanglement
NASA Astrophysics Data System (ADS)
Streltsov, Alexander; Singh, Uttam; Dhar, Himadri Shekhar; Bera, Manabendra Nath; Adesso, Gerardo
2015-07-01
Quantum coherence is an essential ingredient in quantum information processing and plays a central role in emergent fields such as nanoscale thermodynamics and quantum biology. However, our understanding and quantitative characterization of coherence as an operational resource are still very limited. Here we show that any degree of coherence with respect to some reference basis can be converted to entanglement via incoherent operations. This finding allows us to define a novel general class of measures of coherence for a quantum system of arbitrary dimension, in terms of the maximum bipartite entanglement that can be generated via incoherent operations applied to the system and an incoherent ancilla. The resulting measures are proven to be valid coherence monotones satisfying all the requirements dictated by the resource theory of quantum coherence. We demonstrate the usefulness of our approach by proving that the fidelity-based geometric measure of coherence is a full convex coherence monotone, and deriving a closed formula for it on arbitrary single-qubit states. Our work provides a clear quantitative and operational connection between coherence and entanglement, two landmark manifestations of quantum theory and both key enablers for quantum technologies.
Complementarity relations for quantum coherence
NASA Astrophysics Data System (ADS)
Cheng, Shuming; Hall, Michael J. W.
2015-10-01
Various measures have been suggested recently for quantifying the coherence of a quantum state with respect to a given basis. We first use two of these, the l1-norm and relative entropy measures, to investigate tradeoffs between the coherences of mutually unbiased bases. Results include relations between coherence, uncertainty, and purity; tight general bounds restricting the coherences of mutually unbiased bases; and an exact complementarity relation for qubit coherences. We further define the average coherence of a quantum state. For the l1-norm measure this is related to a natural "coherence radius" for the state and leads to a conjecture for an l2-norm measure of coherence. For relative entropy the average coherence is determined by the difference between the von Neumann entropy and the quantum subentropy of the state and leads to upper bounds for the latter quantity. Finally, we point out that the relative entropy of coherence is a special case of G-asymmetry, which immediately yields several operational interpretations in contexts as diverse as frame alignment, quantum communication, and metrology, and suggests generalizing the property of quantum coherence to arbitrary groups of physical transformations.
Assisted Distillation of Quantum Coherence.
Chitambar, E; Streltsov, A; Rana, S; Bera, M N; Adesso, G; Lewenstein, M
2016-02-19
We introduce and study the task of assisted coherence distillation. This task arises naturally in bipartite systems where both parties work together to generate the maximal possible coherence on one of the subsystems. Only incoherent operations are allowed on the target system, while general local quantum operations are permitted on the other; this is an operational paradigm that we call local quantum-incoherent operations and classical communication. We show that the asymptotic rate of assisted coherence distillation for pure states is equal to the coherence of assistance, an analog of the entanglement of assistance, whose properties we characterize. Our findings imply a novel interpretation of the von Neumann entropy: it quantifies the maximum amount of extra quantum coherence a system can gain when receiving assistance from a collaborative party. Our results are generalized to coherence localization in a multipartite setting and possible applications are discussed. PMID:26943512
Assisted Distillation of Quantum Coherence
NASA Astrophysics Data System (ADS)
Chitambar, E.; Streltsov, A.; Rana, S.; Bera, M. N.; Adesso, G.; Lewenstein, M.
2016-02-01
We introduce and study the task of assisted coherence distillation. This task arises naturally in bipartite systems where both parties work together to generate the maximal possible coherence on one of the subsystems. Only incoherent operations are allowed on the target system, while general local quantum operations are permitted on the other; this is an operational paradigm that we call local quantum-incoherent operations and classical communication. We show that the asymptotic rate of assisted coherence distillation for pure states is equal to the coherence of assistance, an analog of the entanglement of assistance, whose properties we characterize. Our findings imply a novel interpretation of the von Neumann entropy: it quantifies the maximum amount of extra quantum coherence a system can gain when receiving assistance from a collaborative party. Our results are generalized to coherence localization in a multipartite setting and possible applications are discussed.
Evolution equation for quantum coherence
Hu, Ming-Liang; Fan, Heng
2016-01-01
The estimation of the decoherence process of an open quantum system is of both theoretical significance and experimental appealing. Practically, the decoherence can be easily estimated if the coherence evolution satisfies some simple relations. We introduce a framework for studying evolution equation of coherence. Based on this framework, we prove a simple factorization relation (FR) for the l1 norm of coherence, and identified the sets of quantum channels for which this FR holds. By using this FR, we further determine condition on the transformation matrix of the quantum channel which can support permanently freezing of the l1 norm of coherence. We finally reveal the universality of this FR by showing that it holds for many other related coherence and quantum correlation measures. PMID:27382933
Evolution equation for quantum coherence
NASA Astrophysics Data System (ADS)
Hu, Ming-Liang; Fan, Heng
2016-07-01
The estimation of the decoherence process of an open quantum system is of both theoretical significance and experimental appealing. Practically, the decoherence can be easily estimated if the coherence evolution satisfies some simple relations. We introduce a framework for studying evolution equation of coherence. Based on this framework, we prove a simple factorization relation (FR) for the l1 norm of coherence, and identified the sets of quantum channels for which this FR holds. By using this FR, we further determine condition on the transformation matrix of the quantum channel which can support permanently freezing of the l1 norm of coherence. We finally reveal the universality of this FR by showing that it holds for many other related coherence and quantum correlation measures.
Evolution equation for quantum coherence.
Hu, Ming-Liang; Fan, Heng
2016-01-01
The estimation of the decoherence process of an open quantum system is of both theoretical significance and experimental appealing. Practically, the decoherence can be easily estimated if the coherence evolution satisfies some simple relations. We introduce a framework for studying evolution equation of coherence. Based on this framework, we prove a simple factorization relation (FR) for the l1 norm of coherence, and identified the sets of quantum channels for which this FR holds. By using this FR, we further determine condition on the transformation matrix of the quantum channel which can support permanently freezing of the l1 norm of coherence. We finally reveal the universality of this FR by showing that it holds for many other related coherence and quantum correlation measures. PMID:27382933
Quantum coherence of steered states
NASA Astrophysics Data System (ADS)
Hu, Xueyuan; Milne, Antony; Zhang, Boyang; Fan, Heng
2016-01-01
Lying at the heart of quantum mechanics, coherence has recently been studied as a key resource in quantum information theory. Quantum steering, a fundamental notion originally considered by Schödinger, has also recently received much attention. When Alice and Bob share a correlated quantum system, Alice can perform a local measurement to ‘steer’ Bob’s reduced state. We introduce the maximal steered coherence as a measure describing the extent to which steering can remotely create coherence; more precisely, we find the maximal coherence of Bob’s steered state in the eigenbasis of his original reduced state, where maximization is performed over all positive-operator valued measurements for Alice. We prove that maximal steered coherence vanishes for quantum-classical states whilst reaching a maximum for pure entangled states with full Schmidt rank. Although invariant under local unitary operations, maximal steered coherence may be increased when Bob performs a channel. For a two-qubit state we find that Bob’s channel can increase maximal steered coherence if and only if it is neither unital nor semi-classical, which coincides with the condition for increasing discord. Our results show that the power of steering for coherence generation, though related to discord, is distinct from existing measures of quantum correlation.
Quantum coherence of steered states
Hu, Xueyuan; Milne, Antony; Zhang, Boyang; Fan, Heng
2016-01-01
Lying at the heart of quantum mechanics, coherence has recently been studied as a key resource in quantum information theory. Quantum steering, a fundamental notion originally considered by Schödinger, has also recently received much attention. When Alice and Bob share a correlated quantum system, Alice can perform a local measurement to ‘steer’ Bob’s reduced state. We introduce the maximal steered coherence as a measure describing the extent to which steering can remotely create coherence; more precisely, we find the maximal coherence of Bob’s steered state in the eigenbasis of his original reduced state, where maximization is performed over all positive-operator valued measurements for Alice. We prove that maximal steered coherence vanishes for quantum-classical states whilst reaching a maximum for pure entangled states with full Schmidt rank. Although invariant under local unitary operations, maximal steered coherence may be increased when Bob performs a channel. For a two-qubit state we find that Bob’s channel can increase maximal steered coherence if and only if it is neither unital nor semi-classical, which coincides with the condition for increasing discord. Our results show that the power of steering for coherence generation, though related to discord, is distinct from existing measures of quantum correlation. PMID:26781214
Quantum Coherence in a Superfluid Josephson Junction
Narayana, Supradeep; Sato, Yuki
2011-02-04
We report a new kind of experiment in which we take an array of nanoscale apertures that form a superfluid {sup 4}He Josephson junction and apply quantum phase gradients directly along the array. We observe collective coherent behaviors from aperture elements, leading to quantum interference. Connections to superconducting and Bose-Einstein condensate Josephson junctions as well as phase coherence among the superfluid aperture array are discussed.
Photoelectric devices with quantum coherence
NASA Astrophysics Data System (ADS)
Shanhe, Su
A phtotoelectric device consisting of a three-level system contacted with two fermionic baths and a photon bath is built. Making the Born-Markov approximation, the equation of motion for the density operator in a Lindblad-like form is derived. We obtain the coherence and the efficiency of the system under the steady-state condition. Results show that quantum coherence can enhance the photoelectric conversion efficiency. The efficiency at maximum power can be larger than the CA efficiency bound with the existence of coherence.
Photoelectric converters with quantum coherence
NASA Astrophysics Data System (ADS)
Su, Shan-He; Sun, Chang-Pu; Li, Sheng-Wen; Chen, Jin-Can
2016-05-01
Photon impingement is capable of liberating electrons in electronic devices and driving the electron flux from the lower chemical potential to higher chemical potential. Previous studies hinted that the thermodynamic efficiency of a nanosized photoelectric converter at maximum power is bounded by the Curzon-Ahlborn efficiency ηCA. In this study, we apply quantum effects to design a photoelectric converter based on a three-level quantum dot (QD) interacting with fermionic baths and photons. We show that, by adopting a pair of suitable degenerate states, quantum coherences induced by the couplings of QDs to sunlight and fermion baths can coexist steadily in nanoelectronic systems. Our analysis indicates that the efficiency at maximum power is no longer limited to ηCA through manipulation of carefully controlled quantum coherences.
Photoelectric converters with quantum coherence.
Su, Shan-He; Sun, Chang-Pu; Li, Sheng-Wen; Chen, Jin-Can
2016-05-01
Photon impingement is capable of liberating electrons in electronic devices and driving the electron flux from the lower chemical potential to higher chemical potential. Previous studies hinted that the thermodynamic efficiency of a nanosized photoelectric converter at maximum power is bounded by the Curzon-Ahlborn efficiency η_{CA}. In this study, we apply quantum effects to design a photoelectric converter based on a three-level quantum dot (QD) interacting with fermionic baths and photons. We show that, by adopting a pair of suitable degenerate states, quantum coherences induced by the couplings of QDs to sunlight and fermion baths can coexist steadily in nanoelectronic systems. Our analysis indicates that the efficiency at maximum power is no longer limited to η_{CA} through manipulation of carefully controlled quantum coherences. PMID:27300826
Quantum variance: A measure of quantum coherence and quantum correlations for many-body systems
NASA Astrophysics Data System (ADS)
Frérot, Irénée; Roscilde, Tommaso
2016-08-01
Quantum coherence is a fundamental common trait of quantum phenomena, from the interference of matter waves to quantum degeneracy of identical particles. Despite its importance, estimating and measuring quantum coherence in generic, mixed many-body quantum states remains a formidable challenge, with fundamental implications in areas as broad as quantum condensed matter, quantum information, quantum metrology, and quantum biology. Here, we provide a quantitative definition of the variance of quantum coherent fluctuations (the quantum variance) of any observable on generic quantum states. The quantum variance generalizes the concept of thermal de Broglie wavelength (for the position of a free quantum particle) to the space of eigenvalues of any observable, quantifying the degree of coherent delocalization in that space. The quantum variance is generically measurable and computable as the difference between the static fluctuations and the static susceptibility of the observable; despite its simplicity, it is found to provide a tight lower bound to most widely accepted estimators of "quantumness" of observables (both as a feature as well as a resource), such as the Wigner-Yanase skew information and the quantum Fisher information. When considering bipartite fluctuations in an extended quantum system, the quantum variance expresses genuine quantum correlations among the two parts. In the case of many-body systems, it is found to obey an area law at finite temperature, extending therefore area laws of entanglement and quantum fluctuations of pure states to the mixed-state context. Hence the quantum variance paves the way to the measurement of macroscopic quantum coherence and quantum correlations in most complex quantum systems.
Coherent dynamics of Landau-Levels in modulation doped GaAs quantum wells at high magnetic fields
NASA Astrophysics Data System (ADS)
Liu, Cunming; Paul, Jagannath; Reno, John; McGill, Stephen; Hilton, David; Karaiskaj, Denis
By using two-dimensional Fourier transform spectroscopy, we investigate the dynamics of Landau-Levels formed in modulation doped GaAs/AlGaAs quantum wells of 18 nm thickness at high magnetic fields and low temperature. The measurements show interesting dephasing dynamics and linewidth dependency as a function of the magnetic field. The work at USF and UAB was supported by the National Science Foundation under grant number DMR-1409473. The work at NHMFL, FSU was supported by the National Science Foundation under grant numbers DMR-1157490 and DMR-1229217. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.
Quantum coherence and quantum phase transitions
NASA Astrophysics Data System (ADS)
Li, Yan-Chao; Lin, Hai-Qing
2016-05-01
We study the connections between local quantum coherence (LQC) based on Wigner-Yanase skew information and quantum phase transitions (QPTs). When applied on the one-dimensional Hubbard, XY spin chain with three-spin interaction, and Su-Schrieffer-Heeger models, the LQC and its derivatives are used successfully to detect different types of QPTs in these spin and fermionic systems. Furthermore, the LQC is effective as the quantum discord (QD) in detecting QPTs at finite temperatures, where the entanglement has lost its effectiveness. We also demonstrate that the LQC can exhibit different behaviors in many forms compared with the QD.
Quantum coherence and quantum phase transitions.
Li, Yan-Chao; Lin, Hai-Qing
2016-01-01
We study the connections between local quantum coherence (LQC) based on Wigner-Yanase skew information and quantum phase transitions (QPTs). When applied on the one-dimensional Hubbard, XY spin chain with three-spin interaction, and Su-Schrieffer-Heeger models, the LQC and its derivatives are used successfully to detect different types of QPTs in these spin and fermionic systems. Furthermore, the LQC is effective as the quantum discord (QD) in detecting QPTs at finite temperatures, where the entanglement has lost its effectiveness. We also demonstrate that the LQC can exhibit different behaviors in many forms compared with the QD. PMID:27193057
Quantum coherence and quantum phase transitions
Li, Yan-Chao; Lin, Hai-Qing
2016-01-01
We study the connections between local quantum coherence (LQC) based on Wigner-Yanase skew information and quantum phase transitions (QPTs). When applied on the one-dimensional Hubbard, XY spin chain with three-spin interaction, and Su-Schrieffer-Heeger models, the LQC and its derivatives are used successfully to detect different types of QPTs in these spin and fermionic systems. Furthermore, the LQC is effective as the quantum discord (QD) in detecting QPTs at finite temperatures, where the entanglement has lost its effectiveness. We also demonstrate that the LQC can exhibit different behaviors in many forms compared with the QD. PMID:27193057
Quantum coherent states in cosmology
NASA Astrophysics Data System (ADS)
Ziaeepour, Houri
2015-07-01
Coherent states consist of superposition of infinite number of particles and do not have a classical analogue. We study their evolution in a FLRW cosmology and show that only when full quantum corrections are considered, they may survive the expansion of the Universe and form a global condensate. This state of matter can be the origin of accelerating expansion of the Universe, generally called dark energy, and inflation in the early universe. Additionally, such a quantum pool may be the ultimate environment for decoherenceat shorter distances. If dark energy is a quantum coherent state, its dominant contribution to the total energy of the Universe at present provides a low entropy state which may be necessary as an initial condition for a new Big Bang in the framework of bouncing cosmology models.
NASA Astrophysics Data System (ADS)
Kononov, A.; Egorov, S. V.; Kvon, Z. D.; Mikhailov, N. N.; Dvoretsky, S. A.; Deviatov, E. V.
2015-06-01
We experimentally investigate spin-polarized electron transport between two ferromagnetic contacts, placed at the edge of a two-dimensional electron system with band inversion. The system is realized in a narrow (8 nm) HgTe quantum well, the ferromagnetic side contacts are formed from a premagnetized permalloy film. In zero magnetic field, we find a significant edge current contribution to the transport between two ferromagnetic contacts. We experimentally demonstrate that this transport is sensitive to the mutual orientation of the magnetization directions of two 200 µm-spaced ferromagnetic leads. This is a direct experimental evidence on the spin-coherent edge transport over the macroscopic distances. Thus, the spin is extremely robust at the edge of a two-dimensional electron system with band inversion, confirming the helical spin-resolved nature of edge currents.
Distribution of Quantum Coherence in Multipartite Systems.
Radhakrishnan, Chandrashekar; Parthasarathy, Manikandan; Jambulingam, Segar; Byrnes, Tim
2016-04-15
The distribution of coherence in multipartite systems is examined. We use a new coherence measure with entropic nature and metric properties, based on the quantum Jensen-Shannon divergence. The metric property allows for the coherence to be decomposed into various contributions, which arise from local and intrinsic coherences. We find that there are trade-off relations between the various contributions of coherence, as a function of parameters of the quantum state. In bipartite systems the coherence resides on individual sites or is distributed among the sites, which contribute in a complementary way. In more complex systems, the characteristics of the coherence can display more subtle changes with respect to the parameters of the quantum state. In the case of the XXZ Heisenberg model, the coherence changes from a monogamous to a polygamous nature. This allows us to define the shareability of coherence, leading to monogamy relations for coherence. PMID:27127948
Distribution of Quantum Coherence in Multipartite Systems
NASA Astrophysics Data System (ADS)
Radhakrishnan, Chandrashekar; Parthasarathy, Manikandan; Jambulingam, Segar; Byrnes, Tim
2016-04-01
The distribution of coherence in multipartite systems is examined. We use a new coherence measure with entropic nature and metric properties, based on the quantum Jensen-Shannon divergence. The metric property allows for the coherence to be decomposed into various contributions, which arise from local and intrinsic coherences. We find that there are trade-off relations between the various contributions of coherence, as a function of parameters of the quantum state. In bipartite systems the coherence resides on individual sites or is distributed among the sites, which contribute in a complementary way. In more complex systems, the characteristics of the coherence can display more subtle changes with respect to the parameters of the quantum state. In the case of the X X Z Heisenberg model, the coherence changes from a monogamous to a polygamous nature. This allows us to define the shareability of coherence, leading to monogamy relations for coherence.
Zory, P.S. Jr.
1993-01-01
The semiconductor quantum well (QW) laser structure is rapidly becoming the preferred design in many applications because of its low threshold, design flexibility, and high reliability. The book begins with a brief, interesting foreword by C.H. Henry on the history of the QW laser concept and its early development. Following this introduction is a 79-page chapter by S.W. Corzine et al. on optical gain in III-V bulk and QW lasers. The next chapter on intraband relaxation and line broadening effects by M. Asada is an excellent expanded review of a topic introduced by Corzine. The remaining chapters describe multiple QW lasers, low-threshold QW laser, special aspects of AlGaAs and (short-wavelength) InGaAsP lasers, valence-band engineering, strained-layer QW lasers, AlGaInP QW lasers, and quantum wire lasers. These chapters are well written by recognized experts in the field.
Hader, K.; Engel, V.
2014-05-14
We study laser excitation processes in a double well potential. The possibility to influence localization via the carrier-envelope phase (CEP) of a laser pulse is investigated for various situations which differ in the nature of the initial state prior to the laser interactions. In more detail, the CEP-dependence of asymmetries in the case where initially the system is described by localized wave packets, eigenstates, or incoherent mixtures are calculated and interpreted within time-dependent perturbation theory. It is investigated which contributions to the asymmetry exist and how they can be modified to reveal a more or less pronounced CEP-effect.
Coherent communication with continuous quantum variables
Wilde, Mark M.; Krovi, Hari; Brun, Todd A.
2007-06-15
The coherent bit (cobit) channel is a resource intermediate between classical and quantum communication. It produces coherent versions of teleportation and superdense coding. We extend the cobit channel to continuous variables by providing a definition of the coherent nat (conat) channel. We construct several coherent protocols that use both a position-quadrature and a momentum-quadrature conat channel with finite squeezing. Finally, we show that the quality of squeezing diminishes through successive compositions of coherent teleportation and superdense coding.
Coherent Control of Quantum Matter
Cavalleri, Andrea
2011-10-05
This talk addresses some recent work aimed at controlling the low-lying electrodynamics of quantum solids using strong field transients. The excitation of selected vibrational resonances to manipulate the many-body physics of one dimensional Mott Hubbard Insulators and to perturb competing orders in High-Tc superconductors is also covered. Finally, the speaker shows how the electrodynamics of layered superconductors can be driven through the orderparameter phase gradient, demonstrating ultrafast transistor action in a layered superconductor. Advances in the use of coherent optics, from tabletop sources to THz and x-ray free-electron lasers are also discussed.
Quantum entanglement and coherence in molecular magnets
NASA Astrophysics Data System (ADS)
Shiddiq, Muhandis
Quantum computers are predicted to outperform classical computers in certain tasks, such as factoring large numbers and searching databases. The construction of a computer whose operation is based on the principles of quantum mechanics appears extremely challenging. Solid state approaches offer the potential to answer this challenge by tailor-making novel nanomaterials for quantum information processing (QIP). Molecular magnets, which are materials whose energy levels and magnetic quantum states are well defined at the molecular level, have been identified as a class of material with properties that make them attractive for quantum computing purpose. In this dissertation, I explore the possibilities and challenges for molecular magnets to be used in quantum computing architecture. The properties of molecular magnets that are critical for applications in quantum computing, i.e., quantum entanglement and coherence, are comprehensively investigated to probe the feasibility of molecular magnets to be used as quantum bits (qubits). Interactions of qubits with photons are at the core of QIP. Photons can be used to detect and manipulate qubits, after which information can then be transferred over long distances. As a potential candidate for qubits, the interactions between Fe8 single-molecule magnets (SMMs) and cavity photons were studied. An earlier report described that a cavity mode splitting was observed in a spectrum of a cavity filled with a single-crystal of Fe8 SMMs. This splitting was interpreted as a vacuum Rabi splitting (VRS), which is a signature of an entanglement between a large number of SMMs and a cavity photon. However, find that large absorption and dispersion of the magnetic susceptibility are the reasons for this splitting. This finding highlights the fact that an observation of a peak splitting in a cavity transmission spectrum neither represents an unambiguous indication of quantum coherence in a large number of spins, nor a signature of
Room temperature quantum coherence in a potential molecular qubit.
Bader, Katharina; Dengler, Dominik; Lenz, Samuel; Endeward, Burkhard; Jiang, Shang-Da; Neugebauer, Petr; van Slageren, Joris
2014-01-01
The successful development of a quantum computer would change the world, and current internet encryption methods would cease to function. However, no working quantum computer that even begins to rival conventional computers has been developed yet, which is due to the lack of suitable quantum bits. A key characteristic of a quantum bit is the coherence time. Transition metal complexes are very promising quantum bits, owing to their facile surface deposition and their chemical tunability. However, reported quantum coherence times have been unimpressive. Here we report very long quantum coherence times for a transition metal complex of 68 μs at low temperature (qubit figure of merit QM=3,400) and 1 μs at room temperature, much higher than previously reported values for such systems. We show that this achievement is because of the rigidity of the lattice as well as removal of nuclear spins from the vicinity of the magnetic ion. PMID:25328006
Room temperature quantum coherence in a potential molecular qubit
NASA Astrophysics Data System (ADS)
Bader, Katharina; Dengler, Dominik; Lenz, Samuel; Endeward, Burkhard; Jiang, Shang-Da; Neugebauer, Petr; van Slageren, Joris
2014-10-01
The successful development of a quantum computer would change the world, and current internet encryption methods would cease to function. However, no working quantum computer that even begins to rival conventional computers has been developed yet, which is due to the lack of suitable quantum bits. A key characteristic of a quantum bit is the coherence time. Transition metal complexes are very promising quantum bits, owing to their facile surface deposition and their chemical tunability. However, reported quantum coherence times have been unimpressive. Here we report very long quantum coherence times for a transition metal complex of 68 μs at low temperature (qubit figure of merit QM=3,400) and 1 μs at room temperature, much higher than previously reported values for such systems. We show that this achievement is because of the rigidity of the lattice as well as removal of nuclear spins from the vicinity of the magnetic ion.
Coherent and incoherent tunneling in asymmetric double-well potentials
Ranfagni, A.; Cacciari, I.; Vitali, M. A.; Viliani, G.; Moretti, P.; Ruggeri, R.
2006-07-15
The determination of the time scale for coherent and incoherent tunneling in asymmetric double-well potentials is reconsidered according to the instanton-bounce method. In particular, by making use of Feynman's transition elements, a different, relatively simpler approach to this problem, with respect to conventional quantum-mechanical treatments, is obtained.
Strong Quantum Coherence between Fermi Liquid Mahan Excitons
NASA Astrophysics Data System (ADS)
Paul, J.; Stevens, C. E.; Liu, C.; Dey, P.; McIntyre, C.; Turkowski, V.; Reno, J. L.; Hilton, D. J.; Karaiskaj, D.
2016-04-01
In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called "Mahan excitons." The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the optical Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system.
Robust quantum receivers for coherent state discrimination
NASA Astrophysics Data System (ADS)
Becerra, Francisco Elohim
2014-05-01
Quantum state discrimination is a central task for quantum information and is a fundamental problem in quantum mechanics. Nonorthogonal states, such as coherent states which have intrinsic quantum noise, cannot be discriminated with total certainty because of their intrinsic overlap. This nonorthogonality is at the heart of quantum key distribution for ensuring absolute secure communications between a transmitter and a receiver, and can enable many quantum information protocols based on coherent states. At the same time, while coherent states are used for communications because of their robustness to loss and simplicity of generation and detection, their nonorthogonality inherently produces errors in the process of decoding the information. The minimum error probability in the discrimination of nonorthogonal coherent states measured by an ideal lossless and noiseless conventional receiver is given by the standard quantum limit (SQL). This limit sets strict bounds on the ultimate performance of coherent communications and many coherent-state-based quantum information protocols. However, measurement strategies based on the quantum properties of these states can allow for better measurements that surpass the SQL and approach the ultimate measurement limits allowed by quantum mechanics. These measurement strategies can allow for optimally extracting information encoded in these states for coherent and quantum communications. We present the demonstration of a receiver based on adaptive measurements and single-photon counting that unconditionally discriminates multiple nonorthogonal coherent states below the SQL. We also discuss the potential of photon-number-resolving detection to provide robustness and high sensitivity under realistic conditions for an adaptive coherent receiver with detectors with finite photon-number resolution.
Quantum coherent oscillations in the early universe
NASA Astrophysics Data System (ADS)
Pikovski, Igor; Loeb, Abraham
2016-05-01
Cosmic inflation is commonly assumed to be driven by quantum fields. Quantum mechanics predicts phenomena such as quantum fluctuations and tunneling of the field. Here, we show an example of a quantum interference effect which goes beyond the semiclassical treatment and which may be of relevance in the early Universe. We study the quantum coherent dynamics for a tilted, periodic potential, which results in genuine quantum oscillations of the inflaton field, analogous to Bloch oscillations in condensed matter and atomic systems. The underlying quantum superpositions are typically very fragile but may persist in the early Universe giving rise to quantum interference phenomena in cosmology.
Quantum coherence, wormholes, and the cosmological constant
Unruh, W.G. )
1989-08-15
Coleman has argued that if wormhole solutions to the Euclidean action coupled to matter dominate the Euclidean path integral for quantum gravity, they do not lead to a loss of quantum coherence for wave functions in our Universe. Furthermore, they also lead to the prediction that the ultimate'' cosmological constant is zero. I analyze the assumptions that go into this result and argue that the presence of wormhole solutions does lead to a loss of quantum coherence and, furthermore, completely destroys the Euclidean quantum theory by producing a highly nonlocal effective Euclidean action which is violently unbounded from below.
Cohering and decohering power of quantum channels
NASA Astrophysics Data System (ADS)
Mani, Azam; Karimipour, Vahid
2015-09-01
We introduce the concepts of cohering and decohering power of quantum channels. Using the axiomatic definition of the coherence measure, we show that the optimization required for calculations of these measures can be restricted to pure input states and hence greatly simplified. We then use two examples of this measure, one based on the skew information and the other based on the l1 norm; we find the cohering and decohering measures of a number of one-, two-, and n -qubit channels. Contrary to the view at first glance, it is seen that quantum channels can have cohering power. It is also shown that a specific property of a qubit unitary map is that it has equal cohering and decohering power in any basis. Finally, we derive simple relations between cohering and decohering powers of unitary qubit gates and their tensor products, results which have physically interesting implications.
Emergence of coherence and the dynamics of quantum phase transitions
Braun, Simon; Friesdorf, Mathis; Hodgman, Sean S.; Schreiber, Michael; Ronzheimer, Jens Philipp; Riera, Arnau; del Rey, Marco; Bloch, Immanuel; Eisert, Jens
2015-01-01
The dynamics of quantum phase transitions pose one of the most challenging problems in modern many-body physics. Here, we study a prototypical example in a clean and well-controlled ultracold atom setup by observing the emergence of coherence when crossing the Mott insulator to superfluid quantum phase transition. In the 1D Bose–Hubbard model, we find perfect agreement between experimental observations and numerical simulations for the resulting coherence length. We, thereby, perform a largely certified analog quantum simulation of this strongly correlated system reaching beyond the regime of free quasiparticles. Experimentally, we additionally explore the emergence of coherence in higher dimensions, where no classical simulations are available, as well as for negative temperatures. For intermediate quench velocities, we observe a power-law behavior of the coherence length, reminiscent of the Kibble–Zurek mechanism. However, we find nonuniversal exponents that cannot be captured by this mechanism or any other known model. PMID:25775515
Sequential quantum teleportation of optical coherent states
Yonezawa, Hidehiro; Furusawa, Akira; Loock, Peter van
2007-09-15
We demonstrate a sequence of two quantum teleportations of optical coherent states, combining two high-fidelity teleporters for continuous variables. In our experiment, the individual teleportation fidelities are evaluated as F{sub 1}=0.70{+-}0.02 and F{sub 2}=0.75{+-}0.02, while the fidelity between the input and the sequentially teleported states is determined as F{sup (2)}=0.57{+-}0.02. This still exceeds the optimal fidelity of one half for classical teleportation of arbitrary coherent states and almost attains the value of the first (unsequential) quantum teleportation experiment with optical coherent states.
Entanglement and Coherence in Quantum State Merging
NASA Astrophysics Data System (ADS)
Streltsov, A.; Chitambar, E.; Rana, S.; Bera, M. N.; Winter, A.; Lewenstein, M.
2016-06-01
Understanding the resource consumption in distributed scenarios is one of the main goals of quantum information theory. A prominent example for such a scenario is the task of quantum state merging, where two parties aim to merge their tripartite quantum state parts. In standard quantum state merging, entanglement is considered to be an expensive resource, while local quantum operations can be performed at no additional cost. However, recent developments show that some local operations could be more expensive than others: it is reasonable to distinguish between local incoherent operations and local operations which can create coherence. This idea leads us to the task of incoherent quantum state merging, where one of the parties has free access to local incoherent operations only. In this case the resources of the process are quantified by pairs of entanglement and coherence. Here, we develop tools for studying this process and apply them to several relevant scenarios. While quantum state merging can lead to a gain of entanglement, our results imply that no merging procedure can gain entanglement and coherence at the same time. We also provide a general lower bound on the entanglement-coherence sum and show that the bound is tight for all pure states. Our results also lead to an incoherent version of Schumacher compression: in this case the compression rate is equal to the von Neumann entropy of the diagonal elements of the corresponding quantum state.
Entanglement and Coherence in Quantum State Merging.
Streltsov, A; Chitambar, E; Rana, S; Bera, M N; Winter, A; Lewenstein, M
2016-06-17
Understanding the resource consumption in distributed scenarios is one of the main goals of quantum information theory. A prominent example for such a scenario is the task of quantum state merging, where two parties aim to merge their tripartite quantum state parts. In standard quantum state merging, entanglement is considered to be an expensive resource, while local quantum operations can be performed at no additional cost. However, recent developments show that some local operations could be more expensive than others: it is reasonable to distinguish between local incoherent operations and local operations which can create coherence. This idea leads us to the task of incoherent quantum state merging, where one of the parties has free access to local incoherent operations only. In this case the resources of the process are quantified by pairs of entanglement and coherence. Here, we develop tools for studying this process and apply them to several relevant scenarios. While quantum state merging can lead to a gain of entanglement, our results imply that no merging procedure can gain entanglement and coherence at the same time. We also provide a general lower bound on the entanglement-coherence sum and show that the bound is tight for all pure states. Our results also lead to an incoherent version of Schumacher compression: in this case the compression rate is equal to the von Neumann entropy of the diagonal elements of the corresponding quantum state. PMID:27367369
Laser-Limited Signatures of Quantum Coherence.
Tempelaar, Roel; Halpin, Alexei; Johnson, Philip J M; Cai, Jianxin; Murphy, R Scott; Knoester, Jasper; Miller, R J Dwayne; Jansen, Thomas L C
2016-05-19
Quantum coherence is proclaimed to promote efficient energy collection by light-harvesting complexes and prototype organic photovoltaics. However, supporting spectroscopic studies are hindered by the problem of distinguishing between the excited state and ground state origin of coherent spectral transients. Coherence amplitude maps, which systematically represent quantum beats observable in two-dimensional (2D) spectroscopy, are currently the prevalent tool for making this distinction. In this article, we present coherence amplitude maps of a molecular dimer, which have become significantly distorted as a result of the finite laser bandwidth used to record the 2D spectra. We argue that under standard spectroscopic conditions similar distortions are to be expected for compounds absorbing over a spectral range similar to, or exceeding, that of the dimer. These include virtually all photovoltaic polymers and certain photosynthetic complexes. With the distortion of coherence amplitude maps, alternative ways to identify quantum coherence are called for. Here, we use numerical simulations that reproduce the essential photophysics of the dimer to unambiguously determine the excited state origin of prominent quantum beats observed in the 2D spectral measurements. This approach is proposed as a dependable method for coherence identification. PMID:26558888
Relating the Resource Theories of Entanglement and Quantum Coherence.
Chitambar, Eric; Hsieh, Min-Hsiu
2016-07-01
Quantum coherence and quantum entanglement represent two fundamental features of nonclassical systems that can each be characterized within an operational resource theory. In this Letter, we unify the resource theories of entanglement and coherence by studying their combined behavior in the operational setting of local incoherent operations and classical communication (LIOCC). Specifically, we analyze the coherence and entanglement trade-offs in the tasks of state formation and resource distillation. For pure states we identify the minimum coherence-entanglement resources needed to generate a given state, and we introduce a new LIOCC monotone that completely characterizes a state's optimal rate of bipartite coherence distillation. This result allows us to precisely quantify the difference in operational powers between global incoherent operations, LIOCC, and local incoherent operations without classical communication. Finally, a bipartite mixed state is shown to have distillable entanglement if and only if entanglement can be distilled by LIOCC, and we strengthen the well-known Horodecki criterion for distillability. PMID:27447493
Relating the Resource Theories of Entanglement and Quantum Coherence
NASA Astrophysics Data System (ADS)
Chitambar, Eric; Hsieh, Min-Hsiu
2016-07-01
Quantum coherence and quantum entanglement represent two fundamental features of nonclassical systems that can each be characterized within an operational resource theory. In this Letter, we unify the resource theories of entanglement and coherence by studying their combined behavior in the operational setting of local incoherent operations and classical communication (LIOCC). Specifically, we analyze the coherence and entanglement trade-offs in the tasks of state formation and resource distillation. For pure states we identify the minimum coherence-entanglement resources needed to generate a given state, and we introduce a new LIOCC monotone that completely characterizes a state's optimal rate of bipartite coherence distillation. This result allows us to precisely quantify the difference in operational powers between global incoherent operations, LIOCC, and local incoherent operations without classical communication. Finally, a bipartite mixed state is shown to have distillable entanglement if and only if entanglement can be distilled by LIOCC, and we strengthen the well-known Horodecki criterion for distillability.
Energy cost of creating quantum coherence
NASA Astrophysics Data System (ADS)
Misra, Avijit; Singh, Uttam; Bhattacharya, Samyadeb; Pati, Arun Kumar
2016-05-01
We consider physical situations where the resource theories of coherence and thermodynamics play competing roles. In particular, we study the creation of quantum coherence using unitary operations with limited thermodynamic resources. We find the maximal coherence that can be created under unitary operations starting from a thermal state and find explicitly the unitary transformation that creates the maximal coherence. Since coherence is created by unitary operations starting from a thermal state, it requires some amount of energy. This motivates us to explore the trade-off between the amount of coherence that can be created and the energy cost of the unitary process. We also find the maximal achievable coherence under the constraint on the available energy. Additionally, we compare the maximal coherence and the maximal total correlation that can be created under unitary transformations with the same available energy at our disposal. We find that when maximal coherence is created with limited energy, the total correlation created in the process is upper bounded by the maximal coherence, and vice versa. For two-qubit systems we show that no unitary transformation exists that creates the maximal coherence and maximal total correlation simultaneously with a limited energy cost.
Photovoltaic quantum well infrared photodetectors
NASA Technical Reports Server (NTRS)
Lyon, Steve A.; Goossen, Keith; Parihar, Sanjay; Alavi, Kambiz; Santos, Mike; Shayegan, Mansour
1990-01-01
Quantum well infrared photodetectors (QWIP) are a promising new approach to long-wavelength infrared detector arrays. Both single-well photovoltaic and multiple-well photoconductive devices have been demonstrated. The author discusses noise considerations as they apply to photovoltaic devices, grating coupling of the infrared light into QWIPs, and recently demonstrated electrically tunable detectors. The use of light trapping to enhance the quantum efficiency and reduce cross-talk in an array is addressed.
Coherent control of atomic tunneling in a driven triple well
Lu Gengbiao; Hai Wenhua; Xie Qiongtao
2011-01-15
Coherent control of quantum tunneling is investigated for a single atom held in a driven triple-well potential without tight-binding approximation. In the high-frequency regime within or without multiphoton resonance, we find the analytical solutions and their numerical correspondences, including the special Floquet states of invariant populations and the non-Floquet states of slowly varying populations. The Floquet quasienergy spectrum exhibits anticrossings and crossings for different values of the driving parameters, which are associated with different tunneling properties described by the non-Floquet states. Applying the presented results, we suggest a scheme for transporting a single atom between nonadjacent wells or between adjacent wells.
Transient quantum coherent response to a partially coherent radiation field
Sadeq, Zaheen S.; Brumer, Paul
2014-02-21
The response of an arbitrary closed quantum system to a partially coherent electric field is investigated, with a focus on the transient coherences in the system. As a model we examine, both perturbatively and numerically, the coherences induced in a three level V system. Both rapid turn-on and pulsed turn-on effects are investigated. The effect of a long and incoherent pulse is also considered, demonstrating that during the pulse the system shows a coherent response which reduces after the pulse is over. Both the pulsed scenario and the thermally broadened CW case approach a mixed state in the long time limit, with rates dictated by the adjacent level spacings and the coherence time of the light, and via a mechanism that is distinctly different from traditional decoherence. These two excitation scenarios are also explored for a minimal “toy” model of the electronic levels in pigment protein complex PC645 by both a collisionally broadened CW laser and by a noisy pulse, where unexpectedly long transient coherence times are observed and explained. The significance of environmentally induced decoherence is noted.
Quantum well multijunction photovoltaic cell
Chaffin, Roger J.; Osbourn, Gordon C.
1987-01-01
A monolithic, quantum well, multilayer photovoltaic cell comprises a p-n junction comprising a p-region on one side and an n-region on the other side, each of which regions comprises a series of at least three semiconductor layers, all p-type in the p-region and all n-type in the n-region; each of said series of layers comprising alternating barrier and quantum well layers, each barrier layer comprising a semiconductor material having a first bandgap and each quantum well layer comprising a semiconductor material having a second bandgap when in bulk thickness which is narrower than said first bandgap, the barrier layers sandwiching each quantum well layer and each quantum well layer being sufficiently thin that the width of its bandgap is between said first and second bandgaps, such that radiation incident on said cell and above an energy determined by the bandgap of the quantum well layers will be absorbed and will produce an electrical potential across said junction.
Quantum well multijunction photovoltaic cell
Chaffin, R.J.; Osbourn, G.C.
1983-07-08
A monolithic, quantum well, multilayer photovoltaic cell comprises a p-n junction comprising a p-region on one side and an n-region on the other side, each of which regions comprises a series of at least three semiconductor layers, all p-type in the p-region and all n-type in the n-region; each of said series of layers comprising alternating barrier and quantum well layers, each barrier layer comprising a semiconductor material having a first bandgap and each quantum well layer comprising a semiconductor material having a second bandgap when in bulk thickness which is narrower than said first bandgap, the barrier layers sandwiching each quantum well layer and each quantum well layer being sufficiently thin that the width of its bandgap is between said first and second bandgaps, such that radiation incident on said cell and above an energy determined by the bandgap of the quantum well layers will be absorbed and will produce an electrical potential across said junction.
Langer, L; Poltavtsev, S V; Yugova, I A; Yakovlev, D R; Karczewski, G; Wojtowicz, T; Kossut, J; Akimov, I A; Bayer, M
2012-10-12
We report on magnetic field-induced oscillations of the photon echo signal from negatively charged excitons in a CdTe/(Cd,Mg)Te semiconductor quantum well. The oscillatory signal is due to Larmor precession of the electron spin about a transverse magnetic field and depends sensitively on the polarization configuration of the exciting and refocusing pulses. The echo amplitude can be fully tuned from the maximum down to zero depending on the time delay between the two pulses and the magnetic-field strength. The results are explained in terms of the optical Bloch equations accounting for the spin level structure of electrons and trions. PMID:23102368
Communication: Fully coherent quantum state hopping
Martens, Craig C.
2015-10-14
In this paper, we describe a new and fully coherent stochastic surface hopping method for simulating mixed quantum-classical systems. We illustrate the approach on the simple but unforgiving problem of quantum evolution of a two-state quantum system in the limit of unperturbed pure state dynamics and for dissipative evolution in the presence of both stationary and nonstationary random environments. We formulate our approach in the Liouville representation and describe the density matrix elements by ensembles of trajectories. Population dynamics are represented by stochastic surface hops for trajectories representing diagonal density matrix elements. These are combined with an unconventional coherent stochastic hopping algorithm for trajectories representing off-diagonal quantum coherences. The latter generalizes the binary (0,1) “probability” of a trajectory to be associated with a given state to allow integers that can be negative or greater than unity in magnitude. Unlike existing surface hopping methods, the dynamics of the ensembles are fully entangled, correctly capturing the coherent and nonlocal structure of quantum mechanics.
Communication: Fully coherent quantum state hopping
NASA Astrophysics Data System (ADS)
Martens, Craig C.
2015-10-01
In this paper, we describe a new and fully coherent stochastic surface hopping method for simulating mixed quantum-classical systems. We illustrate the approach on the simple but unforgiving problem of quantum evolution of a two-state quantum system in the limit of unperturbed pure state dynamics and for dissipative evolution in the presence of both stationary and nonstationary random environments. We formulate our approach in the Liouville representation and describe the density matrix elements by ensembles of trajectories. Population dynamics are represented by stochastic surface hops for trajectories representing diagonal density matrix elements. These are combined with an unconventional coherent stochastic hopping algorithm for trajectories representing off-diagonal quantum coherences. The latter generalizes the binary (0,1) "probability" of a trajectory to be associated with a given state to allow integers that can be negative or greater than unity in magnitude. Unlike existing surface hopping methods, the dynamics of the ensembles are fully entangled, correctly capturing the coherent and nonlocal structure of quantum mechanics.
Intrinsic randomness as a measure of quantum coherence
NASA Astrophysics Data System (ADS)
Yuan, Xiao; Zhou, Hongyi; Cao, Zhu; Ma, Xiongfeng
2015-08-01
Based on the theory of quantum mechanics, intrinsic randomness in measurement distinguishes quantum effects from classical ones. From the perspective of states, this quantum feature can be summarized as coherence or superposition in a specific (classical) computational basis. Recently, by regarding coherence as a physical resource, Baumgratz et al. [Phys. Rev. Lett. 113, 140401 (2014), 10.1103/PhysRevLett.113.140401] presented a comprehensive framework for coherence measures. Here, we propose a quantum coherence measure essentially using the intrinsic randomness of measurement. The proposed coherence measure provides an answer to the open question in completing the resource theory of coherence. Meanwhile, we show that the coherence distillation process can be treated as quantum extraction, which can be regarded as an equivalent process of classical random number extraction. From this viewpoint, the proposed coherence measure also clarifies the operational aspect of quantum coherence. Finally, our results indicate a strong similarity between two types of quantumness—coherence and entanglement.
Coherent quantum effects through dispersive bosonic media
Ye Saiyun; Yang Zhenbiao; Zheng Shibiao; Serafini, Alessio
2010-07-15
The coherent evolution of two qubits mediated by a set of bosonic field modes is investigated. By assuming a specific asymmetric encoding of the quantum states in the internal levels of the qubits, we show that entangling quantum gates can be realized, with high fidelity, even when a large number of mediating modes is involved. The effect of losses and imperfections on the gates' operation is also considered in detail.
Multiconfigurational quantum propagation with trajectory-guided generalized coherent states
NASA Astrophysics Data System (ADS)
Grigolo, Adriano; Viscondi, Thiago F.; de Aguiar, Marcus A. M.
2016-03-01
A generalized version of the coupled coherent states method for coherent states of arbitrary Lie groups is developed. In contrast to the original formulation, which is restricted to frozen-Gaussian basis sets, the extended method is suitable for propagating quantum states of systems featuring diversified physical properties, such as spin degrees of freedom or particle indistinguishability. The approach is illustrated with simple models for interacting bosons trapped in double- and triple-well potentials, most adequately described in terms of SU(2) and SU(3) bosonic coherent states, respectively.
Robustness of Coherence: An Operational and Observable Measure of Quantum Coherence
NASA Astrophysics Data System (ADS)
Napoli, Carmine; Bromley, Thomas R.; Cianciaruso, Marco; Piani, Marco; Johnston, Nathaniel; Adesso, Gerardo
2016-04-01
Quantifying coherence is an essential endeavor for both quantum foundations and quantum technologies. Here, the robustness of coherence is defined and proven to be a full monotone in the context of the recently introduced resource theories of quantum coherence. The measure is shown to be observable, as it can be recast as the expectation value of a coherence witness operator for any quantum state. The robustness of coherence is evaluated analytically on relevant classes of states, and an efficient semidefinite program that computes it on general states is given. An operational interpretation is finally provided: the robustness of coherence quantifies the advantage enabled by a quantum state in a phase discrimination task.
Quantum coherence in semiconductor nanostructures for improved lasers and detectors.
Chow, Weng Wah Dr.; Lyo, Sungkwun Kenneth; Cederberg, Jeffrey George; Modine, Normand Arthur; Biefeld, Robert Malcolm
2006-02-01
The potential for implementing quantum coherence in semiconductor self-assembled quantum dots has been investigated theoretically and experimentally. Theoretical modeling suggests that coherent dynamics should be possible in self-assembled quantum dots. Our experimental efforts have optimized InGaAs and InAs self-assembled quantum dots on GaAs for demonstrating coherent phenomena. Optical investigations have indicated the appropriate geometries for observing quantum coherence and the type of experiments for observing quantum coherence have been outlined. The optical investigation targeted electromagnetically induced transparency (EIT) in order to demonstrate an all optical delay line.
Mesoscopic systems: classical irreversibility and quantum coherence.
Barbara, Bernard
2012-09-28
Mesoscopic physics is a sub-discipline of condensed-matter physics that focuses on the properties of solids in a size range intermediate between bulk matter and individual atoms. In particular, it is characteristic of a domain where a certain number of interacting objects can easily be tuned between classical and quantum regimes, thus enabling studies at the border of the two. In magnetism, such a tuning was first realized with large-spin magnetic molecules called single-molecule magnets (SMMs) with archetype Mn(12)-ac. In general, the mesoscopic scale can be relatively large (e.g. micrometre-sized superconducting circuits), but, in magnetism, it is much smaller and can reach the atomic scale with rare earth (RE) ions. In all cases, it is shown how quantum relaxation can drastically reduce classical irreversibility. Taking the example of mesoscopic spin systems, the origin of irreversibility is discussed on the basis of the Landau-Zener model. A classical counterpart of this model is described enabling, in particular, intuitive understanding of most aspects of quantum spin dynamics. The spin dynamics of mesoscopic spin systems (SMM or RE systems) becomes coherent if they are well isolated. The study of the damping of their Rabi oscillations gives access to most relevant decoherence mechanisms by different environmental baths, including the electromagnetic bath of microwave excitation. This type of decoherence, clearly seen with spin systems, is easily recovered in quantum simulations. It is also observed with other types of qubits such as a single spin in a quantum dot or a superconducting loop, despite the presence of other competitive decoherence mechanisms. As in the molecular magnet V(15), the leading decoherence terms of superconducting qubits seem to be associated with a non-Markovian channel in which short-living entanglements with distributions of two-level systems (nuclear spins, impurity spins and/or charges) leading to 1/f noise induce τ(1)-like
Coherent control in simple quantum systems
NASA Technical Reports Server (NTRS)
Prants, Sergey V.
1995-01-01
Coherent dynamics of two, three, and four-level quantum systems, simultaneously driven by concurrent laser pulses of arbitrary and different forms, is treated by using a nonperturbative, group-theoretical approach. The respective evolution matrices are calculated in an explicit form. General aspects of controllability of few-level atoms by using laser fields are treated analytically.
NASA Astrophysics Data System (ADS)
Naseri, Tayebeh
2016-06-01
A new scheme for investigating electromagnetically induced grating (EIG) in the vanishing two-photon absorption condition in a three-level ladder-configuration n-doped semiconductor quantum well is presented. By applying a standing-wave field interacting with the system, the absorption and dispersion of the probe field will change with the spatial periodical modulation. It is shown that the first-order diffraction intensity sensitively depends on the intensity of coupling fields, detuning of applied laser fields and interaction length. Moreover, it can reach its maximum on varying the system parameters. A novel result shows the considerable efficiency of higher order diffractions is significantly improved via relative phase between applied laser fields. Furthermore, it is found that the intensity of the switching and coupling fields can increase the efficiency of the phase grating in the present model. Such a unique feature of the cooperative Electromagnetic Induced Grating may be extended to further develop diffraction based new photonic devices in quantum information networks and new photonic devices in all-optical switching and optical imaging.
Low-temperature thermodynamics with quantum coherence
Narasimhachar, Varun; Gour, Gilad
2015-01-01
Thermal operations are an operational model of non-equilibrium quantum thermodynamics. In the absence of coherence between energy levels, exact state transition conditions under thermal operations are known in terms of a mathematical relation called thermo-majorization. But incorporating coherence has turned out to be challenging, even under the relatively tractable model wherein all Gibbs state-preserving quantum channels are included. Here we find a mathematical generalization of thermal operations at low temperatures, ‘cooling maps', for which we derive the necessary and sufficient state transition condition. Cooling maps that saturate recently discovered bounds on coherence transfer are realizable as thermal operations, motivating us to conjecture that all cooling maps are thermal operations. Cooling maps, though a less-conservative generalization to thermal operations, are more tractable than Gibbs-preserving operations, suggesting that cooling map-like models at general temperatures could be of use in gaining insight about thermal operations. PMID:26138621
Quantum-Well Thermophotovoltaic Cells
NASA Technical Reports Server (NTRS)
Freudlich, Alex; Ignatiev, Alex
2009-01-01
Thermophotovoltaic cells containing multiple quantum wells have been invented as improved means of conversion of thermal to electrical energy. The semiconductor bandgaps of the quantum wells can be tailored to be narrower than those of prior thermophotovoltaic cells, thereby enabling the cells to convert energy from longer-wavelength photons that dominate the infrared-rich spectra of typical thermal sources with which these cells would be used. Moreover, in comparison with a conventional single-junction thermophotovoltaic cell, a cell containing multiple narrow-bandgap quantum wells according to the invention can convert energy from a wider range of wavelengths. Hence, the invention increases the achievable thermal-to-electrical energy-conversion efficiency. These thermophotovoltaic cells are expected to be especially useful for extracting electrical energy from combustion, waste-heat, and nuclear sources having temperatures in the approximate range from 1,000 to 1,500 C.
Coherence and measurement in quantum thermodynamics
Kammerlander, P.; Anders, J.
2016-01-01
Thermodynamics is a highly successful macroscopic theory widely used across the natural sciences and for the construction of everyday devices, from car engines to solar cells. With thermodynamics predating quantum theory, research now aims to uncover the thermodynamic laws that govern finite size systems which may in addition host quantum effects. Recent theoretical breakthroughs include the characterisation of the efficiency of quantum thermal engines, the extension of classical non-equilibrium fluctuation theorems to the quantum regime and a new thermodynamic resource theory has led to the discovery of a set of second laws for finite size systems. These results have substantially advanced our understanding of nanoscale thermodynamics, however putting a finger on what is genuinely quantum in quantum thermodynamics has remained a challenge. Here we identify information processing tasks, the so-called projections, that can only be formulated within the framework of quantum mechanics. We show that the physical realisation of such projections can come with a non-trivial thermodynamic work only for quantum states with coherences. This contrasts with information erasure, first investigated by Landauer, for which a thermodynamic work cost applies for classical and quantum erasure alike. Repercussions on quantum work fluctuation relations and thermodynamic single-shot approaches are also discussed. PMID:26916503
Coherence and measurement in quantum thermodynamics.
Kammerlander, P; Anders, J
2016-01-01
Thermodynamics is a highly successful macroscopic theory widely used across the natural sciences and for the construction of everyday devices, from car engines to solar cells. With thermodynamics predating quantum theory, research now aims to uncover the thermodynamic laws that govern finite size systems which may in addition host quantum effects. Recent theoretical breakthroughs include the characterisation of the efficiency of quantum thermal engines, the extension of classical non-equilibrium fluctuation theorems to the quantum regime and a new thermodynamic resource theory has led to the discovery of a set of second laws for finite size systems. These results have substantially advanced our understanding of nanoscale thermodynamics, however putting a finger on what is genuinely quantum in quantum thermodynamics has remained a challenge. Here we identify information processing tasks, the so-called projections, that can only be formulated within the framework of quantum mechanics. We show that the physical realisation of such projections can come with a non-trivial thermodynamic work only for quantum states with coherences. This contrasts with information erasure, first investigated by Landauer, for which a thermodynamic work cost applies for classical and quantum erasure alike. Repercussions on quantum work fluctuation relations and thermodynamic single-shot approaches are also discussed. PMID:26916503
Coherence and measurement in quantum thermodynamics
NASA Astrophysics Data System (ADS)
Kammerlander, P.; Anders, J.
2016-02-01
Thermodynamics is a highly successful macroscopic theory widely used across the natural sciences and for the construction of everyday devices, from car engines to solar cells. With thermodynamics predating quantum theory, research now aims to uncover the thermodynamic laws that govern finite size systems which may in addition host quantum effects. Recent theoretical breakthroughs include the characterisation of the efficiency of quantum thermal engines, the extension of classical non-equilibrium fluctuation theorems to the quantum regime and a new thermodynamic resource theory has led to the discovery of a set of second laws for finite size systems. These results have substantially advanced our understanding of nanoscale thermodynamics, however putting a finger on what is genuinely quantum in quantum thermodynamics has remained a challenge. Here we identify information processing tasks, the so-called projections, that can only be formulated within the framework of quantum mechanics. We show that the physical realisation of such projections can come with a non-trivial thermodynamic work only for quantum states with coherences. This contrasts with information erasure, first investigated by Landauer, for which a thermodynamic work cost applies for classical and quantum erasure alike. Repercussions on quantum work fluctuation relations and thermodynamic single-shot approaches are also discussed.
Tunable quantum well infrared detector
NASA Technical Reports Server (NTRS)
Maserjian, Joseph (Inventor)
1990-01-01
A novel infrared detector (20, 20', 20), is provided, which is characterized by photon-assisted resonant tunneling between adjacent quantum wells (22a, 22b) separated by barrier layers (28) in an intrinsic semiconductor layer (24) formed on an n.sup.+ substrate (26), wherein the resonance is electrically tunable over a wide band of wavelengths in the near to long infrared region. An n.sup.+ contacting layer (34) is formed over the intrinsic layer and the substrate is n.sup.+ doped to provide contact to the quantum wells. The detector permits fabrication of arrays (30) (one-dimensional and two-dimensional) for use in imaging and spectroscopy applications.
Strong quantum coherence between Fermi liquid Mahan excitons
Paul, J.; Stevens, C. E.; Liu, C.; Dey, P.; McIntyre, C.; Turkowski, V.; Reno, J. L.; Hilton, D. J.; Karaiskaj, D.
2016-04-14
In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called “Mahan excitons.” The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the opticalmore » Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Furthermore, time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system.« less
Strong Quantum Coherence between Fermi Liquid Mahan Excitons.
Paul, J; Stevens, C E; Liu, C; Dey, P; McIntyre, C; Turkowski, V; Reno, J L; Hilton, D J; Karaiskaj, D
2016-04-15
In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called "Mahan excitons." The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the optical Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system. PMID:27127985
Quantum measurement in coherence-vector representation
NASA Astrophysics Data System (ADS)
Zhou, Tao
2016-04-01
We consider the quantum measurements on a finite quantum system in coherence-vector representation. In this representation, all the density operators of an N-level ( N ⩾ 2) quantum system constitute a convex set M (N) embedded in an ( N 2 - 1)-dimensional Euclidean space R^{N^2 - 1}, and we find that an orthogonal measurement is an ( N - 1)-dimensional projector operator on R^{N^2 - 1}. The states unchanged by an orthogonal measurement form an ( N - 1)-dimensional simplex, and in the case when N is prime or power of prime, the space of the density operator is a direct sum of ( N + 1) such simplices. The mathematical description of quantum measurement is plain in this representation, and this may have further applications in quantum information processing.
Quantum Coherence Arguments for Cosmological Scale
Lindesay, James; /SLAC
2005-05-27
Homogeneity and correlations in the observed CMB are indicative of some form of cosmological coherence in early times. Quantum coherence in the early universe would be expected to give space-like phase coherence to any effects sourced to those times. If dark energy de-coherence is assumed to occur when the rate of expansion of the relevant cosmological scale parameter in the Friedmann-Lemaitre equations is no longer supra-luminal, a critical energy density is immediately defined. It is shown that the general class of dynamical models so defined necessarily requires a spatially flat cosmology in order to be consistent with observed structure formation. The basic assumption is that the dark energy density which is fixed during de-coherence is to be identified with the cosmological constant. It is shown for the entire class of models that the expected amplitude of fluctuations driven by the dark energy de-coherence process is of the order needed to evolve into the fluctuations observed in cosmic microwave background radiation and galactic clustering. The densities involved during de-coherence which correspond to the measured dark energy density turn out to be of the electroweak symmetry restoration scale. In an inflationary cosmology, this choice of the scale parameter in the FL equations directly relates the scale of dark energy decoherence to the De Sitter scales (associated with the positive cosmological constants) at both early and late times.
Coherent versus incoherent sequential quantum measurements
Filip, Radim
2011-03-15
We compare a trade-off between knowledge and decoherence for the incoherent and coherent partial sequential compatible measurements on single-qubit systems. The individual partial measurement nondestructively monitors basis states of the system by single-qubit meter. For the same decoherence caused by this unbiased measurement, the individual coherent measurement gives more knowledge than the incoherent one. For identical sequential coherent measurements, knowledge accumulated not additively increases more slowly than for the incoherent measurements. The overall knowledge can be accumulated using an adaptive measurement strategy on the meters if the single-qubit coherence of meters is kept. On the other hand, preservation of the mutual qubit coherence between the meters necessary for the collective measurement strategy is not required. A loss of single-qubit coherence degrades the coherent measurements back to the incoherent ones. Since the decoherence caused by the measurement process is a quadratic function of knowledge extracted by the individual measurement, Zeno-like behavior can be observed for repetitive weak compatible measurements. This unconditional universal effect does not depend on any dynamics of the qubit and it is a direct consequence of optimally controlled sequential evolution of quantum information.
Quantum coherence and closed timelike curves
NASA Astrophysics Data System (ADS)
Hawking, S. W.
1995-11-01
Various calculations of the S matrix have shown that it seems to be nonunitary for interacting fields when there are closed timelike curves. It is argued that this is because there is loss of quantum coherence caused by the fact that part of the quantum state circulates on the closed timelike curves and is not measured at infinity. A prescription is given for calculating the superscattering matrix on spacetimes whose parameters can be analytically continued to obtain a Euclidean metric. It is illustrated by a discussion of a spacetime in which two disks in flat space are indentified. If the disks have an imaginary time separation, this corresponds to a heat bath. An external field interacting with the heat bath will lose quantum coherence. One can then analytically continue to an almost real separation of the disks. This will give closed timelike curves but one will still get loss of quantum coherence. A comparison is made with the work of authors who find a nonunitary S matrix. It is shown that this is because the does not factor into an S matrix and its adjoint when the spacetime does not have the property of asymptotic completeness.
Quantum State Engineering Via Coherent-State Superpositions
NASA Technical Reports Server (NTRS)
Janszky, Jozsef; Adam, P.; Szabo, S.; Domokos, P.
1996-01-01
The quantum interference between the two parts of the optical Schrodinger-cat state makes possible to construct a wide class of quantum states via discrete superpositions of coherent states. Even a small number of coherent states can approximate the given quantum states at a high accuracy when the distance between the coherent states is optimized, e. g. nearly perfect Fock state can be constructed by discrete superpositions of n + 1 coherent states lying in the vicinity of the vacuum state.
Irreversible degradation of quantum coherence under relativistic motion
NASA Astrophysics Data System (ADS)
Wang, Jieci; Tian, Zehua; Jing, Jiliang; Fan, Heng
2016-06-01
We study the dynamics of quantum coherence under Unruh thermal noise and seek under which condition the coherence can be frozen in a relativistic setting. We find that the frozen condition is either (i) the initial state is prepared as an incoherence state or (ii) the detectors have no interaction with the external field. That is to say, the decoherence of the detectors' quantum state is irreversible under the influence of thermal noise induced by Unruh radiation. It is shown that quantum coherence approaches zero only in the limit of an infinite acceleration, while quantum entanglement could reduce to zero for a finite acceleration. It is also demonstrated that the robustness of quantum coherence is better than entanglement under the influence of the atom-field interaction for an extremely large acceleration. Therefore, quantum coherence is more robust than entanglement in an accelerating system and the coherence-type quantum resources are more accessible for relativistic quantum information processing tasks.
Quantum Zeno control of coherent dissociation
Khripkov, C.; Vardi, A.
2011-08-15
We study the effect of dephasing on the coherent dissociation dynamics of an atom-molecule Bose-Einstein condensate. We show that when phase-noise intensity is strong with respect to the inverse correlation time of the stimulated process, dissociation is suppressed via a Bose enhanced quantum Zeno effect. This is complementary to the quantum Zeno control of phase-diffusion in a bimodal condensate by symmetric noise [Phys. Rev. Lett. 100, 220403 (2008)] in that the controlled process here is phase formation and the required decoherence mechanism for its suppression is purely phase noise.
Quantum Coherence in Photosynthesis for Efficient Solar Energy Conversion
Romero, Elisabet; Augulis, Ramunas; Novoderezhkin, Vladimir I.; Ferretti, Marco; Thieme, Jos; Zigmantas, Donatas; van Grondelle, Rienk
2014-01-01
The crucial step in the conversion of solar to chemical energy in Photosynthesis takes place in the reaction center where the absorbed excitation energy is converted into a stable charge separated state by ultrafast electron transfer events. However, the fundamental mechanism responsible for the near unity quantum efficiency of this process is unknown. Here we elucidate the role of coherence in determining the efficiency of charge separation in the plant photosystem II reaction centre (PSII RC) by comprehensively combining experiment (two-dimensional electronic spectroscopy) and theory (Redfield theory). We reveal the presence of electronic coherence between excitons as well as between exciton and charge transfer states which we argue to be maintained by vibrational modes. Furthermore, we present evidence for the strong correlation between the degree of electronic coherence and efficient and ultrafast charge separation. We propose that this coherent mechanism will inspire the development of new energy technologies. PMID:26870153
Quantum memory with millisecond coherence in circuit QED
NASA Astrophysics Data System (ADS)
Reagor, Matthew; Pfaff, Wolfgang; Axline, Christopher; Heeres, Reinier W.; Ofek, Nissim; Sliwa, Katrina; Holland, Eric; Wang, Chen; Blumoff, Jacob; Chou, Kevin; Hatridge, Michael J.; Frunzio, Luigi; Devoret, Michel H.; Jiang, Liang; Schoelkopf, Robert J.
2016-07-01
Significant advances in coherence render superconducting quantum circuits a viable platform for fault-tolerant quantum computing. To further extend capabilities, highly coherent quantum systems could act as quantum memories for these circuits. A useful quantum memory must be rapidly addressable by Josephson-junction-based artificial atoms, while maintaining superior coherence. We demonstrate a superconducting microwave cavity architecture that is highly robust against major sources of loss that are encountered in the engineering of circuit QED systems. The architecture allows for storage of quantum superpositions in a resonator on the millisecond scale, while strong coupling between the resonator and a transmon qubit enables control, encoding, and readout at MHz rates. This extends the maximum available coherence time attainable in superconducting circuits by almost an order of magnitude compared to earlier hardware. Our design is an ideal platform for studying coherent quantum optics and marks an important step towards hardware-efficient quantum computing in Josephson-junction-based quantum circuits.
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){sup 2} were
Intrinsic quantum correlations of weak coherent states for quantum communication
Sua Yongmeng; Scanlon, Erin; Beaulieu, Travis; Bollen, Viktor; Lee, Kim Fook
2011-03-15
Intrinsic quantum correlations of weak coherent states are observed between two parties through a novel detection scheme, which can be used as a supplement to the existence decoy-state Bennett-Brassard 1984 protocol and the differential phase-shift quantum key distribution (DPS-QKD) protocol. In a proof-of-principle experiment, we generate bipartite correlations of weak coherent states using weak local oscillator fields in two spatially separated balanced homodyne detections. We employ a nonlinearity of postmeasurement method to obtain the bipartite correlations from two single-field interferences at individual homodyne measurements. This scheme is then used to demonstrate bits correlations between two parties over a distance of 10 km through a transmission fiber. We believe that the scheme can add another physical layer of security to these protocols for quantum key distribution.
Quantum speed limits, coherence, and asymmetry
NASA Astrophysics Data System (ADS)
Marvian, Iman; Spekkens, Robert W.; Zanardi, Paolo
2016-05-01
The resource theory of asymmetry is a framework for classifying and quantifying the symmetry-breaking properties of both states and operations relative to a given symmetry. In the special case where the symmetry is the set of translations generated by a fixed observable, asymmetry can be interpreted as coherence relative to the observable eigenbasis, and the resource theory of asymmetry provides a framework to study this notion of coherence. We here show that this notion of coherence naturally arises in the context of quantum speed limits. Indeed, the very concept of speed of evolution, i.e., the inverse of the minimum time it takes the system to evolve to another (partially) distinguishable state, is a measure of asymmetry relative to the time translations generated by the system Hamiltonian. Furthermore, the celebrated Mandelstam-Tamm and Margolus-Levitin speed limits can be interpreted as upper bounds on this measure of asymmetry by functions which are themselves measures of asymmetry in the special case of pure states. Using measures of asymmetry that are not restricted to pure states, such as the Wigner-Yanase skew information, we obtain extensions of the Mandelstam-Tamm bound which are significantly tighter in the case of mixed states. We also clarify some confusions in the literature about coherence and asymmetry, and show that measures of coherence are a proper subset of measures of asymmetry.
Quantum coherence and entanglement control for atom-cavity systems
NASA Astrophysics Data System (ADS)
Shu, Wenchong
Coherence and entanglement play a significant role in the quantum theory. Ideal quantum systems, "closed" to the outside world, remain quantum forever and thus manage to retain coherence and entanglement. Real quantum systems, however, are open to the environment and are therefore susceptible to the phenomenon of decoherence and disentanglement which are major hindrances to the effectiveness of quantum information processing tasks. In this thesis we have theoretically studied the evolution of coherence and entanglement in quantum systems coupled to various environments. We have also studied ways and means of controlling the decay of coherence and entanglement. We have studied the exact qubit entanglement dynamics of some interesting initial states coupled to a high-Q cavity containing zero photon, one photon, two photons and many photons respectively. We have found that an initially correlated environmental state can serve as an enhancer for entanglement decay or generation processes. More precisely, we have demonstrated that the degree of entanglement, including its collapse as well as its revival times, can be significantly modified by the correlated structure of the environmental modes. We have also studied dynamical decoupling (DD) technique --- a prominent strategy of controlling decoherence and preserving entanglement in open quantum systems. We have analyzed several DD control methods applied to qubit systems that can eliminate the system-environment coupling and prolong the quantum coherence time. Particularly, we have proposed a new DD sequence consisting a set of designed control operators that can universally protected an unknown qutrit state against colored phase and amplitude environment noises. In addition, in a non-Markovian regime, we have reformulated the quantum state diffusion (QSD) equation to incorporate the effect of the external control fields. Without any assumptions on the system-environment coupling and the size of environment, we have
Witnessing Quantum Coherence: from solid-state to biological systems
Li, Che-Ming; Lambert, Neill; Chen, Yueh-Nan; Chen, Guang-Yin; Nori, Franco
2012-01-01
Quantum coherence is one of the primary non-classical features of quantum systems. While protocols such as the Leggett-Garg inequality (LGI) and quantum tomography can be used to test for the existence of quantum coherence and dynamics in a given system, unambiguously detecting inherent “quantumness” still faces serious obstacles in terms of experimental feasibility and efficiency, particularly in complex systems. Here we introduce two “quantum witnesses” to efficiently verify quantum coherence and dynamics in the time domain, without the expense and burden of non-invasive measurements or full tomographic processes. Using several physical examples, including quantum transport in solid-state nanostructures and in biological organisms, we show that these quantum witnesses are robust and have a much finer resolution in their detection window than the LGI has. These robust quantum indicators may assist in reducing the experimental overhead in unambiguously verifying quantum coherence in complex systems. PMID:23185690
Quantum theory of optical coherence of nonstationary light in the space-frequency domain
Lahiri, Mayukh; Wolf, Emil
2010-10-15
Classical theories of coherence for statistically stationary, as well as, nonstationary optical fields are frequently discussed both in the space-time and in the space-frequency domains. However, the quantum treatment of coherence theory is generally carried out in the space-time domain. In this paper, we present a quantum-mechanical theory of first-order coherence for statistically nonstationary light in the space-frequency domain.
Quantum Well Infrared Photodetectors (QWIP)
NASA Technical Reports Server (NTRS)
Levine, B. F.
1990-01-01
There has been a lot of interest in III-V long wavelength detectors in the lambda = 8 to 12 micron spectral range as alternatives to HgCdTe. Recently high performance quantum well infrared photodetectors (QWIP) have been demonstrated. They have a responsivity of R = 1.2 A/W, and a detectivity D(exp asterisk) sub lambda = 2 times 10(exp 10) cm Hz(exp 1/2)/W at 68 K for a QWIP with a cutoff wavelength of lambda sub c = 10.7 micron and a R = 1.0 A/W, and D(exp asterisk) sub lambda = 2 times 10(exp 10) cm Hz(exp 1/2)/W at T = 77 K for lambda sub c = 8.4 micron. These detectors consist of 50 periods of molecular beam epitaxy (MBE) grown layers doped n = 1 times 10(exp 18)cm(exp -3) having GaAs quantum well widths of 40 A and barrier widths of 500 A of Al sub x Ga sub 1-x As. Due to the well-established GaAs growth and processing techniques, these detectors have the potential for large, highly uniform, low cost, high performance arrays as well as monolithic integration with GaAs electronics, high speed and radiation hardness. Latest results on the transport physics, device performance and arrays are discussed.
Quantum well infrared photodetector FPA
NASA Astrophysics Data System (ADS)
Kozlowski, L.
1994-03-01
The AT&T/Rockwell team met all the objectives of this collaborative program; AT&T supplied the QWIP detector arrays and Rockwell subsequently fabricated hybrid focal plane arrays using available high performance CMOS multiplexers, tested the hybrids, performed breadboard imaging demonstrations and delivered several hybrid FPA's. Eighteen hybrids were fabricated and evaluated. The collaboration yielded significant improvements in QWIP FPA performance and reliability and many milestones including: first BLIP LWIR FPA sensitivity demonstration at low photon backgrounds (less than 1 x 10(exp 12) photons/sq cm-sec) with the GaAs-based quantum well infrared photodetector (QWIP) technology, high LWIR FPA pixel operability; NE Delta T's as low as 5 mK at LWIR imaging backgrounds at f/1.4 and temperatures consistent with mechanical coolers (approx. 65K), increased coupling efficiency by over an order of magnitude; achieved effective quantum efficiency of approx. 10% with low crosstalk; effective quantum efficiencies of up to 30% under flood illumination, though with high crosstalk; mean D* of 1 x10(exp 14) cm-Hz(exp 1/2)/W at 3.0 x 10(exp 9) photons/sq cm-sec background at 32.5K operating temperature with greater than 98% operability; maximum temperature for 9.5 microns m FPA BLIP sensitivity as high as 62K; excellent hybrid reliability by mechanically thinning the QWIP; and responsivity nonuniformity less than 3% rms, thus enabling greater than 83 dB dynamic range.
Robust Multiple-Range Coherent Quantum State Transfer.
Chen, Bing; Peng, Yan-Dong; Li, Yong; Qian, Xiao-Feng
2016-01-01
We propose a multiple-range quantum communication channel to realize coherent two-way quantum state transport with high fidelity. In our scheme, an information carrier (a qubit) and its remote partner are both adiabatically coupled to the same data bus, i.e., an N-site tight-binding chain that has a single defect at the center. At the weak interaction regime, our system is effectively equivalent to a three level system of which a coherent superposition of the two carrier states constitutes a dark state. The adiabatic coupling allows a well controllable information exchange timing via the dark state between the two carriers. Numerical results show that our scheme is robust and efficient under practically inevitable perturbative defects of the data bus as well as environmental dephasing noise. PMID:27364891
Robust Multiple-Range Coherent Quantum State Transfer
Chen, Bing; Peng, Yan-Dong; Li, Yong; Qian, Xiao-Feng
2016-01-01
We propose a multiple-range quantum communication channel to realize coherent two-way quantum state transport with high fidelity. In our scheme, an information carrier (a qubit) and its remote partner are both adiabatically coupled to the same data bus, i.e., an N-site tight-binding chain that has a single defect at the center. At the weak interaction regime, our system is effectively equivalent to a three level system of which a coherent superposition of the two carrier states constitutes a dark state. The adiabatic coupling allows a well controllable information exchange timing via the dark state between the two carriers. Numerical results show that our scheme is robust and efficient under practically inevitable perturbative defects of the data bus as well as environmental dephasing noise. PMID:27364891
Robust Multiple-Range Coherent Quantum State Transfer
NASA Astrophysics Data System (ADS)
Chen, Bing; Peng, Yan-Dong; Li, Yong; Qian, Xiao-Feng
2016-07-01
We propose a multiple-range quantum communication channel to realize coherent two-way quantum state transport with high fidelity. In our scheme, an information carrier (a qubit) and its remote partner are both adiabatically coupled to the same data bus, i.e., an N-site tight-binding chain that has a single defect at the center. At the weak interaction regime, our system is effectively equivalent to a three level system of which a coherent superposition of the two carrier states constitutes a dark state. The adiabatic coupling allows a well controllable information exchange timing via the dark state between the two carriers. Numerical results show that our scheme is robust and efficient under practically inevitable perturbative defects of the data bus as well as environmental dephasing noise.
Observable measure of quantum coherence in finite dimensional systems.
Girolami, Davide
2014-10-24
Quantum coherence is the key resource for quantum technology, with applications in quantum optics, information processing, metrology, and cryptography. Yet, there is no universally efficient method for quantifying coherence either in theoretical or in experimental practice. I introduce a framework for measuring quantum coherence in finite dimensional systems. I define a theoretical measure which satisfies the reliability criteria established in the context of quantum resource theories. Then, I present an experimental scheme implementable with current technology which evaluates the quantum coherence of an unknown state of a d-dimensional system by performing two programmable measurements on an ancillary qubit, in place of the O(d2) direct measurements required by full state reconstruction. The result yields a benchmark for monitoring quantum effects in complex systems, e.g., certifying nonclassicality in quantum protocols and probing the quantum behavior of biological complexes. PMID:25379903
Kolarczik, Mirco; Owschimikow, Nina; Korn, Julian; Lingnau, Benjamin; Kaptan, Yücel; Bimberg, Dieter; Schöll, Eckehard; Lüdge, Kathy; Woggon, Ulrike
2013-01-01
Coherence in light–matter interaction is a necessary ingredient if light is used to control the quantum state of a material system. Coherent effects are firmly associated with isolated systems kept at low temperature. The exceedingly fast dephasing in condensed matter environments, in particular at elevated temperatures, may well erase all coherent information in the material at timescales shorter than a laser excitation pulse. Here we show for an ensemble of semiconductor quantum dots that even in the presence of ultrafast dephasing, for suitably designed condensed matter systems quantum-coherent effects are robust enough to be observable at room temperature. Our conclusions are based on an analysis of the reshaping an ultrafast laser pulse undergoes on propagation through a semiconductor quantum dot amplifier. We show that this pulse modification contains the signature of coherent light–matter interaction and can be controlled by adjusting the population of the quantum dots via electrical injection. PMID:24336000
Versluis, J H; Kimel, A V; Gridnev, V N; Yakovlev, D R; Karczewski, G; Wojtowicz, T; Kossut, J; Kirilyuk, A; Rasing, Th
2010-03-24
Two temporally non-overlapping linearly cross-polarized 140 fs laser pulses are shown to control the spin polarization in a three-level system. Simultaneous excitation of the two excited states triggers quantum beatings originating from the interference of the wavefunctions corresponding to different spin sublevels of the states. Although the beatings are not seen in the spin densities of the excited states they are clearly observed in the magneto-optical Kerr effect. An analytical expression for the description of the beatings is obtained. Experimental results are in good agreement with theoretical predictions and demonstrate the control of beatings with attosecond resolution. PMID:21389474
NASA Astrophysics Data System (ADS)
Benatti, Fabio; Floreanini, Roberto; Scholes, Greg
2012-08-01
The last years have witnessed fast growing developments in the use of quantum mechanics in technology-oriented and information-related fields, especially in metrology, in the developments of nano-devices and in understanding highly efficient transport processes. The consequent theoretical and experimental outcomes are now driving new experimental tests of quantum mechanical effects with unprecedented accuracies that carry with themselves the concrete possibility of novel technological spin-offs. Indeed, the manifold advances in quantum optics, atom and ion manipulations, spintronics and nano-technologies are allowing direct experimental verifications of new ideas and their applications to a large variety of fields. All of these activities have revitalized interest in quantum mechanics and created a unique framework in which theoretical and experimental physics have become fruitfully tangled with information theory, computer, material and life sciences. This special issue aims to provide an overview of what is currently being pursued in the field and of what kind of theoretical reference frame is being developed together with the experimental and theoretical results. It consists of three sections: 1. Memory effects in quantum dynamics and quantum channels 2. Driven open quantum systems 3. Experiments concerning quantum coherence and/or decoherence The first two sections are theoretical and concerned with open quantum systems. In all of the above mentioned topics, the presence of an external environment needs to be taken into account, possibly in the presence of external controls and/or forcing, leading to driven open quantum systems. The open system paradigm has proven to be central in the analysis and understanding of many basic issues of quantum mechanics, such as the measurement problem, quantum communication and coherence, as well as for an ever growing number of applications. The theory is, however, well-settled only when the so-called Markovian or memoryless
Excitons in asymmetric quantum wells
NASA Astrophysics Data System (ADS)
Grigoryev, P. S.; Kurdyubov, A. S.; Kuznetsova, M. S.; Ignatiev, I. V.; Efimov, Yu. P.; Eliseev, S. A.; Petrov, V. V.; Lovtcius, V. A.; Shapochkin, P. Yu.
2016-09-01
Resonance dielectric response of excitons is studied for the high-quality InGaAs/GaAs heterostructures with wide asymmetric quantum wells (QWs). To highlight effects of the QW asymmetry, we have grown and studied several heterostructures with nominally square QWs as well as with triangle-like QWs. Several quantum confined exciton states are experimentally observed as narrow exciton resonances. A standard approach for the phenomenological analysis of the profiles is generalized by introducing different phase shifts for the light waves reflected from the QWs at different exciton resonances. Good agreement of the phenomenological fit to the experimentally observed exciton spectra for high-quality structures allowed us to reliably obtain parameters of the exciton resonances: the exciton transition energies, the radiative broadenings, and the phase shifts. A direct numerical solution of the Schrödinger equation for the heavy-hole excitons in asymmetric QWs is used for microscopic modeling of the exciton resonances. Remarkable agreement with the experiment is achieved when the effect of indium segregation is taken into account. The segregation results in a modification of the potential profile, in particular, in an asymmetry of the nominally square QWs.
Transient coherent nonlinear spectroscopy of single quantum dots.
Langbein, Wolfgang; Patton, Brian
2007-07-25
We review our recent advances in four-wave mixing spectroscopy of single semiconductor quantum dots using heterodyne spectral interferometry, a novel implementation of transient nonlinear spectroscopy allowing the study of the transient nonlinear polarization emitted from individual electronic transitions in both amplitude and phase. We present experiments on individual excitonic transitions localized in monolayer islands of GaAs/AlAs quantum wells and in self-assembled CdTe/ZnTe quantum dots. We investigate the formation of the photon echo from individual transitions, both with increasing number of transitions in the ensemble, and in the presence of temporal jitter of the energy of a single transition. The detection of amplitude and phase of the signal allows the implementation of a two-dimensional femtosecond spectroscopy, in which mutual coherent coupling of single quantum dot states can observed and quantified. PMID:21483055
External cavity coherent quantum cascade laser array
NASA Astrophysics Data System (ADS)
Vallon, Raphael; Parvitte, Bertrand; Bizet, Laurent; De Naurois, Guy Mael; Simozrag, Bouzid; Maisons, Grégory; Carras, Mathieu; Zeninari, Virginie
2016-05-01
We report on the development of a coherent quantum cascade laser array that consists in the fabrication of multi-stripes array. The main characteristic of this kind of source is that an anti-symmetrical signature with two lobes is obtained in the far field. Taking advantage of this drawback, a grating is aligned with one lobe of the source. Thus a Littrow configuration is designed that permit to obtain a wide tunability of the source. First results are presented and a preliminary test of the source is realized by measurements on acetone.
Silicon Germanium Quantum Well Thermoelectrics
NASA Astrophysics Data System (ADS)
Davidson, Anthony Lee, III
Today's growing energy demands require new technologies to provide high efficiency clean energy. Thermoelectrics that convert heat to electrical energy directly can provide a method for the automobile industry to recover waste heat to power vehicle electronics, hence improving fuel economy. If large enough efficiencies can be obtained then the internal combustion engine could even be replaced. Exhaust temperature for automotive application range from 400 to 800 K. In this temperature range the current state of the art materials are bulk Si1-xGex alloys. By alternating layers of Si and Si1-xGex alloy device performance may be enhanced through quantum well effects and variations in material thermal properties. In this study, superlattices designed for in-plane operation with varying period and crystallinity are examined to determine the effect on electrical and thermal properties. In-plane electrical resistivity of these materials was found to be below the bulk material at a similar doping at room temperature, confirming the role of quantum wells in electron transport. As period is reduced in the structures boundary scattering limits electron propagation leading to increased resistivity. The Seebeck coefficient measured at room temperature is higher than the bulk material, additionally lending proof to the effects of quantum wells. When examining cross-plane operation the low doping in the Si layers of the device produce high resistivity resulting from boundary scattering. Thermal conductivity was measured from 77 K up to 674 K and shows little variation due to periodicity and temperature, however an order of magnitude reduction over bulk Si1-xGex is shown in all samples. A model is developed that suggests a combination of phonon dispersion effects and strong boundary scattering. Further study of the phonon dispersion effects was achieved through the examination of the heat capacity by combining thermal diffusivity with thermal conductivity. All superlattices show a
Robustness of Coherence: An Operational and Observable Measure of Quantum Coherence.
Napoli, Carmine; Bromley, Thomas R; Cianciaruso, Marco; Piani, Marco; Johnston, Nathaniel; Adesso, Gerardo
2016-04-15
Quantifying coherence is an essential endeavor for both quantum foundations and quantum technologies. Here, the robustness of coherence is defined and proven to be a full monotone in the context of the recently introduced resource theories of quantum coherence. The measure is shown to be observable, as it can be recast as the expectation value of a coherence witness operator for any quantum state. The robustness of coherence is evaluated analytically on relevant classes of states, and an efficient semidefinite program that computes it on general states is given. An operational interpretation is finally provided: the robustness of coherence quantifies the advantage enabled by a quantum state in a phase discrimination task. PMID:27127946
Ballistic effects and intersubband excitations in multiple quantum well structures
NASA Astrophysics Data System (ADS)
Schneider, H.; Schönbein, C.; Schwarz, K.; Walther, M.
1998-07-01
We have studied the transport properties of electrons in asymmetric quantum well structures upon far-infrared optical excitation of carriers from the lowest subband into the continuum. Here the photocurrent consists of a coherent component originating from ballistic transport upon excitation, and of an incoherent part associated with asymmetric diffusion and relaxation processes, which occur after the coherence has been lost. The signature of the coherent contribution is provided by a sign reversal of the photocurrent upon changing the excitation energy. This sign reversal arises from the energy-dependent interference between continuum states, which have a twofold degeneracy characterized by positive and negative momenta. The interference effect also allows us to estimate the coherent mean free path ( >20 nm at 77K). In specifically designed device structures, we use both the coherent and incoherent components in order to achieve a pronounced photovoltaic infrared response for detector applications.
Blind Quantum Computing with Weak Coherent Pulses
NASA Astrophysics Data System (ADS)
Dunjko, Vedran; Kashefi, Elham; Leverrier, Anthony
2012-05-01
The universal blind quantum computation (UBQC) protocol [A. Broadbent, J. Fitzsimons, and E. Kashefi, in Proceedings of the 50th Annual IEEE Symposiumon Foundations of Computer Science (IEEE Computer Society, Los Alamitos, CA, USA, 2009), pp. 517-526.] allows a client to perform quantum computation on a remote server. In an ideal setting, perfect privacy is guaranteed if the client is capable of producing specific, randomly chosen single qubit states. While from a theoretical point of view, this may constitute the lowest possible quantum requirement, from a pragmatic point of view, generation of such states to be sent along long distances can never be achieved perfectly. We introduce the concept of ɛ blindness for UBQC, in analogy to the concept of ɛ security developed for other cryptographic protocols, allowing us to characterize the robustness and security properties of the protocol under possible imperfections. We also present a remote blind single qubit preparation protocol with weak coherent pulses for the client to prepare, in a delegated fashion, quantum states arbitrarily close to perfect random single qubit states. This allows us to efficiently achieve ɛ-blind UBQC for any ɛ>0, even if the channel between the client and the server is arbitrarily lossy.
Effects of quantum coherence in metalloprotein electron transfer
NASA Astrophysics Data System (ADS)
Dorner, Ross; Goold, John; Heaney, Libby; Farrow, Tristan; Vedral, Vlatko
2012-09-01
Many intramolecular electron transfer (ET) reactions in biology are mediated by metal centers in proteins. This process is commonly described by a model of diffusive hopping according to the semiclassical theories of Marcus and Hopfield. However, recent studies have raised the possibility that nontrivial quantum mechanical effects play a functioning role in certain biomolecular processes. Here, we investigate the potential effects of quantum coherence in biological ET by extending the semiclassical model to allow for the possibility of quantum coherent phenomena using a quantum master equation based on the Holstein Hamiltonian. We test the model on the structurally defined chain of seven iron-sulfur clusters in nicotinamide adenine dinucleotide plus hydrogen:ubiquinone oxidoreductase (complex I), a crucial respiratory enzyme and one of the longest chains of metal centers in biology. Using experimental parameters where possible, we find that, in limited circumstances, a small quantum mechanical contribution can provide a marked increase in the ET rate above the semiclassical diffusive-hopping rate. Under typical biological conditions, our model reduces to well-known diffusive behavior.
Ultra Thin Quantum Well Materials
Dr Saeid Ghamaty
2012-08-16
This project has enabled Hi-Z technology Inc. (Hi-Z) to understand how to improve the thermoelectric properties of Si/SiGe Quantum Well Thermoelectric Materials. The research that was completed under this project has enabled Hi-Z Technology, Inc. (Hi-Z) to satisfy the project goal to understand how to improve thermoelectric conversion efficiency and reduce costs by fabricating ultra thin Si/SiGe quantum well (QW) materials and measuring their properties. In addition, Hi-Z gained critical new understanding on how thin film fabrication increases the silicon substrate's electrical conductivity, which is important new knowledge to develop critical material fabrication parameters. QW materials are constructed with alternate layers of an electrical conductor, SiGe and an electrical insulator, Si. Film thicknesses were varied, ranging from 2nm to 10nm where 10 nm was the original film thickness prior to this work. The optimum performance was determined at a Si and SiGe thickness of 4nm for an electrical current and heat flow parallel to the films, which was an important conclusion of this work. Essential new information was obtained on how the Si substrate electrical conductivity increases by up to an order of magnitude upon deposition of QW films. Test measurements and calculations are accurate and include both the quantum well and the substrate. The large increase in substrate electrical conductivity means that a larger portion of the electrical current passes through the substrate. The silicon substrate's increased electrical conductivity is due to inherent impurities and thermal donors which are activated during both molecular beam epitaxy and sputtering deposition of QW materials. Hi-Z's forward looking cost estimations based on future high performance QW modules, in which the best Seebeck coefficient and electrical resistivity are taken from separate samples predict that the electricity cost produced with a QW module could be achieved at <$0.35/W. This price would
Quantum well earth science testbed
NASA Astrophysics Data System (ADS)
Johnson, William R.; Hook, Simon J.; Mouroulis, Pantazis; Wilson, Daniel W.; Gunapala, Sarath D.; Hill, Cory J.; Mumolo, Jason M.; Eng, Bjorn T.
2009-11-01
A thermal hyperspectral imager is underdevelopment which utilizes the compact Dyson optical configuration and the broadband (8-12 μm) quantum well infrared photodetector (QWIP) focal plane array technology. The Dyson configuration uses a single monolithic prism-like grating design which allows for a high throughput instrument (F/1.6) with minimal ghosting, stray light and large swath width. The configuration has the potential to be the optimal high resolution imaging spectroscopy solution for aerial and space remote sensing applications due to its small form factor and relatively low power requirements. The planned instrument specifications are discussed as well as thermal design trade-offs. The current design uses a single high power cryocooler which allows operation of the QWIP at 40 K with adequate temperature stability. Calibration testing results (noise equivalent temperature difference, spectral linearity and spectral bandwidth) and laboratory emissivity plots from samples are shown using an operational testbed unit which has similar specifications as the final airborne system. Field testing of the testbed unit was performed to acquire plots of emissivity for various known standard minerals (quartz, opal, alunite). A comparison is made using data from the ASTER spectral library. The current single band (8-9 μm) testbed utilizes the high uniformity and operability of the QWIP array and shows excellent laboratory and field spectroscopic results.
Quantum Coherence, Time-Translation Symmetry, and Thermodynamics
NASA Astrophysics Data System (ADS)
Lostaglio, Matteo; Korzekwa, Kamil; Jennings, David; Rudolph, Terry
2015-04-01
The first law of thermodynamics imposes not just a constraint on the energy content of systems in extreme quantum regimes but also symmetry constraints related to the thermodynamic processing of quantum coherence. We show that this thermodynamic symmetry decomposes any quantum state into mode operators that quantify the coherence present in the state. We then establish general upper and lower bounds for the evolution of quantum coherence under arbitrary thermal operations, valid for any temperature. We identify primitive coherence manipulations and show that the transfer of coherence between energy levels manifests irreversibility not captured by free energy. Moreover, the recently developed thermomajorization relations on block-diagonal quantum states are observed to be special cases of this symmetry analysis.
Arbitrated Quantum Signature Scheme with Continuous-Variable Coherent States
NASA Astrophysics Data System (ADS)
Guo, Ying; Feng, Yanyan; Huang, Dazu; Shi, Jinjing
2016-04-01
Motivated by the revealing features of the continuous-variable (CV) quantum cryptography, we suggest an arbitrated quantum signature (AQS) protocol with CV coherent states. It involves three participants, i.e., the signer Alice, the verifier Bob and the arbitrator Charlie who is trustworthy by Alice and Bob. Three phases initializing phase, signing phase and verifying phase are included in our protocol. The security of the signature scheme is guaranteed by the generation of the shared keys via the CV-based quantum key distribution (CV-QKD) and the implementation process of the CV-based quantum teleportation as well. Security analysis demonstrates that the signature can be neither forged by anyone nor disavowed by the receiver and signer. Moreover, the authenticity and integrality of the transmitted messages can be ensured. The paper shows that a potential high-speed quantum signature scheme with high detection efficiency and repetition rate can be realized when compared to the discrete-variable (DV) quantum signature scheme attributing to the well characteristics of CV-QKD.
Coherent control of diamond defects for quantum information science and quantum sensing
NASA Astrophysics Data System (ADS)
Maurer, Peter
Quantum mechanics, arguably one of the greatest achievements of modern physics, has not only fundamentally changed our understanding of nature but is also taking an ever increasing role in engineering. Today, the control of quantum systems has already had a far-reaching impact on time and frequency metrology. By gaining further control over a large variety of different quantum systems, many potential applications are emerging. Those applications range from the development of quantum sensors and new quantum metrological approaches to the realization of quantum information processors and quantum networks. Unfortunately most quantum systems are very fragile objects that require tremendous experimental effort to avoid dephasing. Being able to control the interaction between a quantum system with its local environment embodies therefore an important aspect for application and hence is at the focus of this thesis. Nitrogen Vacancy (NV) color centers in diamond have recently attracted attention as a room temperature solid state spin system that expresses long coherence times. The electronic spin associated with NV centers can be efficiently manipulated, initialized and readout using microwave and optical techniques. Inspired by these extraordinary properties, much effort has been dedicated to use NV centers as a building block for scalable room temperature quantum information processing and quantum communication as well as a quantum sensing. In the first part of this thesis we demonstrate that by decoupling the spin from the local environment the coherence time of a NV quantum register can be extended by three order of magnitudes. Employing a novel dissipative mechanism in combination with dynamical decoupling, memory times exceeding one second are observed. The second part shows that, based on quantum control, NV centers in nano-diamonds provide a nanoscale temperature sensor with unprecedented accuracy enabling local temperature measurements in living biological cells
Certifying the quantumness of a generalized coherent control scenario.
Scholak, Torsten; Brumer, Paul
2014-11-28
We consider the role of quantum mechanics in a specific coherent control scenario, designing a "coherent control interferometer" as the essential tool that links coherent control to quantum fundamentals. Building upon this allows us to rigorously display the genuinely quantum nature of a generalized weak-field coherent control scenario (utilizing 1 vs. 2 photon excitation) via a Bell-CHSH test. Specifically, we propose an implementation of "quantum delayed-choice" in a bichromatic alkali atom photoionization experiment. The experimenter can choose between two complementary situations, which are characterized by a random photoelectron spin polarization with particle-like behavior on the one hand, and by spin controllability and wave-like nature on the other. Because these two choices are conditioned coherently on states of the driving fields, it becomes physically unknowable, prior to measurement, whether there is control over the spin or not. PMID:25429946
Certifying the quantumness of a generalized coherent control scenario
NASA Astrophysics Data System (ADS)
Scholak, Torsten; Brumer, Paul
2014-11-01
We consider the role of quantum mechanics in a specific coherent control scenario, designing a "coherent control interferometer" as the essential tool that links coherent control to quantum fundamentals. Building upon this allows us to rigorously display the genuinely quantum nature of a generalized weak-field coherent control scenario (utilizing 1 vs. 2 photon excitation) via a Bell-CHSH test. Specifically, we propose an implementation of "quantum delayed-choice" in a bichromatic alkali atom photoionization experiment. The experimenter can choose between two complementary situations, which are characterized by a random photoelectron spin polarization with particle-like behavior on the one hand, and by spin controllability and wave-like nature on the other. Because these two choices are conditioned coherently on states of the driving fields, it becomes physically unknowable, prior to measurement, whether there is control over the spin or not.
Certifying the quantumness of a generalized coherent control scenario
Scholak, Torsten Brumer, Paul
2014-11-28
We consider the role of quantum mechanics in a specific coherent control scenario, designing a “coherent control interferometer” as the essential tool that links coherent control to quantum fundamentals. Building upon this allows us to rigorously display the genuinely quantum nature of a generalized weak-field coherent control scenario (utilizing 1 vs. 2 photon excitation) via a Bell-CHSH test. Specifically, we propose an implementation of “quantum delayed-choice” in a bichromatic alkali atom photoionization experiment. The experimenter can choose between two complementary situations, which are characterized by a random photoelectron spin polarization with particle-like behavior on the one hand, and by spin controllability and wave-like nature on the other. Because these two choices are conditioned coherently on states of the driving fields, it becomes physically unknowable, prior to measurement, whether there is control over the spin or not.
Coherent and passive one dimensional quantum memory
NASA Astrophysics Data System (ADS)
Ping, Yuting; Jefferson, John H.; Lovett, Brendon W.
2014-10-01
We show that the state of a flying qubit may be transferred to a chain of identical, (near) ferromagnetically polarized, but non-interacting, static spin-\\frac{1}{2} particles in a passive way. During this process the flying qubit is coherently polarized, emerging in the direction of the majority static spins. We conjecture that this process is reversible for any number of flying qubits injected sequentially in an arbitrary superposition state, proving this explicitly for an arbitrary state of one and two flying qubits. We also find a special case in which we are able to prove the conjecture for an arbitrary number of qubits. Our architecture thus has the potential to be exploited as a passive quantum memory to encode the flying qubits without the necessity of resetting between successive encoding operations. We also illustrate that the quantum information may be spread over many static spins in the memory chain, making the mechanism resistant to spin decoherence and other imperfections. We discuss implementing the memory system with trapped bosonic atoms, controlled by a spatial light modulator.
Quantum dot spin coherence governed by a strained nuclear environment.
Stockill, R; Le Gall, C; Matthiesen, C; Huthmacher, L; Clarke, E; Hugues, M; Atatüre, M
2016-01-01
The interaction between a confined electron and the nuclei of an optically active quantum dot provides a uniquely rich manifestation of the central spin problem. Coherent qubit control combines with an ultrafast spin-photon interface to make these confined spins attractive candidates for quantum optical networks. Reaching the full potential of spin coherence has been hindered by the lack of knowledge of the key irreversible environment dynamics. Through all-optical Hahn echo decoupling we now recover the intrinsic coherence time set by the interaction with the inhomogeneously strained nuclear bath. The high-frequency nuclear dynamics are directly imprinted on the electron spin coherence, resulting in a dramatic jump of coherence times from few tens of nanoseconds to the microsecond regime between 2 and 3 T magnetic field and an exponential decay of coherence at high fields. These results reveal spin coherence can be improved by applying large magnetic fields and reducing strain inhomogeneity. PMID:27615704
Frobenius-norm-based measures of quantum coherence and asymmetry.
Yao, Yao; Dong, G H; Xiao, Xing; Sun, C P
2016-01-01
We formulate the Frobenius-norm-based measures for quantum coherence and asymmetry respectively. In contrast to the resource theory of coherence and asymmetry, we construct a natural measure of quantum coherence inspired from optical coherence theory while the group theoretical approach is employed to quantify the asymmetry of quantum states. Besides their simple structures and explicit physical meanings, we observe that these quantities are intimately related to the purity (or linear entropy) of the corresponding quantum states. Remarkably, we demonstrate that the proposed coherence quantifier is not only a measure of mixedness, but also an intrinsic (basis-independent) quantification of quantum coherence contained in quantum states, which can also be viewed as a normalized version of Brukner-Zeilinger invariant information. In our context, the asymmetry of N-qubit quantum systems is considered under local independent and collective transformations. In- triguingly, it is illustrated that the collective effect has a significant impact on the asymmetry measure, and quantum correlation between subsystems plays a non-negligible role in this circumstance. PMID:27558009
Frobenius-norm-based measures of quantum coherence and asymmetry
Yao, Yao; Dong, G. H.; Xiao, Xing; Sun, C. P.
2016-01-01
We formulate the Frobenius-norm-based measures for quantum coherence and asymmetry respectively. In contrast to the resource theory of coherence and asymmetry, we construct a natural measure of quantum coherence inspired from optical coherence theory while the group theoretical approach is employed to quantify the asymmetry of quantum states. Besides their simple structures and explicit physical meanings, we observe that these quantities are intimately related to the purity (or linear entropy) of the corresponding quantum states. Remarkably, we demonstrate that the proposed coherence quantifier is not only a measure of mixedness, but also an intrinsic (basis-independent) quantification of quantum coherence contained in quantum states, which can also be viewed as a normalized version of Brukner-Zeilinger invariant information. In our context, the asymmetry of N-qubit quantum systems is considered under local independent and collective transformations. In- triguingly, it is illustrated that the collective effect has a significant impact on the asymmetry measure, and quantum correlation between subsystems plays a non-negligible role in this circumstance. PMID:27558009
Measuring finite quantum geometries via quasi-coherent states
NASA Astrophysics Data System (ADS)
Schneiderbauer, Lukas; Steinacker, Harold C.
2016-07-01
We develop a systematic approach to determine and measure numerically the geometry of generic quantum or ‘fuzzy’ geometries realized by a set of finite-dimensional Hermitian matrices. The method is designed to recover the semi-classical limit of quantized symplectic spaces embedded in {{{R}}}d including the well-known examples of fuzzy spaces, but it applies much more generally. The central tool is provided by quasi-coherent states, which are defined as ground states of Laplace- or Dirac operators corresponding to localized point branes in target space. The displacement energy of these quasi-coherent states is used to extract the local dimension and tangent space of the semi-classical geometry, and provides a measure for the quality and self-consistency of the semi-classical approximation. The method is discussed and tested with various examples, and implemented in an open-source Mathematica package.
Quantum tunneling switch in a planar four-well system
NASA Astrophysics Data System (ADS)
Lu, Gengbiao; Hai, Wenhua
2011-05-01
We investigate the tunneling dynamics of a single atom in a planar four-well potential driven by a high-frequency ac field. The quasienergy spectrum exhibits anticrossing and crossing, which are related to selective coherent destruction of tunneling (CDT) with several selectable directions. By using the CDTs of different directions, the switchlike effect is shown for the six tunneling pathways among the four wells. Applying the present results, we suggest a scheme for designing a single-atom quantum motor with the driving field as a quantum starter.
Quantum tunneling switch in a planar four-well system
Lu Gengbiao; Hai Wenhua
2011-05-15
We investigate the tunneling dynamics of a single atom in a planar four-well potential driven by a high-frequency ac field. The quasienergy spectrum exhibits anticrossing and crossing, which are related to selective coherent destruction of tunneling (CDT) with several selectable directions. By using the CDTs of different directions, the switchlike effect is shown for the six tunneling pathways among the four wells. Applying the present results, we suggest a scheme for designing a single-atom quantum motor with the driving field as a quantum starter.
Quantum coherence and uncertainty in the anisotropic XY chain
NASA Astrophysics Data System (ADS)
Karpat, G.; ćakmak, B.; Fanchini, F. F.
2014-09-01
We explore the local quantum coherence and the local quantum uncertainty, based on Wigner-Yanase skew information, in the ground state of the anisotropic spin-1/2 XY chain in a transverse magnetic field. We show that the skew information, as a figure of merit, supplies the necessary information to reveal the occurrence of the second-order phase transition and the completely factorized ground state in the XY model. Additionally, in the same context, we also discuss the usefulness of a simple experimentally friendly lower bound of local quantum coherence. Furthermore, we demonstrate how the connection between the appearance of nonanalyticities in the local quantum uncertainty of the ground state and the quantum phase transitions does not hold in general, by providing explicit examples of the situation. Lastly, we discuss the ability of the local quantum coherence to accurately estimate the critical point of the phase transition, and we investigate the robustness of the factorization phenomenon at low temperatures.
Manipulating single electron spins and coherence in quantum dots
NASA Astrophysics Data System (ADS)
Awschalom, David
2008-05-01
The non-destructive detection of a single electron spin in a quantum dot (QD) is demonstrated using a time- averaged magneto-optical Kerr rotation measurementootnotetextJ. Berezovsky, M. H. Mikkelsen, O. Gywat, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, Science 314, 1916 (2006).. This technique provides a means to directly probe the spin off- resonance, thus minimally disturbing the system. Furthermore, the ability to sequentially initialize, manipulate, and read out the state of a qubit, such as an electron spin in a quantum dot, is necessary for virtually any scheme for quantum information processing. In addition to the time-averaged measurements, we have extended the single dot KR technique into the time domain with pulsed pump and probe lasers, allowing the observation of the coherent evolution of an electron spin stateootnotetextM. H. Mikkelsen, J. Berezovsky, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, Nature Physics 3, 770 (2007).. The dot is formed by interface fluctuations of a GaAs quantum well and embedded in a diode structure to allow controllable gating/charging of the QD. To enhance the small single spin signal, the QD is positioned within a vertical optical cavity. Observations of coherent single spin precession in an applied magnetic field allow a direct measurement of the electron g-factor and transverse spin lifetime. These measurements reveal information about the relevant spin decoherence mechanisms, while also providing a sensitive probe of the local nuclear spin environment. Finally, we have recently eveloped a scheme for high speed all-optical manipulation of the spin state that enables multiple operations within the coherence timeootnotetextJ. Berezovsky, M. H. Mikkelsen, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, accepted for publication (2008).. The results represent progress toward the control and coupling of single spins and photons for quantum information processingootnotetextS. Ghosh, W.H. Wang, F. M. Mendoza, R. C
Quantum walk coherences on a dynamical percolation graph
NASA Astrophysics Data System (ADS)
Elster, Fabian; Barkhofen, Sonja; Nitsche, Thomas; Novotný, Jaroslav; Gábris, Aurél; Jex, Igor; Silberhorn, Christine
2015-08-01
Coherent evolution governs the behaviour of all quantum systems, but in nature it is often subjected to influence of a classical environment. For analysing quantum transport phenomena quantum walks emerge as suitable model systems. In particular, quantum walks on percolation structures constitute an attractive platform for studying open system dynamics of random media. Here, we present an implementation of quantum walks differing from the previous experiments by achieving dynamical control of the underlying graph structure. We demonstrate the evolution of an optical time-multiplexed quantum walk over six double steps, revealing the intricate interplay between the internal and external degrees of freedom. The observation of clear non-Markovian signatures in the coin space testifies the high coherence of the implementation and the extraordinary degree of control of all system parameters. Our work is the proof-of-principle experiment of a quantum walk on a dynamical percolation graph, paving the way towards complex simulation of quantum transport in random media.
Editorial . Quantum fluctuations and coherence in optical and atomic structures
NASA Astrophysics Data System (ADS)
Eschner, Jürgen; Gatti, Alessandra; Maître, Agnès; Morigi, Giovanna
2003-03-01
From simple interference fringes, over molecular wave packets, to nonlinear optical patterns - the fundamental interaction between light and matter leads to the formation of structures in many areas of atomic and optical physics. Sophisticated technology in experimental quantum optics, as well as modern computational tools available to theorists, have led to spectacular achievements in the investigation of quantum structures. This special issue is dedicated to recent developments in this area. It presents a selection of examples where quantum dynamics, fluctuations, and coherence generate structures in time or in space or where such structures are observed experimentally. The examples range from coherence phenomena in condensed matter, over atoms in optical structures, entanglement in light and matter, to quantum patterns in nonlinear optics and quantum imaging. The combination of such seemingly diverse subjects formed the basis of a successful European TMR network, "Quantum Structures" (visit http://cnqo.phys.strath.ac.uk/~gianluca/QSTRUCT/). This special issue partly re.ects the results and collaborations of the network, going however well beyond its scope by including contributions from a global community and from many related topics which were not addressed directly in the network. The aim of this issue is to present side by side these di.erent topics, all of which are loosely summarized under quantum structures, to highlight their common aspects, their di.erences, and the progress which resulted from the mutual exchange of results, methods, and knowledge. To guide the reader, we have organized the articles into subsections which follow a rough division into structures in material systems and structures in optical .elds. Nevertheless, in the following introduction we point out connections between the contributions which go beyond these usual criteria, thus highlighting the truly interdisciplinary nature of quantum structures. Much of the progress in atom optics
Quantum coherence in a coupled-cavity array
NASA Astrophysics Data System (ADS)
Cao, De-Wei; Zhang, Yixin; Wang, Jicheng; Hu, Zheng-Da
2016-05-01
The dynamical properties of quantum coherence in the system of two-coupled-cavities, each of which resonantly interacts with a two-level atom, is investigated via the relative entropy measure. We focus on the coherences for the atom-atom, atom-cavity and cavity-cavity subsystems and find that the dynamical behaviors of these coherences depend largely on the cavity-cavity coupling, which may indicate the Mott insulator-superfluid transition in the thermodynamic limit. We also study the influences of the initial cavity-cavity correlation on the coherences and show that the initial correlation of the cavity-cavity subsystem can enhance the revival ability for the atom-atom and cavity-cavity coherences while reduce that for the atom-cavity coherence. Besides, we demonstrate the qualitative difference of dynamics between coherence and entanglement. Finally, the influences of dissipations including cavity losses and atomic decays on the coherence are explored.
Atomtronics with holes: Coherent transport of an empty site in a triple-well potential
Benseny, A.; Fernandez-Vidal, S.; Baguda, J.; Corbalan, R.; Picon, A.; Mompart, J.; Roso, L.; Birkl, G.
2010-07-15
We investigate arrays of three traps with two fermionic or bosonic atoms. The tunneling interaction between neighboring sites is used to prepare multisite dark states for the empty site (i.e., the hole) which allows for the coherent manipulation of its external degrees of freedom. By means of an ab initio integration of the Schroedinger equation, we investigate the adiabatic transport of a hole between the two extreme traps of a triple-well potential. Furthermore, a quantum-trajectory approach based on the de Broglie-Bohm formulation of quantum mechanics is used to get physical insight into the transport process. Finally, we discuss the use of the hole for the construction of a coherent single hole diode and a coherent single hole transistor.
Türkpençe, Deniz; Müstecaplıoğlu, Özgür E
2016-01-01
We investigate scaling of work and efficiency of a photonic Carnot engine with a number of quantum coherent resources. Specifically, we consider a generalization of the "phaseonium fuel" for the photonic Carnot engine, which was first introduced as a three-level atom with two lower states in a quantum coherent superposition by M. O. Scully, M. Suhail Zubairy, G. S. Agarwal, and H. Walther [Science 299, 862 (2003)SCIEAS0036-807510.1126/science.1078955], to the case of N+1 level atoms with N coherent lower levels. We take into account atomic relaxation and dephasing as well as the cavity loss and derive a coarse-grained master equation to evaluate the work and efficiency analytically. Analytical results are verified by microscopic numerical examination of the thermalization dynamics. We find that efficiency and work scale quadratically with the number of quantum coherent levels. Quantum coherence boost to the specific energy (work output per unit mass of the resource) is a profound fundamental difference of quantum fuel from classical resources. We consider typical modern resonator set ups and conclude that multilevel phaseonium fuel can be utilized to overcome the decoherence in available systems. Preparation of the atomic coherences and the associated cost of coherence are analyzed and the engine operation within the bounds of the second law is verified. Our results bring the photonic Carnot engines much closer to the capabilities of current resonator technologies. PMID:26871061
NASA Astrophysics Data System (ADS)
Türkpençe, Deniz; Müstecaplıoǧlu, Özgür E.
2016-01-01
We investigate scaling of work and efficiency of a photonic Carnot engine with a number of quantum coherent resources. Specifically, we consider a generalization of the "phaseonium fuel" for the photonic Carnot engine, which was first introduced as a three-level atom with two lower states in a quantum coherent superposition by M. O. Scully, M. Suhail Zubairy, G. S. Agarwal, and H. Walther [Science 299, 862 (2003), 10.1126/science.1078955], to the case of N +1 level atoms with N coherent lower levels. We take into account atomic relaxation and dephasing as well as the cavity loss and derive a coarse-grained master equation to evaluate the work and efficiency analytically. Analytical results are verified by microscopic numerical examination of the thermalization dynamics. We find that efficiency and work scale quadratically with the number of quantum coherent levels. Quantum coherence boost to the specific energy (work output per unit mass of the resource) is a profound fundamental difference of quantum fuel from classical resources. We consider typical modern resonator set ups and conclude that multilevel phaseonium fuel can be utilized to overcome the decoherence in available systems. Preparation of the atomic coherences and the associated cost of coherence are analyzed and the engine operation within the bounds of the second law is verified. Our results bring the photonic Carnot engines much closer to the capabilities of current resonator technologies.
Coherence susceptibility as a probe of quantum phase transitions
NASA Astrophysics Data System (ADS)
Chen, Jin-Jun; Cui, Jian; Zhang, Yu-Ran; Fan, Heng
2016-08-01
We introduce a coherence susceptibility method, based on the fact that it signals quantum fluctuations, for identifying quantum phase transitions, which are induced by quantum fluctuations. This method requires no prior knowledge of order parameter, and there is no need for careful considerations concerning the choice of a bipartition of the system. It can identify different types of quantum phase transition points exactly. At finite temperatures, where quantum criticality is influenced by thermal fluctuations, our method can pinpoint the temperature frame of quantum criticality, which perfectly coincides with recent experiments.
Bound states in continuum: Quantum dots in a quantum well
NASA Astrophysics Data System (ADS)
Prodanović, Nikola; Milanović, Vitomir; Ikonić, Zoran; Indjin, Dragan; Harrison, Paul
2013-11-01
We report on the existence of a bound state in the continuum (BIC) of quantum rods (QR). QRs are novel elongated InGaAs quantum dot nanostructures embedded in the shallower InGaAs quantum well. BIC appears as an excited confined dot state and energetically above the bottom of a well subband continuum. We prove that high height-to-diameter QR aspect ratio and the presence of a quantum well are indispensable conditions for accommodating the BIC. QRs are unique semiconductor nanostructures, exhibiting this mathematical curiosity predicted 83 years ago by Wigner and von Neumann.
Coherent States of Quantum Free Particle on the Spherical Space
NASA Astrophysics Data System (ADS)
Dehdashti, Shahram; Roknizadeh, Rasoul; Mahdifar, Ali; Chen, Hongsheng
2016-01-01
In this paper, we study the quantum free particle on the spherical space by applying da costa approach for quantum particle on the curved space. We obtain the discrete energy eigenvalues and associated normalized eigenfunctions of the free particle on the sphere. In addition, we introduce the Gazeau-Klauder coherent states of free particle on the sphere. Then, the Gaussian coherent states is defined, which is used to describe the localized particle on the spherical space. Finally, we study the relation between the f-deformed coherent states and Gazeau-Klauder ones for this system.
General framework for quantum macroscopicity in terms of coherence
NASA Astrophysics Data System (ADS)
Yadin, Benjamin; Vedral, Vlatko
2016-02-01
We propose a universal language to assess macroscopic quantumness in terms of coherence, with a set of conditions that should be satisfied by any measure of macroscopic coherence. We link the framework to the resource theory of asymmetry. We show that the quantum Fisher information gives a good measure of macroscopic coherence, enabling a rigorous justification of a previously proposed measure of macroscopicity. This picture lets us draw connections between different measures of macroscopicity and evaluate them; we show that another widely studied measure fails one of our criteria.
Quantum dots as active material for quantum cascade lasers: comparison to quantum wells
NASA Astrophysics Data System (ADS)
Michael, Stephan; Chow, Weng W.; Schneider, Hans Christian
2016-03-01
We review a microscopic laser theory for quantum dots as active material for quantum cascade lasers, in which carrier collisions are treated at the level of quantum kinetic equations. The computed characteristics of such a quantum-dot active material are compared to a state-of-the-art quantum-well quantum cascade laser. We find that the current requirement to achieve a comparable gain-length product is reduced compared to that of the quantum-well quantum cascade laser.
Coherent quantum depletion of an interacting atom condensate.
Kira, M
2015-01-01
Sufficiently strong interactions promote coherent quantum transitions in spite of thermalization and losses, which are the adversaries of delicate effects such as reversibility and correlations. In atomic Bose-Einstein condensates (BECs), strong atom-atom interactions can eject atoms from the BEC to the normal component, yielding quantum depletion instead of temperature depletion. A recent experiment has already been verified to overcome losses. Here I show that it also achieves coherent quantum-depletion dynamics in a BEC swept fast enough from weak to strong atom-atom interactions. The elementary coherent process first excites the normal component into a liquid state that evolves into a spherical shell state, where the atom occupation peaks at a finite momentum to shield 50% of the BEC atoms from annihilation. The identified coherent processes resemble ultrafast semiconductor excitations expanding the scope of BEC explorations to many-body non-equilibrium studies. PMID:25767044
Coherent quantum depletion of an interacting atom condensate
Kira, M.
2015-01-01
Sufficiently strong interactions promote coherent quantum transitions in spite of thermalization and losses, which are the adversaries of delicate effects such as reversibility and correlations. In atomic Bose–Einstein condensates (BECs), strong atom–atom interactions can eject atoms from the BEC to the normal component, yielding quantum depletion instead of temperature depletion. A recent experiment has already been verified to overcome losses. Here I show that it also achieves coherent quantum-depletion dynamics in a BEC swept fast enough from weak to strong atom–atom interactions. The elementary coherent process first excites the normal component into a liquid state that evolves into a spherical shell state, where the atom occupation peaks at a finite momentum to shield 50% of the BEC atoms from annihilation. The identified coherent processes resemble ultrafast semiconductor excitations expanding the scope of BEC explorations to many-body non-equilibrium studies. PMID:25767044
Focus on coherent control of complex quantum systems
NASA Astrophysics Data System (ADS)
Whaley, Birgitta; Milburn, Gerard
2015-10-01
The rapid growth of quantum information sciences over the past few decades has fueled a corresponding rise in high profile applications in fields such as metrology, sensors, spintronics, and attosecond dynamics, in addition to quantum information processing. Realizing this potential of today’s quantum science and the novel technologies based on this requires a high degree of coherent control of quantum systems. While early efforts in systematizing methods for high fidelity quantum control focused on isolated or closed quantum systems, recent advances in experimental design, measurement and monitoring, have stimulated both need and interest in the control of complex or large scale quantum systems that may also be coupled to an interactive environment or reservoir. This focus issue brings together new theoretical and experimental work addressing the formulation and implementation of quantum control for a broad range of applications in quantum science and technology today.
Quantum Detection and Invisibility in Coherent Nanostructures
Fransson, J.
2010-04-28
We address quantum invisibility in the context of electronics in nanoscale quantum structures. In analogy with metamaterials, we use the freedom of design that quantum corrals provide and show that quantum mechanical objects can be hidden inside the corral, with respect to inelastic electron scattering spectroscopy in combination with scanning tunneling microscopy, and we propose a design strategy. A simple illustration of the invisibility is given in terms of an elliptic quantum corral containing a molecule, with a local vibrational mode, at one of the foci. Our work has implications to quantum information technology and presents new tools for nonlocal quantum detection and distinguishing between different molecules.
Dynamics of open bosonic quantum systems in coherent state representation
Dalvit, D. A. R.; Berman, G. P.; Vishik, M.
2006-01-15
We consider the problem of decoherence and relaxation of open bosonic quantum systems from a perspective alternative to the standard master equation or quantum trajectories approaches. Our method is based on the dynamics of expectation values of observables evaluated in a coherent state representation. We examine a model of a quantum nonlinear oscillator with a density-density interaction with a collection of environmental oscillators at finite temperature. We derive the exact solution for dynamics of observables and demonstrate a consistent perturbation approach.
Considerations for the extension of coherent optical processors into the quantum computing regime
NASA Astrophysics Data System (ADS)
Young, Rupert C. D.; Birch, Philip M.; Chatwin, Chris R.
2016-04-01
Previously we have examined the similarities of the quantum Fourier transform to the classical coherent optical implementation of the Fourier transform (R. Young et al, Proc SPIE Vol 87480, 874806-1, -11). In this paper, we further consider how superposition states can be generated on coherent optical wave fronts, potentially allowing coherent optical processing hardware architectures to be extended into the quantum computing regime. In particular, we propose placing the pixels of a Spatial Light Modulator (SLM) individually in a binary superposition state and illuminating them with a coherent wave front from a conventional (but low intensity) laser source in order to make a so-called `interaction free' measurement. In this way, the quantum object, i.e. the individual pixels of the SLM in their superposition states, and the illuminating wavefront would become entangled. We show that if this were possible, it would allow the extension of coherent processing architectures into the quantum computing regime and we give an example of such a processor configured to recover one of a known set of images encrypted using the well-known coherent optical processing technique of employing a random Fourier plane phase encryption mask which classically requires knowledge of the corresponding phase conjugate key to decrypt the image. A quantum optical computer would allow interrogation of all possible phase masks in parallel and so immediate decryption.
NASA Astrophysics Data System (ADS)
Viola, Lorenza; Tannor, David
2011-08-01
Precisely characterizing and controlling the dynamics of realistic open quantum systems has emerged in recent years as a key challenge across contemporary quantum sciences and technologies, with implications ranging from physics, chemistry and applied mathematics to quantum information processing (QIP) and quantum engineering. Quantum control theory aims to provide both a general dynamical-system framework and a constructive toolbox to meet this challenge. The purpose of this special issue of Journal of Physics B: Atomic, Molecular and Optical Physics is to present a state-of-the-art account of recent advances and current trends in the field, as reflected in two international meetings that were held on the subject over the last summer and which motivated in part the compilation of this volume—the Topical Group: Frontiers in Open Quantum Systems and Quantum Control Theory, held at the Institute for Theoretical Atomic, Molecular and Optical Physics (ITAMP) in Cambridge, Massachusetts (USA), from 1-14 August 2010, and the Safed Workshop on Quantum Decoherence and Thermodynamics Control, held in Safed (Israel), from 22-27 August 2010. Initial developments in quantum control theory date back to (at least) the early 1980s, and have been largely inspired by the well-established mathematical framework for classical dynamical systems. As the above-mentioned meetings made clear, and as the burgeoning body of literature on the subject testifies, quantum control has grown since then well beyond its original boundaries, and has by now evolved into a highly cross-disciplinary field which, while still fast-moving, is also entering a new phase of maturity, sophistication, and integration. Two trends deserve special attention: on the one hand, a growing emphasis on control tasks and methodologies that are specifically motivated by QIP, in addition and in parallel to applications in more traditional areas where quantum coherence is nevertheless vital (such as, for instance
Coherent tunnelling across a quantum point contact in the quantum Hall regime.
Martins, F; Faniel, S; Rosenow, B; Sellier, H; Huant, S; Pala, M G; Desplanque, L; Wallart, X; Bayot, V; Hackens, B
2013-01-01
The unique properties of quantum hall devices arise from the ideal one-dimensional edge states that form in a two-dimensional electron system at high magnetic field. Tunnelling between edge states across a quantum point contact (QPC) has already revealed rich physics, like fractionally charged excitations, or chiral Luttinger liquid. Thanks to scanning gate microscopy, we show that a single QPC can turn into an interferometer for specific potential landscapes. Spectroscopy, magnetic field and temperature dependences of electron transport reveal a quantitatively consistent interferometric behavior of the studied QPC. To explain this unexpected behavior, we put forward a new model which relies on the presence of a quantum Hall island at the centre of the constriction as well as on different tunnelling paths surrounding the island, thereby creating a new type of interferometer. This work sets the ground for new device concepts based on coherent tunnelling. PMID:23475303
Coherent tunnelling across a quantum point contact in the quantum Hall regime
Martins, F.; Faniel, S.; Rosenow, B.; Sellier, H.; Huant, S.; Pala, M. G.; Desplanque, L.; Wallart, X.; Bayot, V.; Hackens, B.
2013-01-01
The unique properties of quantum hall devices arise from the ideal one-dimensional edge states that form in a two-dimensional electron system at high magnetic field. Tunnelling between edge states across a quantum point contact (QPC) has already revealed rich physics, like fractionally charged excitations, or chiral Luttinger liquid. Thanks to scanning gate microscopy, we show that a single QPC can turn into an interferometer for specific potential landscapes. Spectroscopy, magnetic field and temperature dependences of electron transport reveal a quantitatively consistent interferometric behavior of the studied QPC. To explain this unexpected behavior, we put forward a new model which relies on the presence of a quantum Hall island at the centre of the constriction as well as on different tunnelling paths surrounding the island, thereby creating a new type of interferometer. This work sets the ground for new device concepts based on coherent tunnelling. PMID:23475303
Quantum correlations and coherence in spin-1 Heisenberg chains
NASA Astrophysics Data System (ADS)
Malvezzi, A. L.; Karpat, G.; ćakmak, B.; Fanchini, F. F.; Debarba, T.; Vianna, R. O.
2016-05-01
We explore quantum and classical correlations along with coherence in the ground states of spin-1 Heisenberg chains, namely the one-dimensional XXZ model and the one-dimensional bilinear biquadratic model, with the techniques of density matrix renormalization group theory. Exploiting the tools of quantum information theory, that is, by studying quantum discord, quantum mutual information, and three recently introduced coherence measures in the reduced density matrix of two nearest neighbor spins in the bulk, we investigate the quantum phase transitions and special symmetry points in these models. We point out the relative strengths and weaknesses of correlation and coherence measures as figures of merit to witness the quantum phase transitions and symmetry points in the considered spin-1 Heisenberg chains. In particular, we demonstrate that, as none of the studied measures can detect the infinite-order Kosterlitz-Thouless transition in the XXZ model, they appear to be able to signal the existence of the same type of transition in the biliear biquadratic model. However, we argue that what is actually detected by the measures here is the SU(3) symmetry point of the model rather than the infinite-order quantum phase transition. Moreover, we show in the XXZ model that examining even single site coherence can be sufficient to spotlight the second-order phase transition and the SU(2) symmetry point.
Editorial . Quantum fluctuations and coherence in optical and atomic structures
NASA Astrophysics Data System (ADS)
Eschner, Jürgen; Gatti, Alessandra; Maître, Agnès; Morigi, Giovanna
2003-03-01
From simple interference fringes, over molecular wave packets, to nonlinear optical patterns - the fundamental interaction between light and matter leads to the formation of structures in many areas of atomic and optical physics. Sophisticated technology in experimental quantum optics, as well as modern computational tools available to theorists, have led to spectacular achievements in the investigation of quantum structures. This special issue is dedicated to recent developments in this area. It presents a selection of examples where quantum dynamics, fluctuations, and coherence generate structures in time or in space or where such structures are observed experimentally. The examples range from coherence phenomena in condensed matter, over atoms in optical structures, entanglement in light and matter, to quantum patterns in nonlinear optics and quantum imaging. The combination of such seemingly diverse subjects formed the basis of a successful European TMR network, "Quantum Structures" (visit http://cnqo.phys.strath.ac.uk/~gianluca/QSTRUCT/). This special issue partly re.ects the results and collaborations of the network, going however well beyond its scope by including contributions from a global community and from many related topics which were not addressed directly in the network. The aim of this issue is to present side by side these di.erent topics, all of which are loosely summarized under quantum structures, to highlight their common aspects, their di.erences, and the progress which resulted from the mutual exchange of results, methods, and knowledge. To guide the reader, we have organized the articles into subsections which follow a rough division into structures in material systems and structures in optical .elds. Nevertheless, in the following introduction we point out connections between the contributions which go beyond these usual criteria, thus highlighting the truly interdisciplinary nature of quantum structures. Much of the progress in atom optics
Coherent x-ray diffraction from quantum dots
Vartanyants, I.A.; Robinson, I. K.; Onken, J.D.; Pfeifer, M.A.; Williams, G.J.; Pfeiffer, F.; Metzger, H.; Zhong, Z.; Bauer, G.
2005-06-15
Coherent x-ray diffraction is a new experimental method for studying perfect and imperfect crystals. Instead of incoherent averaging, a coherent sum of amplitudes produces a coherent diffraction pattern originating from the real space arrangement of the sample. We applied this method for studying quantum dot samples that were specially fabricated GeSi islands of nanometer size and in a regular array embedded into a Si substrate. A coherent beam was focused by special Kirkpatric-Baez optics to a micrometer size. In the experiment it was observed that such a microfocused coherent beam produced coherent diffraction pattern with Bragg spots and broad diffuse maxima. The diffuse peak breaks up into a fine speckle pattern. The grazing incidence diffraction pattern has a typical shape resulting from the periodic array of identical islands. We used this diffraction pattern to reconstruct the average shape of the islands using a model independent approach.
NASA Astrophysics Data System (ADS)
Lloyd, Seth
2014-03-01
Femtosecond spectroscopy reveals significant quantum coherence in excitonic transport in photosynthetic organisms. How and why are living systems using quantum mechanics? This talk presents a simple theory of how to optimize energy transport in quantum systems that possess noise and disorder. Too much quantum coherence leads to destructive interference and localization, while too little coherence prevents energy from moving at all, via the watchdog or quantum Zeno effect. With just the right amount of quantum coherence, however, energy can move through photosynthetic complexes with almost 100% efficiency. This talk explains how plants and photosynthetic bacteria attain such high efficiencies for energy transport, and discusses how human-made systems could be designed to attain similar efficiencies.
Resonator-quantum well infrared photodetectors
Choi, K. K. Sun, J.; Olver, K.; Jhabvala, M. D.; Jhabvala, C. A.; Waczynski, A.
2013-11-11
We applied a recent electromagnetic model to design the resonator-quantum well infrared photodetector (R-QWIP). In this design, we used an array of rings as diffractive elements to diffract normal incident light into parallel propagation and used the pixel volume as a resonator to intensify the diffracted light. With a proper pixel size, the detector resonates at certain optical wavelengths and thus yields a high quantum efficiency (QE). To test this detector concept, we fabricated a number of R-QWIPs with different quantum well materials and detector geometries. The experimental result agrees satisfactorily with the prediction, and the highest QE achieved is 71%.
Coherent states and parasupersymmetric quantum mechanics
NASA Technical Reports Server (NTRS)
Debergh, Nathalie
1992-01-01
It is well known that Parafermi and Parabose statistics are natural extensions of the usual Fermi and Bose ones, enhancing trilinear (anti)commutation relations instead of bilinear ones. Due to this generalization, positive parameters appear: the so-called orders of paraquantization p (= 1, 2, 3, ...) and h sub 0 (= 1/2, 1, 3/2, ...), respectively, the first value leading in each case to the usual statistics. The superpostion of the parabosonic and parafermionic operators gives rise to parasupermultiplets for which mixed trilinear relations have already been studied leading to two (nonequivalent) sets: the relative Parabose and the relative Parafermi ones. For the specific values p = 1 = 2h sub 0, these sets reduce to the well known supersymmetry. Coherent states associated with this last model have been recently put in evidence through the annihilation operator point of view and the group theoretical approach or displacement operator context. We propose to realize the corresponding studies within the new context p = 2 = 2h sub 0, being then directly extended to any order of paraquantization.
Quantum repeater based on cavity QED evolutions and coherent light
NASA Astrophysics Data System (ADS)
Gonţa, Denis; van Loock, Peter
2016-05-01
In the framework of cavity QED, we propose a quantum repeater scheme that uses coherent light and chains of atoms coupled to optical cavities. In contrast to conventional repeater schemes, in our scheme there is no need for an explicit use of two-qubit quantum logical gates by exploiting solely the cavity QED evolution. In our previous work (Gonta and van Loock in Phys Rev A 88:052308, 2013), we already proposed a quantum repeater in which the entanglement between two neighboring repeater nodes was distributed using controlled displacements of input coherent light, while the produced low-fidelity entangled pairs were purified using ancillary (four-partite) entangled states. In the present work, the entanglement distribution is realized using a sequence of controlled phase shifts and displacements of input coherent light. Compared to previous coherent-state-based distribution schemes for two-qubit entanglement, our scheme here relies only upon a simple discrimination of two coherent states with opposite signs, which can be performed in a quantum mechanically optimal fashion via a beam splitter and two on-off detectors. For the entanglement purification, we employ a method that avoids the use of extra entangled ancilla states. Our repeater scheme exhibits reasonable fidelities and repeater rates providing an attractive platform for long-distance quantum communication.
Coherent radiation by quantum dots and magnetic nanoclusters
Yukalov, V. I.; Yukalova, E. P.
2014-03-31
The assemblies of either quantum dots or magnetic nanoclusters are studied. It is shown that such assemblies can produce coherent radiation. A method is developed for solving the systems of nonlinear equations describing the dynamics of such assemblies. The method is shown to be general and applicable to systems of different physical nature. Despite mathematical similarities of dynamical equations, the physics of the processes for quantum dots and magnetic nanoclusters is rather different. In a quantum dot assembly, coherence develops due to the Dicke effect of dot interactions through the common radiation field. For a system of magnetic clusters, coherence in the spin motion appears due to the Purcell effect caused by the feedback action of a resonator. Self-organized coherent spin radiation cannot arise without a resonator. This principal difference is connected with the different physical nature of dipole forces between the objects. Effective dipole interactions between the radiating quantum dots, appearing due to photon exchange, collectivize the dot radiation. While the dipolar spin interactions exist from the beginning, yet before radiation, and on the contrary, they dephase spin motion, thus destroying the coherence of moving spins. In addition, quantum dot radiation exhibits turbulent photon filamentation that is absent for radiating spins.
Quantum communication with coherent states and linear optics
NASA Astrophysics Data System (ADS)
Arrazola, Juan Miguel; Lütkenhaus, Norbert
2014-10-01
We introduce a general mapping for encoding quantum communication protocols involving pure states of multiple qubits, unitary transformations, and projective measurements into another set of protocols that employ a coherent state of light in a linear combination of optical modes, linear-optics transformations, and measurements with single-photon threshold detectors. This provides a general framework for transforming protocols in quantum communication into a form in which they can be implemented with current technology. We explore the similarity between properties of the original qubit protocols and the coherent-state protocols obtained from the mapping and make use of the mapping to construct additional protocols in the context of quantum communication complexity and quantum digital signatures. Our results have the potential of bringing a wide class of quantum communication protocols closer to their experimental demonstration.
Generation of infrared entangled light in asymmetric semiconductor quantum wells
NASA Astrophysics Data System (ADS)
Lü, Xin-You; Wu, Jing; Zheng, Li-Li; Huang, Pei
2010-12-01
We proposed a scheme to achieve two-mode CV entanglement with the frequencies of entangled modes in the infrared range in an asymmetric semiconductor double-quantum-wells (DQW), where the required quantum coherence is obtained by inducing the corresponding intersubband transitions (ISBTs) with a classical field. By numerically simulating the dynamics of system, we show that the entanglement period can be prolonged via enhancing the intensity of classical field, and the generation of entanglement doesn't depend intensively on the initial condition of system in our scheme. Moreover, we also show that a bipartite entanglement amplifier can be realized in our scheme. The present research provides an efficient approach to achieve infrared entangled light in the semiconductor nanostructure, which may have significant impact on the progress of solid-state quantum information theory.
Quantum mirages formed by coherent projection of electronic structure
Manoharan; Lutz; Eigler
2000-02-01
Image projection relies on classical wave mechanics and the use of natural or engineered structures such as lenses or resonant cavities. Well-known examples include the bending of light to create mirages in the atmosphere, and the focusing of sound by whispering galleries. However, the observation of analogous phenomena in condensed matter systems is a more recent development, facilitated by advances in nanofabrication. Here we report the projection of the electronic structure surrounding a magnetic Co atom to a remote location on the surface of a Cu crystal; electron partial waves scattered from the real Co atom are coherently refocused to form a spectral image or 'quantum mirage'. The focusing device is an elliptical quantum corral, assembled on the Cu surface. The corral acts as a quantum mechanical resonator, while the two-dimensional Cu surface-state electrons form the projection medium. When placed on the surface, Co atoms display a distinctive spectroscopic signature, known as the many-particle Kondo resonance, which arises from their magnetic moment. By positioning a Co atom at one focus of the ellipse, we detect a strong Kondo signature not only at the atom, but also at the empty focus. This behaviour contrasts with the usual spatially-decreasing response of an electron gas to a localized perturbation. PMID:10676952
Coherent eavesdropping strategies for the four state quantum cryptography protocol
NASA Astrophysics Data System (ADS)
Cirac, J. I.; Gisin, N.
1997-02-01
An elementary derivation of best eavesdropping strategies for the four state BB84 quantum cryptography protocol is presented, for both incoherent and two-qubit coherent attacks. While coherent attacks do not help Eve to obtain more information, they are more powerful to reveal the whole message sent by Alice. Our results are based on symmetric eavesdropping strategies, which we show to be sufficient to analyze these kind of problems.
Waveguide switches using asymmetric coupled quantum wells
NASA Astrophysics Data System (ADS)
Ritter, Kenneth J.; Horst, Scott C.
1994-07-01
This report contains the results of a three-year effort to investigate the use of Asymmetric Coupled Quantum Well in optical waveguide cross bar switches. The two types of devices investigated are the standard delta beta switch and the delta alpha switch. The delta alpha switch uses the imaginary part of the refractive index to modulate the intensity along different waveguide paths in the switch structure. Both types of switch were fabricated and tested. The delta beta switches produced are suitable as 1-input 2-output devices. The delta alpha switches were demonstrated as 2 by 2 cross bar switches with up to 40% throughput. To compensate for losses in the switches the use of amplifying elements was investigated. To provide gain at a longer wavelength than that of the excitons in the modulation waveguides, the quantum wells in the modulation waveguides were blue shifted using vacancy induced disordering (VID). The VID shifted quantum wells showed less Stark shift than the unshifted quantum wells. This effect is explained by the nearly parabolic shape of the disordered wells. Coupled quantum wells can be used to create a structure that will maintain a strongly Stark shifted spatially indirect transition even after VID. Modeling of the various waveguide structures used is also discussed.
Experimental quantum fingerprinting with weak coherent pulses.
Xu, Feihu; Arrazola, Juan Miguel; Wei, Kejin; Wang, Wenyuan; Palacios-Avila, Pablo; Feng, Chen; Sajeed, Shihan; Lütkenhaus, Norbert; Lo, Hoi-Kwong
2015-01-01
Quantum communication holds the promise of creating disruptive technologies that will play an essential role in future communication networks. For example, the study of quantum communication complexity has shown that quantum communication allows exponential reductions in the information that must be transmitted to solve distributed computational tasks. Recently, protocols that realize this advantage using optical implementations have been proposed. Here we report a proof-of-concept experimental demonstration of a quantum fingerprinting system that is capable of transmitting less information than the best-known classical protocol. Our implementation is based on a modified version of a commercial quantum key distribution system using off-the-shelf optical components over telecom wavelengths, and is practical for messages as large as 100 Mbits, even in the presence of experimental imperfections. Our results provide a first step in the development of experimental quantum communication complexity. PMID:26515586
Experimental quantum fingerprinting with weak coherent pulses
NASA Astrophysics Data System (ADS)
Xu, Feihu; Arrazola, Juan Miguel; Wei, Kejin; Wang, Wenyuan; Palacios-Avila, Pablo; Feng, Chen; Sajeed, Shihan; Lütkenhaus, Norbert; Lo, Hoi-Kwong
2015-10-01
Quantum communication holds the promise of creating disruptive technologies that will play an essential role in future communication networks. For example, the study of quantum communication complexity has shown that quantum communication allows exponential reductions in the information that must be transmitted to solve distributed computational tasks. Recently, protocols that realize this advantage using optical implementations have been proposed. Here we report a proof-of-concept experimental demonstration of a quantum fingerprinting system that is capable of transmitting less information than the best-known classical protocol. Our implementation is based on a modified version of a commercial quantum key distribution system using off-the-shelf optical components over telecom wavelengths, and is practical for messages as large as 100 Mbits, even in the presence of experimental imperfections. Our results provide a first step in the development of experimental quantum communication complexity.
Experimental quantum fingerprinting with weak coherent pulses
Xu, Feihu; Arrazola, Juan Miguel; Wei, Kejin; Wang, Wenyuan; Palacios-Avila, Pablo; Feng, Chen; Sajeed, Shihan; Lütkenhaus, Norbert; Lo, Hoi-Kwong
2015-01-01
Quantum communication holds the promise of creating disruptive technologies that will play an essential role in future communication networks. For example, the study of quantum communication complexity has shown that quantum communication allows exponential reductions in the information that must be transmitted to solve distributed computational tasks. Recently, protocols that realize this advantage using optical implementations have been proposed. Here we report a proof-of-concept experimental demonstration of a quantum fingerprinting system that is capable of transmitting less information than the best-known classical protocol. Our implementation is based on a modified version of a commercial quantum key distribution system using off-the-shelf optical components over telecom wavelengths, and is practical for messages as large as 100 Mbits, even in the presence of experimental imperfections. Our results provide a first step in the development of experimental quantum communication complexity. PMID:26515586
Microscopic quantum coherence in a photosynthetic-light-harvesting antenna.
Dawlaty, Jahan M; Ishizaki, Akihito; De, Arijit K; Fleming, Graham R
2012-08-13
We briefly review the coherent quantum beats observed in recent two-dimensional electronic spectroscopy experiments in a photosynthetic-light-harvesting antenna. We emphasize that the decay of the quantum beats in these experiments is limited by ensemble averaging. The in vivo dynamics of energy transport depends upon the local fluctuations of a single photosynthetic complex during the energy transfer time (a few picoseconds). Recent analyses suggest that it remains possible that the quantum-coherent motion may be robust under individual realizations of the environment-induced fluctuations contrary to intuition obtained from condensed phase spectroscopic measurements and reduced density matrices. This result indicates that the decay of the observed quantum coherence can be understood as ensemble dephasing. We propose a fluorescence-detected single-molecule experiment with phase-locked excitation pulses to investigate the coherent dynamics at the level of a single molecule without hindrance by ensemble averaging. We discuss the advantages and limitations of this method. We report our initial results on bulk fluorescence-detected coherent spectroscopy of the Fenna-Mathews-Olson complex. PMID:22753820
The high-order quantum coherence of thermal light
NASA Astrophysics Data System (ADS)
Chen, Hui
Turbulence-free Imaging System, which provides high contrast as well as high resolution. Based on the experiments of the simulations of quantum interference in thermal light and the dramatically improvement of the contrast in quantum imaging, this study of the high-order coherence of thermal light has a great significance both for fundamental physics and applications.
Coherent coupling of multiple transverse modes in quantum cascade lasers.
Yu, Nanfang; Diehl, Laurent; Cubukcu, Ertugrul; Bour, David; Corzine, Scott; Höfler, Gloria; Wojcik, Aleksander K; Crozier, Kenneth B; Belyanin, Alexey; Capasso, Federico
2009-01-01
Quantum cascade lasers are a unique laboratory for studying nonlinear laser dynamics because of their high intracavity intensity, strong intersubband optical nonlinearity, and an unusual combination of relaxation time scales. Here we investigate the nonlinear coupling between the transverse modes of quantum cascade lasers. We present evidence for stable phase coherence of multiple transverse modes over a large range of injection currents. We explain the phase coherence by a four-wave mixing interaction originating from the strong optical nonlinearity of the gain transition. The phase-locking conditions predicted by theory are supported by spectral data and both near- and far-field mode measurements. PMID:19257192
Control of atomic spin squeezing via quantum coherence
NASA Astrophysics Data System (ADS)
Shao, Xuping; Ling, Yang; Yang, Xihua; Xiao, Min
2016-06-01
We propose a scheme to generate and control atomic spin squeezing via atomic coherence induced by the strong coupling and probe fields in the Λ-type electromagnetically-induced-transparency configuration in an atomic ensemble. Manipulation of squeezing of the two components in the plane orthogonal to the mean atomic spin direction and generation of nearly perfect squeezing in either component can be achieved by varying the relative intensities of the coupling and probe fields. This method provides a flexible and convenient way to create and control atomic spin squeezing, which may find potential applications in high-precision atomic-physics measurement, quantum coherent control, and quantum information processing.
Large Scale Quantum Coherence of Nearly Circular Wavepackets
Reinhold, Carlos O; Yoshida, S.; Burgdorfer, J.; Wyker, B.; Mestayer, J. J.; Dunning, F. B.
2009-01-01
We demonstrate that the quantum coherence of mesoscopic very-high-n, n {approx} 305, Rydberg wave packets travelling along nearly circular orbits can be maintained on microsecond time scales corresponding to hundreds of classical orbital periods. The coherence is probed through collapses and revivals of periodic oscillations in the average electron position. The temporal interferences of spatially separated Schroedinger cat-like wave packets are also observed. A novel hybrid quantum-classical trajectory method is employed to simulate the wave packet dynamics.
Signatures of discrete breathers in coherent state quantum dynamics
Igumenshchev, Kirill; Ovchinnikov, Misha; Prezhdo, Oleg; Maniadis, Panagiotis
2013-02-07
In classical mechanics, discrete breathers (DBs) - a spatial time-periodic localization of energy - are predicted in a large variety of nonlinear systems. Motivated by a conceptual bridging of the DB phenomena in classical and quantum mechanical representations, we study their signatures in the dynamics of a quantum equivalent of a classical mechanical point in phase space - a coherent state. In contrast to the classical point that exhibits either delocalized or localized motion, the coherent state shows signatures of both localized and delocalized behavior. The transition from normal to local modes have different characteristics in quantum and classical perspectives. Here, we get an insight into the connection between classical and quantum perspectives by analyzing the decomposition of the coherent state into system's eigenstates, and analyzing the spacial distribution of the wave-function density within these eigenstates. We find that the delocalized and localized eigenvalue components of the coherent state are separated by a mixed region, where both kinds of behavior can be observed. Further analysis leads to the following observations. Considered as a function of coupling, energy eigenstates go through avoided crossings between tunneling and non-tunneling modes. The dominance of tunneling modes in the high nonlinearity region is compromised by the appearance of new types of modes - high order tunneling modes - that are similar to the tunneling modes but have attributes of non-tunneling modes. Certain types of excitations preferentially excite higher order tunneling modes, allowing one to study their properties. Since auto-correlation functions decrease quickly in highly nonlinear systems, short-time dynamics are sufficient for modeling quantum DBs. This work provides a foundation for implementing modern semi-classical methods to model quantum DBs, bridging classical and quantum mechanical signatures of DBs, and understanding spectroscopic experiments that
Phase-controlled coherent population trapping in superconducting quantum circuits
NASA Astrophysics Data System (ADS)
Cheng, Guang-Ling; Wang, Yi-Ping; Chen, Ai-Xi
2015-04-01
We investigate the influences of the-applied-field phases and amplitudes on the coherent population trapping behavior in superconducting quantum circuits. Based on the interactions of the microwave fields with a single Δ-type three-level fluxonium qubit, the coherent population trapping could be obtainable and it is very sensitive to the relative phase and amplitudes of the applied fields. When the relative phase is tuned to 0 or π, the maximal atomic coherence is present and coherent population trapping occurs. While for the choice of π/2, the atomic coherence becomes weak. Meanwhile, for the fixed relative phase π/2, the value of coherence would decrease with the increase of Rabi frequency of the external field coupled with two lower levels. The responsible physical mechanism is quantum interference induced by the control fields, which is indicated in the dressed-state representation. The microwave coherent phenomenon is present in our scheme, which will have potential applications in optical communication and nonlinear optics in solid-state devices. Project supported by the National Natural Science Foundation of China (Grant Nos. 11165008 and 11365009), the Foundation of Young Scientist of Jiangxi Province, China (Grant No. 20142BCB23011), and the Scientific Research Foundation of Jiangxi Provincial Department of Education (Grant No. GJJ13348).
Robustness of asymmetry and coherence of quantum states
NASA Astrophysics Data System (ADS)
Piani, Marco; Cianciaruso, Marco; Bromley, Thomas R.; Napoli, Carmine; Johnston, Nathaniel; Adesso, Gerardo
2016-04-01
Quantum states may exhibit asymmetry with respect to the action of a given group. Such an asymmetry of states can be considered a resource in applications such as quantum metrology, and it is a concept that encompasses quantum coherence as a special case. We introduce explicitly and study the robustness of asymmetry, a quantifier of asymmetry of states that we prove to have many attractive properties, including efficient numerical computability via semidefinite programming and an operational interpretation in a channel discrimination context. We also introduce the notion of asymmetry witnesses, whose measurement in a laboratory detects the presence of asymmetry. We prove that properly constrained asymmetry witnesses provide lower bounds to the robustness of asymmetry, which is shown to be a directly measurable quantity itself. We then focus our attention on coherence witnesses and the robustness of coherence, for which we prove a number of additional results; these include an analysis of its specific relevance in phase discrimination and quantum metrology, an analytical calculation of its value for a relevant class of quantum states, and tight bounds that relate it to another previously defined coherence monotone.
Loss of quantum coherence through scattering off virtual black holes
NASA Astrophysics Data System (ADS)
Hawking, S. W.; Ross, Simon F.
1997-11-01
In quantum gravity, fields may lose quantum coherence by scattering off vacuum fluctuations in which virtual black hole pairs appear and disappear. Although it is not possible to properly compute the scattering off such fluctuations, we argue that one can get useful qualitative results, which provide a guide to the possible effects of such scattering, by considering a quantum field on the C metric, which has the same topology as a virtual black hole pair. We study a scalar field on the Lorentzian C metric background, with the scalar field in the analytically continued Euclidean vacuum state. We find that there are a finite number of particles at infinity in this state, contrary to recent claims made by Yi. Thus, this state is not determined by data at infinity, and there is loss of quantum coherence in this semiclassical calculation.
Coherent pulse position modulation quantum cipher
Sohma, Masaki; Hirota, Osamu
2014-12-04
On the basis of fundamental idea of Yuen, we present a new type of quantum random cipher, where pulse position modulated signals are encrypted in the picture of quantum Gaussian wave form. We discuss the security of our proposed system with a phase mask encryption.
Finite-temperature scaling of quantum coherence near criticality in a spin chain
NASA Astrophysics Data System (ADS)
Cheng, Weiwen; Zhang, Zhijun; Gong, Longyan; Zhao, Shengmei
2016-06-01
We explore quantum coherence, inherited from Wigner-Yanase skew information, to analyze quantum criticality in the anisotropic XY chain model at finite temperature. Based on the exact solutions of the Hamiltonian, the quantum coherence contained in a nearest-neighbor spin pairs reduced density matrix ρ is obtained. The first-order derivative of the quantum coherence is non-analytic around the critical point at sufficient low temperature. The finite-temperature scaling behavior and the universality are verified numerically. In particular, the quantum coherence can also detect the factorization transition in such a model at sufficient low temperature. We also show that quantum coherence contained in distant spin pairs can characterize quantum criticality and factorization phenomena at finite temperature. Our results imply that quantum coherence can serve as an efficient indicator of quantum criticality in such a model and shed considerable light on the relationships between quantum phase transitions and quantum information theory at finite temperature.
The set of triple-resonance sequences with a multiple quantum coherence evolution period
NASA Astrophysics Data System (ADS)
Koźmiński, Wiktor; Zhukov, Igor
2004-12-01
The new pulse sequence building block that relies on evolution of heteronuclear multiple quantum coherences is proposed. The particular chemical shifts are obtained in multiple quadrature, using linear combinations of frequencies taken from spectra measured at different quantum levels. The pulse sequences designed in this way consist of small number of RF-pulses, are as short as possible, and could be applied for determination of coupling constants. The examples presented involve 2D correlations H NCO, H NCA, H N(CO) CA, and H(N) COCA via heteronuclear zero and double coherences, as well as 2D H NCOCA technique with simultaneous evolution of triple and three distinct single quantum coherences. Applications of the new sequences are presented for 13C, 15N-labeled ubiquitin.
A Calculation of Cosmological Scale from Quantum Coherence
Lindesay, J
2004-07-23
We use general arguments to examine the energy scales for which a quantum coherent description of gravitating quantum energy units is necessary. The cosmological dark energy density is expected to decouple from the Friedman-Lemaitre energy density when the Friedman-Robertson-Walker scale expansion becomes sub-luminal at R = c, at which time the usual microscopic interactions of relativistic quantum mechanics (QED, QCD, etc) open new degrees of freedom. We assume that these microscopic interactions cannot signal with superluminal exchanges, only superluminal quantum correlations. The expected gravitational vacuum energy density at that scale would be expected to freeze out due to the loss of gravitational coherence. We define the vacuum energy which generates this cosmological constant to be that of a zero temperature Bose condensate at this gravitational de-coherence scale. We presume a universality throughout the universe in the available degrees of freedom determined by fundamental constants during its evolution. Examining the reverse evolution of the universe from the present, long before reaching Planck scale dynamics one expects major modifications from the de-coherent thermal equations of state, suggesting that the pre-coherent phase has global coherence properties. Since the arguments presented involve primarily counting of degrees of freedom, we expect the statistical equilibrium states of causally disconnected regions of space to be independently identical. Thus, there is no horizon problem associated with the lack of causal influences between spatially separated regions in this approach. The scale of the amplitude of fluctuations produced during de-coherence of cosmological vacuum energy are found to evolve to values consistent with those observed in cosmic microwave background radiation and galactic clustering.
Robb, G. R. M.; Bonifacio, R.
2013-03-15
We extend previous analyses of spontaneous emission in a quantum free electron laser (QFEL) and competition between spontaneous and coherent QFEL emission to include a broad distribution of photon frequencies and momenta appropriate for spontaneous undulator radiation. We show that although the predictions of monochromatic and broadband models predict different electron momentum distributions for the quantum regime due to spontaneous emission alone after many photon emissions, the inclusion of broadband spontaneous emission has a negligible effect on the competition between spontaneous and coherent emission in the QFEL. Numerical results from both models are well described by the same condition for the threshold/critical value of spontaneous emission rate.
Bipartite quantum channels using multipartite cluster-type entangled coherent states
Munhoz, P. P.; Semiao, F. L.; Roversi, J. A.; Vidiella-Barranco, A.
2010-04-15
We propose a particular encoding for bipartite entangled states derived from multipartite cluster-type entangled coherent states (CTECSs). We investigate the effects of amplitude damping on the entanglement content of this bipartite state, as well as its usefulness as a quantum channel for teleportation. We find interesting relationships among the amplitude of the coherent states constituting the CTECSs, the number of subsystems forming the logical qubits (redundancy), and the extent to which amplitude damping affects the entanglement of the channel. For instance, in the sense of sudden death of entanglement, given a fixed value of the initial coherent state amplitude, the entanglement life span is shortened if redundancy is increased.
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. PMID:23624695
Quantum walk coherences on a dynamical percolation graph
Elster, Fabian; Barkhofen, Sonja; Nitsche, Thomas; Novotný, Jaroslav; Gábris, Aurél; Jex, Igor; Silberhorn, Christine
2015-01-01
Coherent evolution governs the behaviour of all quantum systems, but in nature it is often subjected to influence of a classical environment. For analysing quantum transport phenomena quantum walks emerge as suitable model systems. In particular, quantum walks on percolation structures constitute an attractive platform for studying open system dynamics of random media. Here, we present an implementation of quantum walks differing from the previous experiments by achieving dynamical control of the underlying graph structure. We demonstrate the evolution of an optical time-multiplexed quantum walk over six double steps, revealing the intricate interplay between the internal and external degrees of freedom. The observation of clear non-Markovian signatures in the coin space testifies the high coherence of the implementation and the extraordinary degree of control of all system parameters. Our work is the proof-of-principle experiment of a quantum walk on a dynamical percolation graph, paving the way towards complex simulation of quantum transport in random media. PMID:26311434
Quantum coherent switch utilizing commensurate nanoelectrode and charge density periodicities
Harrison, Neil; Singleton, John; Migliori, Albert
2008-08-05
A quantum coherent switch having a substrate formed from a density wave (DW) material capable of having a periodic electron density modulation or spin density modulation, a dielectric layer formed onto a surface of the substrate that is orthogonal to an intrinsic wave vector of the DW material; and structure for applying an external spatially periodic electrostatic potential over the dielectric layer.
Coherent controllers for optical-feedback cooling of quantum oscillators
NASA Astrophysics Data System (ADS)
Hamerly, Ryan; Mabuchi, Hideo
2013-01-01
We study the cooling performance of optical-feedback controllers for open optical and mechanical resonators in the linear quadratic Gaussian setting of stochastic control theory. We utilize analysis and numerical optimization of closed-loop models based on quantum stochastic differential equations to show that coherent control schemes, where we embed the resonator in an interferometer to achieve all-optical feedback, can outperform optimal measurement-based feedback control schemes in the quantum regime of low steady-state excitation number. These performance gains are attributed to the coherent controller's ability to simultaneously process both quadratures of an optical probe field without measurement or loss of fidelity, and may guide the design of coherent feedback schemes for more general problems of robust nonlinear and robust control.
Dirac Cones in Periodically Modulated Quantum Wells
NASA Astrophysics Data System (ADS)
Yao, Yuanzhao; Sakoda, Kazuaki
2016-06-01
We show by a degenerate k · p perturbation theory and group theory that Dirac cones in the Brillouin-zone center can be materialized for the electronic bands of periodically modulated quantum wells. We examine in particular the periodic modulation of the C4v and C6v symmetries. The analytical conclusions are confirmed by numerical calculations using the finite element method.
Spectroscopy of GaAs quantum wells
West, L.C.
1985-07-01
A new type of optical dipole transition in GaAs quantum wells has been observed. The dipole occurs between two envelope states of the conduction band electron wavefunction, and is called a quantum well envelope state transition (QWEST). The QWEST is observed by infrared absorption in three different samples with quantum well thicknesses 65, 82, and 92 A and resonant energies of 152, 121, and 108 MeV, respectively. The oscillator strength is found to have values of over 12, in good agreement with prediction. The linewidths are seen as narrow as 10 MeV at room temperature and 7 MeV at low temperature, thus proving a narrow line resonance can indeed occur between transitions of free electrons. Techniques for the proper growth of these quantum well samples to enable observation of the QWEST have also been found using (AlGa)As compounds. This QWEST is considered to be an ideal material for an all optical digital computer. The QWEST can be made frequency matched to the inexpensive Carbon Dioxide laser with an infrared wavelength of 10 microns. The nonlinearity and fast relaxation time of the QWEST indicate a logic element with a subpicosecond switch time can be built in the near future, with a power level which will eventually be limited only by the noise from a lack of quanta to above approximately 10 microwatts. 64 refs., 35 figs., 6 tabs.
Fractional quantum conductance in edge channels of silicon quantum wells
Bagraev, Nikolay; Klyachkin, Leonid; Kudryavtsev, Andrey; Malyarenko, Anna
2013-12-04
We present the findings for the fractional quantum conductance of holes that is caused by the edge channels in the silicon nanosandwich prepared within frameworks of the Hall geometry. This nanosandwich represents the ultra-narrow p-type silicon quantum well (Si-QW), 2 nm, confined by the δ-barriers heavily doped with boron on the n-type Si (100) surface. The edge channels in the Si-QW plane are revealed by measuring the longitudinal quantum conductance staircase, G{sub xx}, as a function of the voltage applied to the Hall contacts, V{sub xy}, to a maximum of 4e{sup 2}/h. In addition to the standard plateau, 2e{sup 2}/h, the variations of the V{sub xy} voltage appear to exhibit the fractional form of the quantum conductance staircase with the plateaus and steps that bring into correlation respectively with the odd and even fractions.
Quantum Discord and Entanglement of Quasi-Werner States Based on Bipartite Entangled Coherent States
NASA Astrophysics Data System (ADS)
Mishra, Manoj K.; Maurya, Ajay K.; Prakash, Hari
2016-06-01
Present work is an attempt to compare quantum discord and quantum entanglement of quasi-Werner states formed with the four bipartite entangled coherent states (ECS) used recently for quantum teleportation of a qubit encoded in superposed coherent state. Out of these, the quasi-Werner states based on maximally ECS due to its invariant nature under local operation is independent of measurement basis and mean photon numbers, while for quasi-Werner states based on non-maximally ECS, it depends upon measurement basis as well as on mean photon number. However, for large mean photon numbers since non-maximally ECS becomes almost maximally entangled therefore dependence of quantum discord for non-maximally ECS based quasi-Werner states on the measurement basis disappears.
Gaussian private quantum channel with squeezed coherent states.
Jeong, Kabgyun; Kim, Jaewan; Lee, Su-Yong
2015-01-01
While the objective of conventional quantum key distribution (QKD) is to secretly generate and share the classical bits concealed in the form of maximally mixed quantum states, that of private quantum channel (PQC) is to secretly transmit individual quantum states concealed in the form of maximally mixed states using shared one-time pad and it is called Gaussian private quantum channel (GPQC) when the scheme is in the regime of continuous variables. We propose a GPQC enhanced with squeezed coherent states (GPQCwSC), which is a generalization of GPQC with coherent states only (GPQCo) [Phys. Rev. A 72, 042313 (2005)]. We show that GPQCwSC beats the GPQCo for the upper bound on accessible information. As a subsidiary example, it is shown that the squeezed states take an advantage over the coherent states against a beam splitting attack in a continuous variable QKD. It is also shown that a squeezing operation can be approximated as a superposition of two different displacement operations in the small squeezing regime. PMID:26364893
Gaussian private quantum channel with squeezed coherent states
Jeong, Kabgyun; Kim, Jaewan; Lee, Su-Yong
2015-01-01
While the objective of conventional quantum key distribution (QKD) is to secretly generate and share the classical bits concealed in the form of maximally mixed quantum states, that of private quantum channel (PQC) is to secretly transmit individual quantum states concealed in the form of maximally mixed states using shared one-time pad and it is called Gaussian private quantum channel (GPQC) when the scheme is in the regime of continuous variables. We propose a GPQC enhanced with squeezed coherent states (GPQCwSC), which is a generalization of GPQC with coherent states only (GPQCo) [Phys. Rev. A 72, 042313 (2005)]. We show that GPQCwSC beats the GPQCo for the upper bound on accessible information. As a subsidiary example, it is shown that the squeezed states take an advantage over the coherent states against a beam splitting attack in a continuous variable QKD. It is also shown that a squeezing operation can be approximated as a superposition of two different displacement operations in the small squeezing regime. PMID:26364893
Gaussian private quantum channel with squeezed coherent states
NASA Astrophysics Data System (ADS)
Jeong, Kabgyun; Kim, Jaewan; Lee, Su-Yong
2015-09-01
While the objective of conventional quantum key distribution (QKD) is to secretly generate and share the classical bits concealed in the form of maximally mixed quantum states, that of private quantum channel (PQC) is to secretly transmit individual quantum states concealed in the form of maximally mixed states using shared one-time pad and it is called Gaussian private quantum channel (GPQC) when the scheme is in the regime of continuous variables. We propose a GPQC enhanced with squeezed coherent states (GPQCwSC), which is a generalization of GPQC with coherent states only (GPQCo) [Phys. Rev. A 72, 042313 (2005)]. We show that GPQCwSC beats the GPQCo for the upper bound on accessible information. As a subsidiary example, it is shown that the squeezed states take an advantage over the coherent states against a beam splitting attack in a continuous variable QKD. It is also shown that a squeezing operation can be approximated as a superposition of two different displacement operations in the small squeezing regime.
Nonlocal entanglement of coherent states, complementarity, and quantum erasure
Gerry, Christopher C.; Grobe, R.
2007-03-15
We describe a nonlocal method for generating entangled coherent states of a two-mode field wherein the field modes never meet. The proposed method is an extension of an earlier proposal [C. C. Gerry, Phys. Rev. A 59, 4095 (1999)] for the generation of superpositions of coherent states. A single photon injected into a Mach-Zehnder interferometer with cross-Kerr media in both arms coupling with two external fields in coherent states produces entangled coherent states upon detection at one of the output ports. We point out that our proposal can be alternatively viewed as a 'which path' experiment, and in the case of only one external field, we describe the implementation of a quantum eraser.
Semiconductor Lasers Containing Quantum Wells in Junctions
NASA Technical Reports Server (NTRS)
Yang, Rui Q.; Qiu, Yueming
2004-01-01
In a recent improvement upon In(x)Ga(1-x)As/InP semiconductor lasers of the bipolar cascade type, quantum wells are added to Esaki tunnel junctions, which are standard parts of such lasers. The energy depths and the geometric locations and thicknesses of the wells are tailored to exploit quantum tunneling such that, as described below, electrical resistances of junctions and concentrations of dopants can be reduced while laser performances can be improved. In(x)Ga(1-x)As/InP bipolar cascade lasers have been investigated as sources of near-infrared radiation (specifically, at wavelengths of about 980 and 1,550 nm) for photonic communication systems. The Esaki tunnel junctions in these lasers have been used to connect adjacent cascade stages and to enable transport of charge carriers between them. Typically, large concentrations of both n (electron-donor) and p (electron-acceptor) dopants have been necessary to impart low electrical resistances to Esaki tunnel junctions. Unfortunately, high doping contributes free-carrier absorption, thereby contributing to optical loss and thereby, further, degrading laser performance. In accordance with the present innovation, quantum wells are incorporated into the Esaki tunnel junctions so that the effective heights of barriers to quantum tunneling are reduced (see figure).
Cavity-enhanced coherent light scattering from a quantum dot
Bennett, Anthony J.; Lee, James P.; Ellis, David J. P.; Meany, Thomas; Murray, Eoin; Floether, Frederik F.; Griffths, Jonathan P.; Farrer, Ian; Ritchie, David A.; Shields, Andrew J.
2016-01-01
The generation of coherent and indistinguishable single photons is a critical step for photonic quantum technologies in information processing and metrology. A promising system is the resonant optical excitation of solid-state emitters embedded in wavelength-scale three-dimensional cavities. However, the challenge here is to reject the unwanted excitation to a level below the quantum signal. We demonstrate this using coherent photon scattering from a quantum dot in a micropillar. The cavity is shown to enhance the fraction of light that is resonantly scattered toward unity, generating antibunched indistinguishable photons that are 16 times narrower than the time-bandwidth limit, even when the transition is near saturation. Finally, deterministic excitation is used to create two-photon N00N states with which we make superresolving phase measurements in a photonic circuit. PMID:27152337
Cavity-enhanced coherent light scattering from a quantum dot.
Bennett, Anthony J; Lee, James P; Ellis, David J P; Meany, Thomas; Murray, Eoin; Floether, Frederik F; Griffths, Jonathan P; Farrer, Ian; Ritchie, David A; Shields, Andrew J
2016-04-01
The generation of coherent and indistinguishable single photons is a critical step for photonic quantum technologies in information processing and metrology. A promising system is the resonant optical excitation of solid-state emitters embedded in wavelength-scale three-dimensional cavities. However, the challenge here is to reject the unwanted excitation to a level below the quantum signal. We demonstrate this using coherent photon scattering from a quantum dot in a micropillar. The cavity is shown to enhance the fraction of light that is resonantly scattered toward unity, generating antibunched indistinguishable photons that are 16 times narrower than the time-bandwidth limit, even when the transition is near saturation. Finally, deterministic excitation is used to create two-photon N00N states with which we make superresolving phase measurements in a photonic circuit. PMID:27152337
Quantum Coherence between Two Atoms beyond Q=10{sup 15}
Chou, C. W.; Hume, D. B.; Thorpe, M. J.; Wineland, D. J.; Rosenband, T.
2011-04-22
We place two atoms in quantum superposition states and observe coherent phase evolution for 3.4x10{sup 15} cycles. Correlation signals from the two atoms yield information about their relative phase even after the probe radiation has decohered. This technique allowed a frequency comparison of two {sup 27}Al{sup +} ions with fractional uncertainty 3.7{sub -0.8}{sup +1.0}x10{sup -16}/{radical}({tau}/s). Two measures of the Q factor are reported: The Q factor derived from quantum coherence is 3.4{sub -1.1}{sup +2.4}x10{sup 16}, and the spectroscopic Q factor for a Ramsey time of 3 s is 6.7x10{sup 15}. We demonstrate a method to detect the individual quantum states of two Al{sup +} ions in a Mg{sup +}-Al{sup +}-Al{sup +} linear ion chain without spatially resolving the ions.
Numerical study on dynamical behavior in oscillatory driven quantum double-well systems
NASA Astrophysics Data System (ADS)
Igarashi, Akira; Yamada, Hiroaki
2008-08-01
We numerically investigate quantum dynamics in a one-dimensional double-well system emphasizing influence of a parametrically polychromatic perturbation on the dynamics. It is found that time dependence of transition probability for an initially localized wave packet between the wells shows two types of motion, coherent and incoherent motion, depending on the perturbation parameters. As the strength and/or the number of frequency components of the perturbation increase, coherent motion changes into incoherent one. The former is related to coherent tunneling of the wave packet due to coherence; the latter is related to a delocalized state caused by decoherence. In coherent motion, by virtue of coherence of the dynamics, the expectation value and the standard deviation of a dynamical variable such as the energy of the system show oscillatory time dependence around the initial values. On the contrary in incoherent motion, because of the decoherence, the time dependence fluctuates irregularly around a certain value after a rapid increase due to the resonance. We find that negativity of the Wigner function also show similar time dependence in each type of motion. We compare the classification of the quantum dynamics based on regularity of the time dependence with the one of corresponding classical dynamics based on the Lyapunov exponent. The classifications of the quantum and classical dynamics overlap well in the parameter space. Furthermore, we confirm decoherence of quantum dynamics in a kicked double-well system.
Incoherent and coherent eavesdropping in the six-state protocol of quantum cryptography
NASA Astrophysics Data System (ADS)
Bechmann-Pasquinucci, H.; Gisin, N.
1999-06-01
All incoherent as well as 2- and 3-qubit coherent eavesdropping strategies on the six-state protocol of quantum cryptography are classified. For a disturbance of 1/6, the optimal incoherent eavesdropping strategy reduces to the universal quantum cloning machine. Coherent eavesdropping cannot increase Eve's Shannon information, neither on the entire string of bits, nor on the set of bits received undisturbed by Bob. However, coherent eavesdropping can increase as well Eve's Renyi information as her probability of guessing correctly all bits. The case that Eve delays the measurement of her probe until after the public discussion on error correction and privacy amplification is also considered. It is argued that by doing so, Eve gains only negligibly small additional information.
Quantum mechanical coherence, resonance, and mind
Stapp, H.P.
1995-03-26
Norbert Wiener and J.B.S. Haldane suggested during the early thirties that the profound changes in our conception of matter entailed by quantum theory opens the way for our thoughts, and other experiential or mind-like qualities, to play a role in nature that is causally interactive and effective, rather than purely epiphenomenal, as required by classical mechanics. The mathematical basis of this suggestion is described here, and it is then shown how, by giving mind this efficacious role in natural process, the classical character of our perceptions of the quantum universe can be seen to be a consequence of evolutionary pressures for the survival of the species.
Coherent and conventional gravidynamic quantum 1/f noise
NASA Astrophysics Data System (ADS)
Handel, Peter H.; George, Thomas F.
2008-04-01
Quantum 1/f noise is a fundamental fluctuation of currents, physical cross sections or process rates, caused by infrared coupling of the current carriers to very low frequency (soft) quanta, also known as infraquanta. The latter are soft gravitons in the gravidynamic case with the coupling constant g= pGM2/Nch considered here -- soft photons in the electrodynamic case and soft transversal piezo-phonons in the lattice-dynamical case. Here p=3.14 and F=psi. Quantum 1/f noise is a new aspect of quantum mechanics expressed mainly through the coherent quantum 1/f effect 2g/pf derived here for large systems, and mainly through the conventional quantum 1/f effect for small systems or individual particles. Both effects are present in general, and their effects are superposed in a first approximation with the help of a coherence (weight) parameter s" that will be derived elsewhere for the gravitational case. The spectral density of fractional fluctuations S(dj/j,f) for j=e(hk/2pm)|F|2 is S(F2,f)/<|F|2> = S(j,f)/
Strained quantum well photovoltaic energy converter
NASA Technical Reports Server (NTRS)
Freundlich, Alexandre (Inventor); Renaud, Philippe (Inventor); Vilela, Mauro Francisco (Inventor); Bensaoula, Abdelhak (Inventor)
1998-01-01
An indium phosphide photovoltaic cell is provided where one or more quantum wells are introduced between the conventional p-conductivity and n-conductivity indium phosphide layer. The approach allows the cell to convert the light over a wider range of wavelengths than a conventional single junction cell and in particular convert efficiently transparency losses of the indium phosphide conventional cell. The approach hence may be used to increase the cell current output. A method of fabrication of photovoltaic devices is provided where ternary InAsP and InGaAs alloys are used as well material in the quantum well region and results in an increase of the cell current output.
Quantum confinement in transition metal oxide quantum wells
Choi, Miri; Lin, Chungwei; Butcher, Matthew; Posadas, Agham B.; Demkov, Alexander A.; Rodriguez, Cesar; Zollner, Stefan; He, Qian; Borisevich, Albina Y.
2015-05-11
We report on the quantum confinement in SrTiO{sub 3} (STO) quantum wells (QWs) grown by molecular beam epitaxy. The QW structure consists of LaAlO{sub 3} (LAO) and STO layers grown on LAO substrate. Structures with different QW thicknesses ranging from two to ten unit cells were grown and characterized. Optical properties (complex dielectric function) were measured by spectroscopic ellipsometry in the range of 1.0 eV–6.0 eV at room temperature. We observed that the absorption edge was blue-shifted by approximately 0.39 eV as the STO quantum well thickness was reduced to two unit cells. This demonstrates that the energy level of the first sub-band can be controlled by the QW thickness in a complex oxide material.
Negative refraction without absorption via quantum coherence
NASA Astrophysics Data System (ADS)
Fang, Ai-Ping; Ge, Wenchao; Wang, Meng; Li, Fu-li; Zubairy, M. Suhail
2016-02-01
Negative refraction of a probe field is studied in a dense gas consisting of cascade-type four-level atoms. By coupling the magnetic component of the probe field to a Λ scheme with initially prepared coherence in the two lower levels, strong negative permeability with minimal absorption can be obtained. The permittivity of the gas to the electric component of the probe field can be made negative by taking into account the local field effect of the dense atoms. Strong negative refraction with zero absorption can be achieved in a wide range of parameters in our scheme. A possible experimental realization is also discussed.
Extending quantum coherence of superconducting flux
NASA Astrophysics Data System (ADS)
Yan, Fei; Kamal, Archana; Orlando, Terry; Gustavsson, Simon; Oliver, William; Engineering Quantum Systems, MIT Team
We present the design of a superconducting qubit with multiple Josephson junctions. The design starts with a capacitively shunted flux qubit, and it incorporates particular junction parameter choices for the purpose of simultaneously optimizing over transition frequency, anharmonicity, flux- and charge-noise sensitivity around flux degeneracy. By studying the scaling properties with design parameters, we identify directions to extend coherence substantially. This research was funded by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) via MIT Lincoln Laboratory under Air Force Contract No. FA8721-05-C-0002.
Rabi model as a quantum coherent heat engine: From quantum biology to superconducting circuits
NASA Astrophysics Data System (ADS)
Altintas, Ferdi; Hardal, Ali Ü. C.; Müstecaplıoǧlu, Özgür E.
2015-02-01
We propose a multilevel quantum heat engine with a working medium described by a generalized Rabi model which consists of a two-level system coupled to a single-mode bosonic field. The model is constructed to be a continuum limit of a quantum biological description of light-harvesting complexes so that it can amplify quantum coherence by a mechanism which is a quantum analog of classical Huygens clocks. The engine operates in a quantum Otto cycle where the working medium is coupled to classical heat baths in the isochoric processes of the four-stroke cycle, while either the coupling strength or the resonance frequency is changed in the adiabatic stages. We found that such an engine can produce work with an efficiency close to the Carnot bound when it operates at low temperatures and in the ultrastrong-coupling regime. The interplay of the effects of quantum coherence and quantum correlations on the engine performance is discussed in terms of second-order coherence, quantum mutual information, and the logarithmic negativity of entanglement. We point out that the proposed quantum Otto engine can be implemented experimentally with modern circuit quantum electrodynamic systems where flux qubits can be coupled ultrastrongly to superconducting transmission-line resonators.
Time evolution of multiple quantum coherences in NMR
NASA Astrophysics Data System (ADS)
Sánchez, Claudia M.; Pastawski, Horacio M.; Levstein, Patricia R.
2007-09-01
In multiple quantum NMR, individual spins become correlated with one another over time through their dipolar couplings. In this way, the usual Zeeman selection rule can be overcome and forbidden transitions can be excited. Experimentally, these multiple quantum coherences (MQC) are formed by the application of appropriate sequences of radio frequency pulses that force the spins to act collectively. 1H spin coherences of even order up to 16 were excited in a polycrystalline sample of ferrocene (C 5H 5) 2Fe and up to 32 in adamantane (C 10H 16) and their evolutions studied in different conditions: (a) under the natural dipolar Hamiltonian, H ZZ (free evolution) and with H ZZ canceled out by (b) time reversion or (c) with the MREV8 sequence. The results show that when canceling H ZZ the coherences decay with characteristic times ( τc≈200 μs), which are more than one order of magnitude longer than those under free evolution ( τc≈10 μs). In addition, it is observed that with both MREV8 and time reversion sequences, the higher the order of the coherence (larger number of correlated spins) the faster the speed of degradation, as it happens during the evolution with H ZZ. In both systems, it is observed that the sequence of time reversion of the dipolar Hamiltonian preserves coherences for longer times than MREV8.
Molecular Spintronics: Wiring Spin Coherence between Semiconductor Quantum Dots
NASA Astrophysics Data System (ADS)
Ouyang, Min
2004-03-01
Semiconductor quantum dots (QDs) are attractive candidates for scalable solid state implementations of quantum information processing based on electron spin states, where a crucial requirement for practical devices is to have efficient and tunable spin coupling between them. We focus on recent femtosecond time-resolved Faraday rotation studies of self-assembled multilayer spintronic devices based on colloidal quantum dots bridged by conjugated molecules (M. Ouyang et al., Science 301, 1074 (2003)). The data reveal the instantaneous transfer of spin coherence through conjugated molecular bridges spanning quantum dots of different size over a broad range of temperature. The room temperature spin transfer efficiency exceeds 20%, which approximately doubles the value measured at T=4.5K. A molecular π-orbital mediated spin coherence transfer mechanism is proposed to provide a qualitative insight into the experimental observations, suggesting a correlation between the stereochemistry of molecules and the transfer process. The results show that conjugated molecules can be used not only as physical links for the assembly of functional networks, but also as efficient channels for shuttling quantum information. This class of structures may be useful as two-spin quantum devices operating at ambient temperatures and may offer promising opportunities for future versatile molecule-based spintronic technologies.
Cavity-based architecture to preserve quantum coherence and entanglement
Man, Zhong-Xiao; Xia, Yun-Jie; Lo Franco, Rosario
2015-01-01
Quantum technology relies on the utilization of resources, like quantum coherence and entanglement, which allow quantum information and computation processing. This achievement is however jeopardized by the detrimental effects of the environment surrounding any quantum system, so that finding strategies to protect quantum resources is essential. Non-Markovian and structured environments are useful tools to this aim. Here we show how a simple environmental architecture made of two coupled lossy cavities enables a switch between Markovian and non-Markovian regimes for the dynamics of a qubit embedded in one of the cavity. Furthermore, qubit coherence can be indefinitely preserved if the cavity without qubit is perfect. We then focus on entanglement control of two independent qubits locally subject to such an engineered environment and discuss its feasibility in the framework of circuit quantum electrodynamics. With up-to-date experimental parameters, we show that our architecture allows entanglement lifetimes orders of magnitude longer than the spontaneous lifetime without local cavity couplings. This cavity-based architecture is straightforwardly extendable to many qubits for scalability. PMID:26351004
Cavity-based architecture to preserve quantum coherence and entanglement.
Man, Zhong-Xiao; Xia, Yun-Jie; Lo Franco, Rosario
2015-01-01
Quantum technology relies on the utilization of resources, like quantum coherence and entanglement, which allow quantum information and computation processing. This achievement is however jeopardized by the detrimental effects of the environment surrounding any quantum system, so that finding strategies to protect quantum resources is essential. Non-Markovian and structured environments are useful tools to this aim. Here we show how a simple environmental architecture made of two coupled lossy cavities enables a switch between Markovian and non-Markovian regimes for the dynamics of a qubit embedded in one of the cavity. Furthermore, qubit coherence can be indefinitely preserved if the cavity without qubit is perfect. We then focus on entanglement control of two independent qubits locally subject to such an engineered environment and discuss its feasibility in the framework of circuit quantum electrodynamics. With up-to-date experimental parameters, we show that our architecture allows entanglement lifetimes orders of magnitude longer than the spontaneous lifetime without local cavity couplings. This cavity-based architecture is straightforwardly extendable to many qubits for scalability. PMID:26351004
Interacting Electrodynamics of Short Coherent Conductors in Quantum Circuits
NASA Astrophysics Data System (ADS)
Altimiras, C.; Portier, F.; Joyez, P.
2016-07-01
When combining lumped mesoscopic electronic components to form a circuit, quantum fluctuations of electrical quantities lead to a nonlinear electromagnetic interaction between the components, which is generally not understood. The Landauer-Büttiker formalism that is frequently used to describe noninteracting coherent mesoscopic components is not directly suited to describe such circuits since it assumes perfect voltage bias, i.e., the absence of fluctuations. Here, we show that for short coherent conductors of arbitrary transmission, the Landauer-Büttiker formalism can be extended to take into account quantum voltage fluctuations similarly to what is done for tunnel junctions. The electrodynamics of the whole circuit is then formally worked out disregarding the non-Gaussianity of fluctuations. This reveals how the aforementioned nonlinear interaction operates in short coherent conductors: Voltage fluctuations induce a reduction of conductance through the phenomenon of dynamical Coulomb blockade, but they also modify their internal density of states, leading to an additional electrostatic modification of the transmission. Using this approach, we can quantitatively account for conductance measurements performed on quantum point contacts in series with impedances of the order of RK=h /e2 . Our work should enable a better engineering of quantum circuits with targeted properties.
Electron spin coherence near room temperature in magnetic quantum dots.
Moro, Fabrizio; Turyanska, Lyudmila; Wilman, James; Fielding, Alistair J; Fay, Michael W; Granwehr, Josef; Patanè, Amalia
2015-01-01
We report on an example of confined magnetic ions with long spin coherence near room temperature. This was achieved by confining single Mn(2+) spins in colloidal semiconductor quantum dots (QDs) and by dispersing the QDs in a proton-spin free matrix. The controlled suppression of Mn-Mn interactions and minimization of Mn-nuclear spin dipolar interactions result in unprecedentedly long phase memory (TM ~ 8 μs) and spin-lattice relaxation (T1 ~ 10 ms) time constants for Mn(2+) ions at T = 4.5 K, and in electron spin coherence observable near room temperature (TM ~ 1 μs). PMID:26040432
Energy loss rate in disordered quantum well
Tripathi, P.; Ashraf, S. S. Z.; Hasan, S. T.; Sharma, A. C.
2014-04-24
We report the effect of dynamically screened deformation potential on the electron energy loss rate in disordered semiconductor quantum well. Interaction of confined electrons with bulk acoustic phonons has been considered in the deformation coupling. The study concludes that the dynamically screened deformation potential coupling plays a significant role as it substantially affects the power dependency of electron relaxation on temperature and mean free path.
Functionalized Graphene Nanoroads for Quantum Well Device
Zhou, Yungang; Yang, Ping; Wang, Zhiguo; Xiao, Hai Yan; Zu, Xiaotao T.; Sun, Xin; Khaleel, Mohammad A.; Gao, Fei
2011-03-02
Using density functional theory, a series of calculations of structural and electronic properties of Si-substituted graphene were conducted. Through substituting C atoms by Si atoms on graphene in the present study, we found that the band gap of graphene can be continuously tuned with differently substitutional concentration. To utilize such substitution-induced band gap changes, we proposed a special design to fabricate graphene-based quantum well device.
Spin-dependent coherent transport in a double quantum dot system
NASA Astrophysics Data System (ADS)
Petrosyan, L. S.; Shahbazyan, T. V.
2015-09-01
We study spin-resolved resonant tunneling in a system of two quantum dots sandwiched between doped quantum wells. In the coherent (Dicke) regime, i.e., when quantum dot separation is smaller than the Fermi wavelength in a two-dimensional electron gas in quantum wells, application of an in-plane magnetic field leads to a pronounced spin-resolved structure of conductance peak line shape even for very small Zeeman splitting of the quantum dots' resonant levels. In the presence of electron-gas spin-orbit coupling, this spin-resolved structure is washed out due to Fermi surface deformation in the momentum space. We also show that Aharonov-Bohm flux penetrating the area enclosed by tunneling electron pathways completely destroys the conductance spin structure.
External-field effect on quantum features of radiation emitted by a quantum well in a microcavity
Sete, Eyob A.; Das, Sumanta; Eleuch, H.
2011-02-15
We consider a semiconductor quantum well in a microcavity driven by coherent light and coupled to a squeezed vacuum reservoir. By systematically solving the pertinent quantum Langevin equations in the strong-coupling and low-excitation regimes, we study the effect of exciton-photon detuning, external coherent light, and the squeezed vacuum reservoir on vacuum Rabi splitting and on quantum statistical properties of the light emitted by the quantum well. We show that the exciton-photon detuning leads to a shift in polariton resonance frequencies and a decrease in fluorescence intensity. We also show that the fluorescent light exhibits quadrature squeezing, which predominately depends on the exciton-photon detuning and the degree of the squeezing of the input field.
Fractional Quantum Hall States in a Ge Quantum Well
NASA Astrophysics Data System (ADS)
Mironov, O. A.; d'Ambrumenil, N.; Dobbie, A.; Leadley, D. R.; Suslov, A. V.; Green, E.
2016-04-01
Measurements of the Hall and dissipative conductivity of a strained Ge quantum well on a SiGe /(001 )Si substrate in the quantum Hall regime are reported. We analyze the results in terms of thermally activated quantum tunneling of carriers from one internal edge state to another across saddle points in the long-range impurity potential. This shows that the gaps for different filling fractions closely follow the dependence predicted by theory. We also find that the estimates of the separation of the edge states at the saddle are in line with the expectations of an electrostatic model in the lowest spin-polarized Landau level (LL), but not in the spin-reversed LL where the density of quasiparticle states is not high enough to accommodate the carriers required.
Fractional Quantum Hall States in a Ge Quantum Well.
Mironov, O A; d'Ambrumenil, N; Dobbie, A; Leadley, D R; Suslov, A V; Green, E
2016-04-29
Measurements of the Hall and dissipative conductivity of a strained Ge quantum well on a SiGe/(001)Si substrate in the quantum Hall regime are reported. We analyze the results in terms of thermally activated quantum tunneling of carriers from one internal edge state to another across saddle points in the long-range impurity potential. This shows that the gaps for different filling fractions closely follow the dependence predicted by theory. We also find that the estimates of the separation of the edge states at the saddle are in line with the expectations of an electrostatic model in the lowest spin-polarized Landau level (LL), but not in the spin-reversed LL where the density of quasiparticle states is not high enough to accommodate the carriers required. PMID:27176531
Spectroscopy of Single Free Standing Quantum Wells
Williams, M D; Hollars, C W; Huser, T; Jallow, N; Cochran, A; Bryant, R
2006-03-14
We investigated the interaction of quantum confined exciton states GaAs quantum wells with native surface states. Single molecule photoluminescence (PL) spectroscopy, developed by T. Huser at LLNL was used to probe the unique bare quantum wells in the free standing quantum well structure. The latter was developed by the M. D. Williams at Clark Atlanta University. The goals of the project during this budget cycle were to procure samples containing GaAs free standing QWs, identify suitable regions for PL analysis at Lawrence Livermore, analyze the structures at room temperature and at liquid nitrogen temperatures. The specific regions of interest on the sample structures were identified by scanning electron microscopy at Clark Atlanta prior to transport to LLNL. Previous attempts at other facilities using NSOM, cathodoluminescence, and conventional PL showed little luminescence activity at room temperature from the 200 {angstrom} thick wells. This suggested either excess recombination due to surface states in the quantum well region or insufficient absorption length for photoluminescence. The literature suggested that the effect of the defects could be eliminated by reducing the sample temperature below their associated activation energies. In our previous subcontract work with LLNL, a significant amount of effort was expended to modify the apparatus to allow low temperature measurements. The modifications were not successful and we concluded that in order to do the measurements at low temperature we would need to purchase a commercial optical cryostat to get reliable results. Ms. Rochelle Bryant worked during the summer as an intern at LLNL on the project under the supervision of C. Hollars and in collaboration with T. Huser and found that PL emission could be obtained at room temperature. This was a surprising result as the literature and our experience shows that there is no PL emission from GaAs at room temperature. We speculate that this is due to the small
Generalized coherent states under deformed quantum mechanics with maximum momentum
NASA Astrophysics Data System (ADS)
Ching, Chee Leong; Ng, Wei Khim
2013-10-01
Following the Gazeau-Klauder approach, we construct generalized coherent states (GCS) as the quantum simulator to examine the deformed quantum mechanics, which exhibits an intrinsic maximum momentum. We study deformed harmonic oscillators and compute their probability distribution and entropy of states exactly. Also, a particle in an infinite potential box is studied perturbatively. In particular, unlike usual quantum mechanics, the present deformed case increases the entropy of the Planck scale quantum optical system. Furthermore, for simplicity, we obtain the modified uncertainty principle (MUP) with the perturbative treatment up to leading order. MUP turns out to increase generally. However, for certain values of γ (a parameter of GCS), it is possible that the MUP will vanish and hence will exhibit the classical characteristic. This is interpreted as the manifestation of the intrinsic high-momentum cutoff at lower momentum in a perturbative treatment. Although the GCS saturates the minimal uncertainty in a simultaneous measurement of physical position and momentum operators, thus constituting the squeezed states, complete coherency is impossible in quantum gravitational physics. The Mandel Q number is calculated, and it is shown that the statistics can be Poissonian and super-/sub-Poissonian depending on γ. The equation of motion is studied, and both Ehrenfest’s theorem and the correspondence principle are recovered. Fractional revival times are obtained through the autocorrelation, and they indicate that the superposition of a classical-like subwave packet is natural in GCS. We also contrast our results with the string-motivated (Snyder) type of deformed quantum mechanics, which incorporates a minimum position uncertainty rather than a maximum momentum. With the advances of quantum optics technology, it might be possible to realize some of these distinguishing quantum-gravitational features within the domain of future experiments.
Coherent versus Measurement Feedback: Linear Systems Theory for Quantum Information
NASA Astrophysics Data System (ADS)
Yamamoto, Naoki
2014-10-01
To control a quantum system via feedback, we generally have two options in choosing a control scheme. One is the coherent feedback, which feeds the output field of the system, through a fully quantum device, back to manipulate the system without involving any measurement process. The other one is measurement-based feedback, which measures the output field and performs a real-time manipulation on the system based on the measurement results. Both schemes have advantages and disadvantages, depending on the system and the control goal; hence, their comparison in several situations is important. This paper considers a general open linear quantum system with the following specific control goals: backaction evasion, generation of a quantum nondemolished variable, and generation of a decoherence-free subsystem, all of which have important roles in quantum information science. Some no-go theorems are proven, clarifying that those goals cannot be achieved by any measurement-based feedback control. On the other hand, it is shown that, for each control goal there exists a coherent feedback controller accomplishing the task. The key idea to obtain all the results is system theoretic characterizations of the above three notions in terms of controllability and observability properties or transfer functions of linear systems, which are consistent with their standard definitions.
Realization of a scalable coherent quantum Fourier transform
NASA Astrophysics Data System (ADS)
Debnath, Shantanu; Linke, Norbert; Figgatt, Caroline; Landsman, Kevin; Wright, Ken; Monroe, Chris
2016-05-01
The exponential speed-up in some quantum algorithms is a direct result of parallel function-evaluation paths that interfere through a quantum Fourier transform (QFT). We report the implementation of a fully coherent QFT on five trapped Yb+ atomic qubits using sequences of fundamental quantum logic gates. These modular gates can be used to program arbitrary sequences nearly independent of system size and distance between qubits. We use this capability to first perform a Deutsch-Jozsa algorithm where several instances of three-qubit balanced and constant functions are implemented and then examined using single qubit QFTs. Secondly, we apply a fully coherent five-qubit QFT as a part of a quantum phase estimation protocol. Here, the QFT operates on a five-qubit superposition state with a particular phase modulation of its coefficients and directly produces the corresponding phase to five-bit precision. Finally, we examine the performance of the QFT in the period finding problem in the context of Shor's factorization algorithm. This work is supported by the ARO with funding from the IARPA MQCO program and the AFOSR MURI on Quantum Measurement and Verification.
Coherent state quantum key distribution based on entanglement sudden death
NASA Astrophysics Data System (ADS)
Jaeger, Gregg; Simon, David; Sergienko, Alexander V.
2016-03-01
A method for quantum key distribution (QKD) using entangled coherent states is discussed which is designed to provide key distribution rates and transmission distances surpassing those of traditional entangled photon pair QKD by exploiting entanglement sudden death. The method uses entangled electromagnetic signal states of `macroscopic' average photon numbers rather than single photon or entangled photon pairs, which have inherently limited rate and distance performance as bearers of quantum key data. Accordingly, rather than relying specifically on Bell inequalities as do entangled photon pair-based methods, the security of this method is based on entanglement witnesses and related functions.
Quantum Communication between Atomic Ensembles Using Coherent Light
NASA Astrophysics Data System (ADS)
Duan, Lu-Ming; Cirac, J. I.; Zoller, P.; Polzik, E. S.
2000-12-01
Protocols for quantum communication between massive particles, such as atoms, are usually based on making use of nonclassical light, and/or superhigh finesse optical cavities are normally needed to enhance interaction between atoms and photons. We demonstrate a remarkable result: by using only coherent light, entanglement can be generated between distant free space atomic ensembles, and an unknown quantum state can thus be teleported from one to another. Neither nonclassical light nor cavities are needed in the scheme, which greatly simplifies its experimental implementation.
Probing mechanical quantum coherence with an ultracold-atom meter
Lo Gullo, N.; Busch, Th.; Palma, G. M.; Paternostro, M.
2011-12-15
We propose a scheme to probe quantum coherence in the state of a nanocantilever based on its magnetic coupling (mediated by a magnetic tip) with a spinor Bose Einstein condensate (BEC). By mapping the BEC into a rotor, its coupling with the cantilever results in a gyroscopic motion whose properties depend on the state of the cantilever: the dynamics of one of the components of the rotor angular momentum turns out to be strictly related to the presence of quantum coherence in the state of the cantilever. We also suggest a detection scheme relying on Faraday rotation, which produces only a very small back-action on the BEC and is thus suitable for a continuous detection of the cantilever's dynamics.
Electron transfer and capture dynamics in ZnSe quantum wells grown on GaAs
Dongol, A.; Wagner, H. P.
2013-12-04
We investigate the transfer and capture dynamics of electrons in phase coherent photorefractive ZnSe quantum wells grown on GaAs using degenerate three-beam four-wave-mixing. The measurements reveal electron capture times by the quantum well in the order of several tens of picoseconds and a transit time of approximately 5 picoseconds from the GaAs substrate through the ZnMgSe barrier.
Coherent two-dimensional photocurrent spectroscopy in a PbS quantum dot photocell.
Karki, Khadga J; Widom, Julia R; Seibt, Joachim; Moody, Ian; Lonergan, Mark C; Pullerits, Tõnu; Marcus, Andrew H
2014-01-01
Recently there has been growing interest in the role of coherence in electronic dynamics. Coherent multidimensional spectroscopy has been used to reveal coherent phenomena in numerous material systems. Here we utilize a recent implementation of coherent multidimensional spectroscopy--two-dimensional photocurrent spectroscopy--in which we detect the photocurrent from a PbS quantum dot photocell resulting from its interactions with a sequence of four ultrafast laser pulses. We observe sub-picosecond evolution of two-dimensional spectra consistent with multiple exciton generation. Moreover, a comparison with two-dimensional fluorescence spectra of the quantum dots demonstrates the potential of two-dimensional photocurrent spectroscopy to elucidate detailed origins of photocurrent generating electronic state coherence pathways. Since the measurement is based on detecting the photocell current in situ, the method is well suited to study the fundamental ultrafast processes that affect the function of the device. This opens new avenues to investigate and implement coherent optimization strategies directly within devices. PMID:25519819
Control of Population Flow in Coherently Driven Quantum Ladders
Garcia-Fernandez, Ruth; Bergmann, Klaas; Ekers, Aigars; Yatsenko, Leonid P.; Vitanov, Nikolay V.
2005-07-22
A technique for adiabatic control of the population flow through a preselected decaying excited level in a three-level quantum ladder is presented. The population flow through the intermediate or upper level is controlled efficiently and robustly by varying the pulse delay between a pair of partly overlapping coherent laser pulses. The technique is analyzed theoretically and demonstrated in an experiment with Na{sub 2} molecules.
Quantum-coherence quantifiers based on the Tsallis relative α entropies
NASA Astrophysics Data System (ADS)
Rastegin, Alexey E.
2016-03-01
The concept of coherence is one of cornerstones in physics. The development of quantum information science has lead to renewed interest in properly approaching the coherence at the quantum level. Various measures could be proposed to quantify coherence of a quantum state with respect to the prescribed orthonormal basis. To be a proper measure of coherence, each candidate should enjoy certain properties. It seems that the monotonicity property plays a crucial role here. Indeed, there is known an intuitive measure of coherence that does not share this condition. We study coherence measures induced by quantum divergences of the Tsallis type. Basic properties of the considered coherence quantifiers are derived. Tradeoff relations between coherence and mixedness are examined. The property of monotonicity under incoherent selective measurements has to be reformulated. The proposed formulation can naturally be treated as a parametric extension of its standard form. Finally, two coherence measures quadratic in moduli of matrix elements are compared from the monotonicity viewpoint.
Electronic quantum confinement in cylindrical potential well
NASA Astrophysics Data System (ADS)
Baltenkov, Arkadiy S.; Msezane, Alfred Z.
2016-04-01
The effects of quantum confinement on the momentum distribution of electrons confined within a cylindrical potential well have been analyzed. The motivation is to understand specific features of the momentum distribution of electrons when the electron behavior is completely controlled by the parameters of a non-isotropic potential cavity. It is shown that studying the solutions of the wave equation for an electron confined in a cylindrical potential well offers the possibility to analyze the confinement behavior of an electron executing one- or two-dimensional motion in the three-dimensional space within the framework of the same mathematical model. Some low-lying electronic states with different symmetries have been considered and the corresponding wave functions have been calculated; the behavior of their nodes and their peak positions with respect to the parameters of the cylindrical well has been analyzed. Additionally, the momentum distributions of electrons in these states have been calculated. The limiting cases of the ratio of the cylinder length H and its radius R0 have been considered; when the cylinder length H significantly exceeds its radius R0 and when the cylinder radius is much greater than its length. The cylindrical quantum confinement effects on the momentum distribution of electrons in these potential wells have been analyzed. The possible application of the results obtained here for the description of the general features in the behavior of electrons in nanowires with metallic type of conductivity (or nanotubes) and ultrathin epitaxial films (or graphene sheets) are discussed. Possible experiments are suggested where the quantum confinement can be manifested. Contribution to the Topical Issue "Atomic Cluster Collisions (7th International Symposium)", edited by Gerardo Delgado Barrio, Andrey Solov'Yov, Pablo Villarreal, Rita Prosmiti.
Quantum-well lasers for direct solar photopumping
NASA Technical Reports Server (NTRS)
Unnikrishnan, Sreenath; Anderson, Neal G.
1993-01-01
Semiconductor lasers directly photopumped by focused sunlight may be viable sources of coherent light for intersatellite communications and other low-power spaceborne applications. In this work, we theoretically explore the possibility of realizing such devices. We specifically assess solar pumped operation of separate-confinement-quantum-well heterostructure (SCQWH) lasers based on InGaAs, GaAs, and AlGaA, as fabrication technology for these lasers is mature and they can operate at very low thresholds. We develop a model for step-index single-well SCQWH lasers photopumped by sunlight, examine how threshold solar photoexcitation intensities depend upon material and structure parameters, design optimum structures for solar-pumped operation, and identify design tradeoffs. Our results suggest that laser action should be possible in properly designed structures at readily achievable solar concentrations and that optimum designs for solar-pumped SCQWH lasers differ significantly from those for analogous current injection devices.