Coherent state polarons in quantum wells
NASA Astrophysics Data System (ADS)
Thilagam, A.; Lohe, M. A.
2005-01-01
We investigate the polaronic effects of an electron confined in a quantum well, which we describe through its algebraic properties using su (1 , 1), taking into account the electron-bulk longitudinal-optical phonon interaction. We construct the variational wave function as the direct product of an electronic part and a part describing coherent phonons generated by the Low-Lee-Pines transformation from the vacuum state. We use two explicit forms of coherent states, Perelomov and Barut-Girardello states, to represent the electronic part in the quantum well spectrum. Our results show that in a coherent state basis for electrons the basic polaron parameters such as the energy gap shift and effective mass are further enhanced compared to those obtained with the conventional sinusoidal form of the basis. The difference between the two types of quantum well coherent states appears in polaronic interactions in quantum wells. We extend the calculations in order to estimate polaron lifetimes for a variety of different material systems.
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.
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.
Coherent Pump-Probe Interactions and Terahertz Intersubband Gain in Semiconductor Quantum Wells
NASA Technical Reports Server (NTRS)
Liu, Ansheng; Ning, Cun-Zheng
1999-01-01
In recent years there has been considerable interest in intersubband-transition-based infrared semiconductor quantum well (QW) lasers because of their potential applications. In the mid-infrared range, both electrically-injected quantum cascade lasers [1] and optically-pumped multiple QW lasers [2] have been experimentally realized. In these studies, optical gain is due to population inversion between the lasing subbands. It was also proposed that stimulated Raman scattering in QW systems can produce net infrared optical gain [3j. In such a nonlinear optical scheme, the appearance of optical gain that may lead to intersubband Raman lasers does not rely on the population inversion. Since, in tile resonant Raman process (Raman gain is the largest in this case), the pump field induces population redistribution among subbands in the QW s ystem, it seems that a realistic estimate of the optical gain has to include this effect. Perturbative calculations used in the previous work [3] may overestimate the Raman gain. In this paper we present a nonperturbative calculation of terahertz gain of optically-pumped semiconductor step quantum wells. Limiting optical transitions within the conduction band of QW, we solve the pump-field-induced nonequilibrium distribution function for each subband of the QW system from a set of coupled rate equations. Both intrasubband and intersubband relaxation processes in the quantum well system are included. Taking into account the coherent interactions between pump and THz (signal) waves, we we derive the susceptibility of the QW system for the THz field. For a GaAs/AlGaAs step QW, we calculate the Thz gain spectrum for different pump frequencies and intensities. Under moderately strong pumping (approximately 0.3 MW/sq cm), a significant THz gain (approximately 300/m) is predicted. It is also shown that the coherent wave interactions (resonant stimulated Raman processes) contribute significantly to the THz gain.
Two-Color Coherent Control of Optical Bistability in Asymmetric Semiconductor Quantum Wells
NASA Astrophysics Data System (ADS)
Li, Jia-Hua; Hao, Xiang-Ying
We investigate optical bistability in intersubband transitions of an asymmetric semiconductor quantum well structure that has equidistant transitions between three subbands of the system and is placed in a unidirectional cavity. The system is simultaneously coupled by a fundamental field and its second harmonic. The second harmonic field acts as a control field and significantly influences the optical bistability. In addition, the two-color coherent control of optical bistability by the relative phase of the fundamental and the second harmonic fields is shown. The influence of the electronic cooperation parameter on the OB behavior is also discussed. This investigation may be used for optimizing and controlling the optical switching process in the SQW solid-state system, which is much more practical than that in the atomic system because of its flexible design and the controllable interference strength.
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.
Andrianov, A. V. Alekseev, P. S.; Klimko, G. V.; Ivanov, S. V.; Shcheglov, V. L.; Sedova, M. A.; Zakhar'in, A. O.
2013-11-15
The generation of coherent terahertz radiation upon the band-to-band femtosecond laser photoexcitation of GaAs/AlGaAs multiple-quantum-well structures in a transverse electric field at room temperature is investigated. The properties of the observed terahertz radiation suggest that it is generated on account of the excitation of a time-dependent dipole moment as a result of the polarization of nonequilibrium electron-hole pairs in quantum wells by the electric field. The proposed theoretical model taking into account the dynamic screening of the electric field in the quantum wells by nonequilibrium charge carriers describes the properties of the observed terahertz signal.
Quantum coherence: Reciprocity and distribution
NASA Astrophysics Data System (ADS)
Kumar, Asutosh
2017-03-01
Quantum coherence is the outcome of the superposition principle. Recently, it has been theorized as a quantum resource, and is the premise of quantum correlations in multipartite systems. It is therefore interesting to study the coherence content and its distribution in a multipartite quantum system. In this work, we show analytically as well as numerically the reciprocity between coherence and mixedness of a quantum state. We find that this trade-off is a general feature in the sense that it is true for large spectra of measures of coherence and of mixedness. We also study the distribution of coherence in multipartite systems by looking at monogamy-type relation-which we refer to as additivity relation-between coherences of different parts of the system. We show that for the Dicke states, while the normalized measures of coherence violate the additivity relation, the unnormalized ones satisfy the same.
NASA Astrophysics Data System (ADS)
Raheli, A.; Afshari, H.; Hamedi, H. R.
2015-10-01
This paper deals with optical bistability (OB) and optical multistability (OM) behaviors for a triple semiconductor quantum well (SQW) structure driven coherently with two control fields, confined in a unidirectional ring cavity. The effect of different system parameters on OB and OM is explored. It is found that the threshold of onset of the OB can be controlled by manipulating the Rabi frequency of control fields. In this case, OB can be converted to OM. Then we investigate the effect of probe and control field detunings on OB behaviors. We found that the frequency detuning of probe field affects only the upper-lower branches of the OB curves but has no specific impact on OB threshold. By manipulating the first control field detuning, neither the OB threshold intensity nor upper-lower branches change. Finally, it is found that increasing the second control field detuning can reduce merely the OB threshold intensity, while no change happens in upper-lower OB branches. The results may be applicable in real experiments for realizing an all-optical switching or coding element in a solid-state platform.
NASA Astrophysics Data System (ADS)
Naseri, Tayebeh
2017-04-01
A new scheme for obtaining an electromagnetically induced grating (EIG) via biexciton coherence in quantum well nanostructures is developed. It is theoretically shown that exciton spin relaxation and biexciton binding energy have important roles in producing efficient dual electromagnetically induced phase grating. In this structure, due to biexciton coherence, the higher order diffraction intensities of the grating can be observed. Furthermore, it is shown that the efficiency of different orders in the grating patterns could be controlled by biexciton energy renormalization (ESR) and relative phase between the applied laser fields.
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.
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
Converting Coherence to Quantum Correlations.
Ma, Jiajun; Yadin, Benjamin; Girolami, Davide; Vedral, Vlatko; Gu, Mile
2016-04-22
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.
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)
de Vicente, Julio I.; Streltsov, Alexander
2017-01-01
Any quantum resource theory is based on free states and free operations, i.e. states and operations which can be created and performed at no cost. In the resource theory of coherence free states are diagonal in some fixed basis, and free operations are those which cannot create coherence for some particular experimental realization. Recently, some problems of this approach have been discussed, and new sets of operations have been proposed to resolve these problems. We propose here the framework of genuine quantum coherence. This approach is based on a simple principle: we demand that a genuinely incoherent operation preserves all incoherent states. This framework captures coherence under additional constrains such as energy preservation and all genuinely incoherent operations are incoherent regardless of their particular experimental realization. We also introduce the full class of operations with this property, which we call fully incoherent. We analyze in detail the mathematical structure of these classes and also study possible state transformations. We show that deterministic manipulation is severely limited, even in the asymptotic settings. In particular, this framework does not have a unique golden unit, i.e. there is no single state from which all other states can be created deterministically with the free operations. This suggests that any reasonably powerful resource theory of coherence must contain free operations which can potentially create coherence in some experimental realization.
Extracting quantum coherence via steering
Hu, Xueyuan; Fan, Heng
2016-01-01
As the precious resource for quantum information processing, quantum coherence can be created remotely if the involved two sites are quantum correlated. It can be expected that the amount of coherence created should depend on the quantity of the shared quantum correlation, which is also a resource. Here, we establish an operational connection between coherence induced by steering and the quantum correlation. We find that the steering-induced coherence quantified by such as relative entropy of coherence and trace-norm of coherence is bounded from above by a known quantum correlation measure defined as the one-side measurement-induced disturbance. The condition that the upper bound saturated by the induced coherence varies for different measures of coherence. The tripartite scenario is also studied and similar conclusion can be obtained. Our results provide the operational connections between local and non-local resources in quantum information processing. PMID:27682450
Extracting quantum coherence via steering
NASA Astrophysics Data System (ADS)
Hu, Xueyuan; Fan, Heng
2016-09-01
As the precious resource for quantum information processing, quantum coherence can be created remotely if the involved two sites are quantum correlated. It can be expected that the amount of coherence created should depend on the quantity of the shared quantum correlation, which is also a resource. Here, we establish an operational connection between coherence induced by steering and the quantum correlation. We find that the steering-induced coherence quantified by such as relative entropy of coherence and trace-norm of coherence is bounded from above by a known quantum correlation measure defined as the one-side measurement-induced disturbance. The condition that the upper bound saturated by the induced coherence varies for different measures of coherence. The tripartite scenario is also studied and similar conclusion can be obtained. Our results provide the operational connections between local and non-local resources in quantum information processing.
Perspective: Quantum or classical coherence?
Miller, William H
2012-06-07
Some coherence effects in chemical dynamics are described correctly by classical mechanics, while others only appear in a quantum treatment--and when these are observed experimentally it is not always immediately obvious whether their origin is classical or quantum. Semiclassical theory provides a systematic way of adding quantum coherence to classical molecular dynamics and thus provides a useful way to distinguish between classical and quantum coherence. Several examples are discussed which illustrate both cases. Particularly interesting is the situation with electronically non-adiabatic processes, where sometimes whether the coherence effects are classical or quantum depends on what specific aspects of the process are observed.
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.
NASA Astrophysics Data System (ADS)
Bagaev, V. S.; Davletov, E. T.; Krivobok, V. S.; Nikolaev, S. N.; Novikov, A. V.; Onishchenko, E. E.; Pruchkina, A. A.; Skorikov, M. L.
2015-12-01
The measured stationary and time-resolved photoluminescence is used to study the properties of the exciton gas in a second-order 5-nm-thick Si0.905Ge0.095/Si quantum well. It is shown that, despite the presence of an electron barrier in the Si0.905Ge0.095 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 1.3 m 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.
Quantum chaos meets coherent control.
Gong, Jiangbin; Brumer, Paul
2005-01-01
Coherent control of atomic and molecular processes has been a rapidly developing field. Applications of coherent control to large and complex molecular systems are expected to encounter the effects of chaos in the underlying classical dynamics, i.e., quantum chaos. Hence, recent work has focused on examining control in model chaotic systems. This work is reviewed, with an emphasis on a variety of new quantum phenomena that are of interest to both areas of quantum chaos and coherent control.
Quantum coherence in two dimensions
Hawking, S.W.; Hayward, J.D. California Institute of Technology, Pasadena, California 91125 )
1994-05-15
The formation and evaporation of two-dimensional black holes are discussed. It is shown that if the radiation in minimal scalars has positive energy, there must be a global event horizon or a naked singularity. The former would imply loss of quantum coherence while the latter would lead to an even worse breakdown of predictability. [ital CPT] invariance would suggest that there ought to be past horizons as well. A way in which this could happen with wormholes is described.
Supersymmetric infinite wells and coherent states
NASA Astrophysics Data System (ADS)
Fiset, M.-A.; Hussin, V.
2015-06-01
Gaussian Klauder coherent states are discussed in the context of the infinite well quantum model, otherwise known as the particle in a box. A supersymmetric partner system is also presented, as well as a construction of coherent states in this new system. We show that these states can be chosen, in both systems to have many properties usually expected for coherent states. In particular, they yield highly localised wave packets for a short period of time, which evolve in a quasi-classical manner and which saturate approximately Heisenberg uncertainty relation. These studies are elaborated in one- and two-dimensional contexts. Finally, some relations are established between the Gaussian states being mostly used here and the generalised coherent states, which are more standardly found in the literature.
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
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 of steered states.
Hu, Xueyuan; Milne, Antony; Zhang, Boyang; Fan, Heng
2016-01-19
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 Processes Which Do Not Use Coherence
NASA Astrophysics Data System (ADS)
Yadin, Benjamin; Ma, Jiajun; Girolami, Davide; Gu, Mile; Vedral, Vlatko
2016-10-01
A major signature of quantum mechanics beyond classical physics is coherence, the existence of superposition states. The recently developed resource theory of quantum coherence allows the formalization of incoherent operations—those operations which cannot create coherence. We identify the set of operations which additionally do not use coherence. These are such that coherence cannot be exploited by a classical observer, who measures incoherent properties of the system, to go beyond classical dynamics. We give a physical interpretation in terms of interferometry and prove a dilation theorem, showing how these operations can always be constructed by the system interacting, in an incoherent way, with an ancilla. Such a physical justification is not known for the incoherent operations; thus, our results lead to a physically well-motivated resource theory of coherence. Next, we investigate the implications for coherence in multipartite systems. We show that quantum correlations can be defined naturally with respect to a fixed basis, providing a link between coherence and quantum discord. We demonstrate the interplay between these two quantities in the operations that we study and suggest implications for the theory of quantum discord by relating these operations to those which cannot create discord.
Toward a superconducting quantum computer. Harnessing macroscopic quantum coherence.
Tsai, Jaw-Shen
2010-01-01
Intensive research on the construction of superconducting quantum computers has produced numerous important achievements. The quantum bit (qubit), based on the Josephson junction, is at the heart of this research. This macroscopic system has the ability to control quantum coherence. This article reviews the current state of quantum computing as well as its history, and discusses its future. Although progress has been rapid, the field remains beset with unsolved issues, and there are still many new research opportunities open to physicists and engineers.
Quantum coherence of relic neutrinos.
Fuller, George M; Kishimoto, Chad T
2009-05-22
We argue that in at least a portion of the history of the Universe the relic background neutrinos are spatially extended, coherent superpositions of mass states. We show that an appropriate quantum mechanical treatment affects the neutrino mass values derived from cosmological data. The coherence scale of these neutrino flavor wave packets can be an appreciable fraction of the causal horizon size, raising the possibility of spacetime curvature-induced decoherence.
NASA Astrophysics Data System (ADS)
Barnham, K. W. J.; Ballard, I.; Connolly, J. P.; Ekins-Daukes, N. J.; Kluftinger, B. G.; Nelson, J.; Rohr, C.
2002-04-01
This paper reviews the experimental and theoretical studies of quantum well solar cells with an aim of providing the background to the more detailed papers on this subject in these proceedings. It discusses the way quantum wells enhance efficiency in real, lattice matched material systems and fundamental studies of radiative recombination relevant to the question of whether such enhancements are possible in ideal cells. A number of theoretical models for quantum well solar cells (QWSCs) are briefly reviewed and more detail is given of our own group's model of the dark-currents. The temperature and field dependence of QWSCs are all briefly reviewed.
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.
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.
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.
Estimation on Geometric Measure of Quantum Coherence
NASA Astrophysics Data System (ADS)
Zhang, Hai-Jun; Chen, Bin; Li, Ming; Fei, Shao-Ming; Long, Gui-Lu
2017-02-01
We study the geometric measure of quantum coherence recently proposed in [Phys. Rev. Lett. 115, 020403 (2015)]. Both lower and upper bounds of this measure are provided. These bounds are shown to be tight for a class of important coherent states -- maximally coherent mixed states. The trade-off relation between quantum coherence and mixedness for this measure is also discussed.
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
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.
Nonlocal advantage of quantum coherence
NASA Astrophysics Data System (ADS)
Mondal, Debasis; Pramanik, Tanumoy; Pati, Arun Kumar
2017-01-01
A bipartite state is said to be steerable if and only if it does not have a single-system description, i.e., the bipartite state cannot be explained by a local hidden state model. Several steering inequalities have been derived using different local uncertainty relations to verify the ability to control the state of one subsystem by the other party. Here, we derive complementarity relations between coherences measured on mutually unbiased bases using various coherence measures such as the l1-norm, relative entropy, and skew information. Using these relations, we derive conditions under which a nonlocal advantage of quantum coherence can be achieved and the state is steerable. We show that not all steerable states can achieve such an advantage.
The complementarity relations of quantum coherence in quantum information processing.
Pan, Fei; Qiu, Liang; Liu, Zhi
2017-03-08
We establish two complementarity relations for the relative entropy of coherence in quantum information processing, i.e., quantum dense coding and teleportation. We first give an uncertainty-like expression relating local quantum coherence to the capacity of optimal dense coding for bipartite system. The relation can also be applied to the case of dense coding by using unital memoryless noisy quantum channels. Further, the relation between local quantum coherence and teleportation fidelity for two-qubit system is given.
The complementarity relations of quantum coherence in quantum information processing
NASA Astrophysics Data System (ADS)
Pan, Fei; Qiu, Liang; Liu, Zhi
2017-03-01
We establish two complementarity relations for the relative entropy of coherence in quantum information processing, i.e., quantum dense coding and teleportation. We first give an uncertainty-like expression relating local quantum coherence to the capacity of optimal dense coding for bipartite system. The relation can also be applied to the case of dense coding by using unital memoryless noisy quantum channels. Further, the relation between local quantum coherence and teleportation fidelity for two-qubit system is given.
The complementarity relations of quantum coherence in quantum information processing
Pan, Fei; Qiu, Liang; Liu, Zhi
2017-01-01
We establish two complementarity relations for the relative entropy of coherence in quantum information processing, i.e., quantum dense coding and teleportation. We first give an uncertainty-like expression relating local quantum coherence to the capacity of optimal dense coding for bipartite system. The relation can also be applied to the case of dense coding by using unital memoryless noisy quantum channels. Further, the relation between local quantum coherence and teleportation fidelity for two-qubit system is given. PMID:28272481
Quantum coherence in the dynamical Casimir effect
NASA Astrophysics Data System (ADS)
Samos-Sáenz de Buruaga, D. N.; Sabín, Carlos
2017-02-01
We propose to use quantum coherence as the ultimate proof of the quantum nature of the radiation that appears by means of the dynamical Casimir effect in experiments with superconducting microwave waveguides. We show that, unlike previously considered measurements such as entanglement and discord, quantum coherence does not require a threshold value of the external pump amplitude and is highly robust to thermal noise.
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 well nonlinear microcavities
NASA Astrophysics Data System (ADS)
Oudar, J. L.; Kuszelewicz, R.; Sfez, B.; Pellat, D.; Azoulay, R.
We report on recent progress in reducing the power threshold of all-optical bistable quantum well vertical microcavities. Significant improvements are achieved through an increase of the cavity finesse, together with a reduction of the device active layer thickness. A critical intensity of 5 μW/μm 2 has been observed on a microcavity of finesse 250, with a nonlinear medium of only 18 GaAs quantum wells of 10 nm thickness. Further improvements of the Bragg mirror quality resulted in a finesse of 700 and a power-lifetime product of 15 fJ/μm 2. Microresonator pixellation allows to obtain 2-dimensional arrays. A thermally-induced alloy-mixing technique is described, which produced a 110 meV carrier confinement energy, together with a refractive index change of -.012, averaged over the 2.6 μm nonlinear medium thickness. The resulting electrical and optical confinement is shown to improve the nonlinear characteristics, by limiting lateral carrier diffusion and light diffraction.
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
2014-10-20
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.
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.
Cavity quantum electrodynamics: coherence in context.
Mabuchi, H; Doherty, A C
2002-11-15
Modern cavity quantum electrodynamics (cavity QED) illuminates the most fundamental aspects of coherence and decoherence in quantum mechanics. Experiments on atoms in cavities can be described by elementary models but reveal intriguing subtleties of the interplay of coherent dynamics with external couplings. Recent activity in this area has pioneered powerful new approaches to the study of quantum coherence and has fueled the growth of quantum information science. In years to come, the purview of cavity QED will continue to grow as researchers build on a rich infrastructure to attack some of the most pressing open questions in micro- and mesoscopic physics.
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.
Theory of coherent control with quantum light
NASA Astrophysics Data System (ADS)
Schlawin, Frank; Buchleitner, Andreas
2017-01-01
We develop a coherent control theory for multimode quantum light. It allows us to examine a fundamental problem in quantum optics: what is the optimal pulse form to drive a two-photon-transition? In formulating the question as a coherent control problem, we show that—and quantify how much—the strong frequency quantum correlations of entangled photons enhance the transition compared to shaped classical pulses. In ensembles of collectively driven two-level systems, such enhancement requires nonvanishing interactions.
Coherent semiclassical states for loop quantum cosmology
NASA Astrophysics Data System (ADS)
Corichi, Alejandro; Montoya, Edison
2011-08-01
The spatially flat Friedmann-Robertson-Walker cosmological model with a massless scalar field in loop quantum cosmology admits a description in terms of a completely solvable model. This has been used to prove that: (i) the quantum bounce that replaces the big bang singularity is generic; (ii) there is an upper bound on the energy density for all states, and (iii) semiclassical states at late times had to be semiclassical before the bounce. Here we consider a family of exact solutions to the theory, corresponding to generalized coherent Gaussian and squeezed states. We analyze the behavior of basic physical observables and impose restrictions on the states based on physical considerations. These turn out to be enough to select, from all the generalized coherent states, those that behave semiclassical at late times. We study then the properties of such states near the bounce where the most “quantum behavior” is expected. As it turns out, the states remain sharply peaked and semiclassical at the bounce and the dynamics is very well approximated by the “effective theory” throughout the time evolution. We compare the semiclassicality properties of squeezed states to those of the Gaussian semiclassical states and conclude that the Gaussians are better behaved. In particular, the asymmetry in the relative fluctuations before and after the bounce are negligible, thus ruling out claims of so-called “cosmic forgetfulness.”
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
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; Schneider, Ulrich
2015-03-24
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.
Quantifying the coherence of pure quantum states
NASA Astrophysics Data System (ADS)
Chen, Jianxin; Grogan, Shane; Johnston, Nathaniel; Li, Chi-Kwong; Plosker, Sarah
2016-10-01
In recent years, several measures have been proposed for characterizing the coherence of a given quantum state. We derive several results that illuminate how these measures behave when restricted to pure states. Notably, we present an explicit characterization of the closest incoherent state to a given pure state under the trace distance measure of coherence. We then use this result to show that the states maximizing the trace distance of coherence are exactly the maximally coherent states. We define the trace distance of entanglement and show that it coincides with the trace distance of coherence for pure states. Finally, we give an alternate proof to a recent result that the ℓ1 measure of coherence of a pure state is never smaller than its relative entropy of coherence.
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.
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.
Coherent control of quantum systems as a resource theory
NASA Astrophysics Data System (ADS)
Matera, J. M.; Egloff, D.; Killoran, N.; Plenio, M. B.
2016-08-01
Control at the interface between the classical and the quantum world is fundamental in quantum physics. In particular, how classical control is enhanced by coherence effects is an important question both from a theoretical as well as from a technological point of view. In this work, we establish a resource theory describing this setting and explore relations to the theory of coherence, entanglement and information processing. Specifically, for the coherent control of quantum systems, the relevant resources of entanglement and coherence are found to be equivalent and closely related to a measure of discord. The results are then applied to the DQC1 protocol and the precision of the final measurement is expressed in terms of the available resources.
Coherent spaces, Boolean rings and quantum gates
NASA Astrophysics Data System (ADS)
Vourdas, A.
2016-10-01
Coherent spaces spanned by a finite number of coherent states, are introduced. Their coherence properties are studied, using the Dirac contour representation. It is shown that the corresponding projectors resolve the identity, and that they transform into projectors of the same type, under displacement transformations, and also under time evolution. The set of these spaces, with the logical OR and AND operations is a distributive lattice, and with the logical XOR and AND operations is a Boolean ring (Stone's formalism). Applications of this Boolean ring into classical CNOT gates with n-ary variables, and also quantum CNOT gates with coherent states, are discussed.
One-way quantum deficit and quantum coherence in the anisotropic XY chain
NASA Astrophysics Data System (ADS)
Ye, Biao-Liang; Li, Bo; Zhao, Li-Jun; Zhang, Hai-Jun; Fei, Shao-Ming
2017-03-01
In this study, we investigate pairwise non-classical correlations measured using a one-way quantum deficit as well as quantum coherence in the XY spin-1/2 chain in a transverse magnetic field for both zero and finite temperatures. The analytical and numerical results of our investigations are presented. In the case when the temperature is zero, it is shown that the one-way quantum deficit can characterize quantum phase transitions as well as quantum coherence. We find that these measures have a clear critical point at λ = 1. When λ ≤ 1, the one-way quantum deficit has an analytical expression that coincides with the relative entropy of coherence. We also study an XX model and an Ising chain at the finite temperatures.
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.
NASA Astrophysics Data System (ADS)
Liu, Zhi; Qiu, Liang; Pan, Fei
2017-04-01
We consider the enhancement effect of quantum partially collapsing measurements, i.e., weak measurement and quantum measurement reversal, on quantum coherence and quantum Fisher information, both of which are transmitted through a spin-chain channel. For the state parameter lying in the region (π /2, π ), weak measurement can enhance quantum coherence and quantum Fisher information. For the state parameter lying in the region (0, π /2), quantum coherence and quantum Fisher information can be enhanced by quantum measurement reversal combined with weak measurement. We assume the probabilistic nature of the method should be responsible for the enhancement.
Signatures of Quantum Coherences in Rydberg Excitons
NASA Astrophysics Data System (ADS)
Grünwald, P.; Aßmann, M.; Heckötter, J.; Fröhlich, D.; Bayer, M.; Stolz, H.; Scheel, S.
2016-09-01
Coherent optical control of individual particles has been demonstrated both for atoms and semiconductor quantum dots. Here we demonstrate the emergence of quantum coherent effects in semiconductor Rydberg excitons in bulk Cu2O . Because of the spectral proximity between two adjacent Rydberg exciton states, a single-frequency laser may pump both resonances with little dissipation from the detuning. As a consequence, additional resonances appear in the absorption spectrum that correspond to dressed states consisting of two Rydberg exciton levels coupled to the excitonic vacuum, forming a V -type three-level system, but driven only by one laser light source. We show that the level of pure dephasing in this system is extremely low. These observations are a crucial step towards coherently controlled quantum technologies in a bulk semiconductor.
Coherent quantum states from classical oscillator amplitudes
NASA Astrophysics Data System (ADS)
Briggs, John S.; Eisfeld, Alexander
2012-05-01
In the first days of quantum mechanics Dirac pointed out an analogy between the time-dependent coefficients of an expansion of the Schrödinger equation and the classical position and momentum variables solving Hamilton's equations. Here it is shown that the analogy can be made an equivalence in that, in principle, systems of classical oscillators can be constructed whose position and momenta variables form time-dependent amplitudes which are identical to the complex quantum amplitudes of the coupled wave function of an N-level quantum system with real coupling matrix elements. Hence classical motion can reproduce quantum coherence.
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.
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.
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.
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.
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.
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.
Optimal Protection of Quantum Coherence in Noisy Environment
NASA Astrophysics Data System (ADS)
Huang, Zhiming; Situ, Haozhen
2017-02-01
In this paper, we analyse the quantum coherence dynamics of a single qubit locally interacting with a zero-temperature reservoir. We compare the behaviors of quantum coherence in Markovian and non-Markovian regime. We find that the system coherence is transferred to the reservoir and decreases with time. In non-Markovian regime, quantum coherence exists instantaneous disappearance at some discrete time points. Furthermore, we propose an optimal scheme to protect quantum coherence by executing prior weak measurement and post-measurement reversal. It is worth noticing that the scheme can get better quantum coherence with the larger weak measurement strength, while at the cost of reducing success probability.
Experimental Demonstration of Coherent Control in Quantum Chaotic Systems
NASA Astrophysics Data System (ADS)
Bitter, M.; Milner, V.
2017-01-01
We experimentally demonstrate coherent control of a quantum system, whose dynamics is chaotic in the classical limit. Interaction of diatomic molecules with a periodic sequence of ultrashort laser pulses leads to the dynamical localization of the molecular angular momentum, a characteristic feature of the chaotic quantum kicked rotor. By changing the phases of the rotational states in the initially prepared coherent wave packet, we control the rotational distribution of the final localized state and its total energy. We demonstrate the anticipated sensitivity of control to the exact parameters of the kicking field, as well as its disappearance in the classical regime of excitation.
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.
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.
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
2016-02-26
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.
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.
Efficient quantum computing using coherent photon conversion.
Langford, N K; Ramelow, S; Prevedel, R; Munro, W J; Milburn, G J; Zeilinger, A
2011-10-12
Single photons are excellent quantum information carriers: they were used in the earliest demonstrations of entanglement and in the production of the highest-quality entanglement reported so far. However, current schemes for preparing, processing and measuring them are inefficient. For example, down-conversion provides heralded, but randomly timed, single photons, and linear optics gates are inherently probabilistic. Here we introduce a deterministic process--coherent photon conversion (CPC)--that provides a new way to generate and process complex, multiquanta states for photonic quantum information applications. The technique uses classically pumped nonlinearities to induce coherent oscillations between orthogonal states of multiple quantum excitations. One example of CPC, based on a pumped four-wave-mixing interaction, is shown to yield a single, versatile process that provides a full set of photonic quantum processing tools. This set satisfies the DiVincenzo criteria for a scalable quantum computing architecture, including deterministic multiqubit entanglement gates (based on a novel form of photon-photon interaction), high-quality heralded single- and multiphoton states free from higher-order imperfections, and robust, high-efficiency detection. It can also be used to produce heralded multiphoton entanglement, create optically switchable quantum circuits and implement an improved form of down-conversion with reduced higher-order effects. Such tools are valuable building blocks for many quantum-enabled technologies. Finally, using photonic crystal fibres we experimentally demonstrate quantum correlations arising from a four-colour nonlinear process suitable for CPC and use these measurements to study the feasibility of reaching the deterministic regime with current technology. Our scheme, which is based on interacting bosonic fields, is not restricted to optical systems but could also be implemented in optomechanical, electromechanical and superconducting
Strong quantum coherence between Fermi liquid Mahan excitons
Paul, J.; Stevens, C. E.; Liu, C.; ...
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.
2016-04-14
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 optical 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.
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.
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.
Interface effect in coupled quantum wells
Hao, Ya-Fei
2014-06-28
This paper intends to theoretically investigate the effect of the interfaces on the Rashba spin splitting of two coupled quantum wells. The results show that the interface related Rashba spin splitting of the two coupled quantum wells is both smaller than that of a step quantum well which has the same structure with the step quantum well in the coupled quantum wells. And the influence of the cubic Dresselhaus spin-orbit interaction of the coupled quantum wells is larger than that of a step quantum well. It demonstrates that the spin relaxation time of the two coupled quantum wells will be shorter than that of a step quantum well. As for the application in the spintronic devices, a step quantum well may be better than the coupled quantum wells, which is mentioned in this paper.
Fundamental Principles of Coherent-Feedback Quantum Control
2014-12-08
AFRL-OSR-VA-TR-2015-0009 FUNDAMENTAL PRINCIPLES OF COHERENT- FEEDBACK QUANTUM CONTROL Hideo Mabuchi LELAND STANFORD JUNIOR UNIV CA Final Report 12/08...foundations and potential applications of coherent- feedback quantum control. We have focused on potential applications in quantum-enhanced metrology and...picture of how coherent feedback can provide a kind of circuit/network theory for quantum engineering, enabling rigorous analysis and numerical simulation
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 key distribution device with coherent states
NASA Astrophysics Data System (ADS)
Lodewyck, Jérôme; Bloch, Matthieu; García-Patrón, Raúl; Fossier, Simon; Karpov, Evgueni; Diamanti, Eleni; Debuisschert, Thierry; Cerf, Nicolas J.; Tualle-Brouri, Rosa; McLaughlin, Steven W.; Grangier, Philippe
2007-09-01
We report on both theoretical and experimental aspects of a fully implemented quantum key distribution device with coherent states. This system features a final key rate of more than 2 kb/s over 25 km of optical fiber. It comprises all required elements for field operation: a compact optical setup, a fast secret bit extraction using efficient LDPC codes, privacy amplification algorithms and a classical channel software. Both hardware and software are operated in real time.
Spectral coherent-state quantum cryptography.
Cincotti, Gabriella; Spiekman, Leo; Wada, Naoya; Kitayama, Ken-ichi
2008-11-01
A novel implementation of quantum-noise optical cryptography is proposed, which is based on a simplified architecture that allows long-haul, high-speed transmission in a fiber optical network. By using a single multiport encoder/decoder and 16 phase shifters, this new approach can provide the same confidentiality as other implementations of Yuen's encryption protocol, which use a larger number of phase or polarization coherent states. Data confidentiality and error probability for authorized and unauthorized receivers are carefully analyzed.
Spatially indirect excitons in coupled quantum wells
Lai, Chih-Wei Eddy
2004-03-01
Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer)^{2} were
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 coherence of spin-boson model at finite temperature
NASA Astrophysics Data System (ADS)
Wu, Wei; Xu, Jing-Bo
2017-02-01
We investigate the dynamical behavior of quantum coherence in spin-boson model, which consists of a qubit coupled to a finite-temperature bosonic bath with power-law spectral density beyond rotating wave approximation, by employing l1-norm as well as quantum relative entropy. It is shown that the temperature of bosonic bath and counter-rotating terms significantly affect the decoherence rate in sub-Ohmic, Ohmic and super-Ohmic baths. At high temperature, we find the counter-rotating terms of spin-boson model are able to increase the decoherence rate for sub-Ohmic baths, however, for Ohmic and super-Ohmic baths, the counter-rotating terms tend to decrease the value of decoherence rate. At low temperature, we find the counter-rotating terms always play a positive role in preserving the qubit's quantum coherence regardless of sub-Ohmic, Ohmic and super-Ohmic baths.
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.
Effect of Multiple Scattering in a Quantum Well
NASA Astrophysics Data System (ADS)
Sheng, Hanyu; Chua, Soo-Jin; Sinkkonen, Juha
This paper gives a potentially useful application to quantum well of the theory of scattering in the Born approximation. The simple formulae for multiple scattering in a quantum well of double barrier structure are derived. The multiple scattering parameter is the complex mean free path. We show that the amplitude of the coherent wave will be exponentially attenuated and the phase of the wave will be delayed because of the scattering.
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.
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.
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.
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
Quantum well cells for thermophotovoltaics
NASA Astrophysics Data System (ADS)
Connolly, J. P.; Rohr, C.
2003-05-01
Quantum well cells (QWCs) are p-i-n photovoltaic devices with multiple quantum wells (MQWs) inserted in the intrinsic region. They show the desirable characteristic for thermophotovoltaics (TPV) of an easily tuneable bandgap. This is highly relevant because the bandgaps of homostructures are restricted by the range of lattice matched substrates available. As a consequence bulk structures cannot be tuned to desirable narrow band emission spectra. QWCs, however, display tuneable bandgaps from several points of view. Firstly, the MQW bandedge is inherently tuneable by changing well depth and dimension. More significantly, strain compensated MQW systems allow lattice mismatched layers to be deposited without the formation of dislocations. This permits a much greater range of compositions and hence bandgaps than are obtainable with lattice matched systems. We first review the fundamental understanding of spectral response and dark current of the QWC and describe the initial proposal of QWCs for TPV before examining subsequent work on lattice matched InGaAsP/InP. Design limitations of the lattice matched materials can be eliminated by applying strain compensation techniques to strained materials. We show dark current measurements for lattice matched and strained systems with dark currents lower than other promising TPV designs despite longer wavelength absorption edges. This work has recently produced a QWC capable of reaching the Holmia narrow band emission peak at wavelength 1950 nm which shows the promise of this approach for TPV.
Ultrashort Pulse Interaction with Intersubband Transitions of Semiconductor Quantum Wells
NASA Astrophysics Data System (ADS)
Katsantonis, Ioannis; Stathatos, Elias; Paspalakis, Emmanuel
2015-09-01
We study coherent ultrashort pulse propagation in a two-subband system in a symmetric semiconductor quantum well structure, performing calculations beyond the rotating wave approximation and the slowly varying envelope approximation and taking into account the effects of electron-electron interactions. The interaction of the quantum well structure with the electromagnetic fields is studied with modified, nonlinear, Bloch equations. These equations are combined with the full-wave Maxwell equations for the study of pulse propagation. We present results for the pulse propagation and the population inversion dynamics in the quantum well structure for different electron sheet densities.
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
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
NASA Astrophysics Data System (ADS)
Hui, Ning-Ju; Xu, Yang-Yang; Wang, Jicheng; Zhang, Yixin; Hu, Zheng-Da
2017-04-01
We investigate the properties of geometric quantum coherence in the XY spin-1/2 chain with staggered Dzyaloshinsky-Moriya interaction via the quantum renormalization-group approach. It is shown that the geometric quantum coherence and its coherence susceptibility are effective to detect the quantum phase transition. In the thermodynamic limit, the geometric quantum coherence exhibits a sudden jump. The coherence susceptibilities versus the anisotropy parameter and the Dzyaloshinsky-Moriya interaction are infinite and vanishing, respectively, illustrating the distinct roles of the anisotropy parameter and the Dzyaloshinsky-Moriya interaction in quantum phase transition. Moreover, we also explore the finite-size scaling behaviors of the coherence susceptibilities. For a finite-size chain, the coherence susceptibility versus the phase-transition parameter is always maximal at the critical point, indicating the dramatic quantum fluctuation. Besides, we show that the correlation length can be revealed by the scaling exponent for the coherence susceptibility versus the Dzyaloshinsky-Moriya interaction.
Quantum coherence effects in a Raman amplifier
NASA Astrophysics Data System (ADS)
Ooi, C. H. Raymond; Harun, S. W.; Ahmad, H.
2011-01-01
We have studied optical pulse propagation in a Raman fiber amplifier doped with a three-level medium and driven by a control laser pulse. We analyze the spatial-temporal dynamics of pulse propagation for different atomic initial conditions. The propagation of an optical pulse through the amplifier can be sustained by a control laser that induces transparency via quantum coherence, which is useful for extending the distance between optical repeaters. Under certain conditions, amplification is achieved without population inversion. The results could be useful for laser control of optical pulses in amplifiers and waveguides.
Quantum memory receiver for superadditive communication using binary coherent states
Klimek, Aleksandra; Jachura, Michał; Wasilewski, Wojciech; Banaszek, Konrad
2016-01-01
We propose a simple architecture based on multimode quantum memories for collective readout of classical information keyed using a pair coherent states, exemplified by the well-known binary phase shift keying format. Such a configuration enables demonstration of the superadditivity effect in classical communication over quantum channels, where the transmission rate becomes enhanced through joint detection applied to multiple channel uses. The proposed scheme relies on the recently introduced idea to prepare Hadamard sequences of input symbols that are mapped by a linear optical transformation onto the pulse position modulation format [Guha, S. Phys. Rev. Lett. 2011, 106, 240502]. We analyze two versions of readout based on direct detection and an optional Dolinar receiver which implements the minimum-error measurement for individual detection of a binary coherent state alphabet. PMID:27695200
Localization and coherent states in a quantum DNLS trimer
NASA Astrophysics Data System (ADS)
Martínez-Galicia, Ricardo; Panayotaros, Panayotis
2016-11-01
We compare quantum states obtained from the integration of exact and approximate evolution equations for a quantized discrete nonlinear Schrödinger system (DNLS) with three lattice sites (trimer). The initial conditions are Glauber coherent states, and their projections to subspaces with a definite number of particles, and we are especially interested in coherent states that correspond to classical states that are in the neighborhood of breather solutions of the classical system. The breathers are well defined periodic orbits of the classical DNLS that we heuristically view as examples of spatially localized solutions. The two evolution equations give converging results in the subspaces with an increasing number of particles. This is no longer the case for normalized projections of Glauber states, where we see that the distance between the quantum states obtained by the exact and approximate equations shows recurrence phenomena that depend on the number of quanta and on the dynamical properties of the classical trajectory.
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
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.
Blind quantum computing with weak coherent pulses.
Dunjko, Vedran; Kashefi, Elham; Leverrier, Anthony
2012-05-18
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 ϵ blindness for UBQC, in analogy to the concept of ϵ 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 ϵ-blind UBQC for any ϵ>0, even if the channel between the client and the server is arbitrarily lossy.
Quantum simulation. Coherent imaging spectroscopy of a quantum many-body spin system.
Senko, C; Smith, J; Richerme, P; Lee, A; Campbell, W C; Monroe, C
2014-07-25
Quantum simulators, in which well-controlled quantum systems are used to reproduce the dynamics of less understood ones, have the potential to explore physics inaccessible to modeling with classical computers. However, checking the results of such simulations also becomes classically intractable as system sizes increase. Here, we introduce and implement a coherent imaging spectroscopic technique, akin to magnetic resonance imaging, to validate a quantum simulation. We use this method to determine the energy levels and interaction strengths of a fully connected quantum many-body system. Additionally, we directly measure the critical energy gap near a quantum phase transition. We expect this general technique to become a verification tool for quantum simulators once experiments advance beyond proof-of-principle demonstrations and exceed the resources of conventional computers.
Quantum discord of bipartite entangled non-linear coherent states
NASA Astrophysics Data System (ADS)
Castro, E.; Zambrano, A.; Ladera, C. L.; Gómez, R.
2013-11-01
Quantum discord measures the fraction of the pair-wise mutual information that is locally inaccessible in a multipartite system. Nonzero quantum discord has interesting and significant applications because although non-zero entanglement guarantees the existence of quantum correlation in a bipartite quantum system, zero entanglement does not guarantee the absence of a quantum correlation. On the other hand, many quantum optics systems can be described as deformed quantum oscillators. In this work, we investigate the quantum discord of bipartite entangled nonlinear coherent states, in the context of the so-called f-deformed coherent states algebra. To calculate the quantum discord, we consider quasi- Werner mixed states bases on bipartite entangled f-deformed coherent states. Two explicit analytic expressions are derived for the quantum discord of two different nonlinear deformed coherent states. The first one considers deformed coherent states obtained as eigenstates of the annihilation deformed operator, and the second one is obtained by using a deformed displacement operator. We compare the quantum discord of those states, when the nonlinear deformation function is either associated with the SU(1,1) coherent states in the Gilmore-Perelomov or Barut-Girardello representations, respectively.
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
Coherent manipulation of single quantum systems in the solid state
NASA Astrophysics Data System (ADS)
Childress, Lilian Isabel
2007-12-01
The controlled, coherent manipulation of quantum-mechanical systems is an important challenge in modern science and engineering, with significant applications in quantum information science. Solid-state quantum systems such as electronic spins, nuclear spins, and superconducting islands are among the most promising candidates for realization of quantum bits (qubits). However, in contrast to isolated atomic systems, these solid-state qubits couple to a complex environment which often results in rapid loss of coherence, and, in general, is difficult to understand. Additionally, the strong interactions which make solid-state quantum systems attractive can typically only occur between neighboring systems, leading to difficulties in coupling arbitrary pairs of quantum bits. This thesis presents experimental progress in understanding and controlling the complex environment of a solid-state quantum bit, and theoretical techniques for extending the distance over which certain quantum bits can interact coherently. Coherent manipulation of an individual electron spin associated with a nitrogen-vacancy center in diamond is used to gain insight into its mesoscopic environment. Furthermore, techniques for exploiting coherent interactions between the electron spin and a subset of the environment are developed and demonstrated, leading to controlled interactions with single isolated nuclear spins. The quantum register thus formed by a coupled electron and nuclear spin provides the basis for a theoretical proposal for fault-tolerant long-distance quantum communication with minimal physical resource requirements. Finally, we consider a mechanism for long-distance coupling between quantum dots based on chip-scale cavity quantum electrodynamics.
Quantum dot spin coherence governed by a strained nuclear environment
NASA Astrophysics Data System (ADS)
Stockill, R.; Le Gall, C.; Matthiesen, C.; Huthmacher, L.; Clarke, E.; Hugues, M.; Atatüre, M.
2016-09-01
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.
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
NASA Astrophysics Data System (ADS)
Yao, Yao; Dong, G. H.; Xiao, Xing; Sun, C. P.
2016-08-01
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.
Experimental demonstration of macroscopic quantum coherence in Gaussian states
Marquardt, Christoph; Leuchs, Gerd; Andersen, Ulrik L.; Takeno, Yuishi; Yukawa, Mitsuyoshi; Yonezawa, Hidehiro; Furusawa, Akira
2007-09-15
We witness experimentally the presence of macroscopic coherence in Gaussian quantum states using a recently proposed criterion [E. G. Cavalcanti and M. D. Reid, Phys. Rev. Lett. 97 170405 (2006)]. The macroscopic coherence stems from interference between macroscopically distinct states in phase space, and we prove experimentally that a coherent state contains these features with a distance in phase space of 0.51{+-}0.02 shot noise units. This is surprising because coherent states are generally considered being at the border between classical and quantum states, not yet displaying any nonclassical effect. For squeezed and entangled states the effect may be larger but depends critically on the state purity.
Quantum Correlations, Separability, and Quantum Coherence Length in Equilibrium Many-Body Systems
NASA Astrophysics Data System (ADS)
Malpetti, Daniele; Roscilde, Tommaso
2016-09-01
Nonlocality is a fundamental trait of quantum many-body systems, both at the level of pure states, as well as at the level of mixed states. Because of nonlocality, mixed states of any two subsystems are correlated in a stronger way than what can be accounted for by considering the correlated probabilities of occupying some microstates. In the case of equilibrium mixed states, we explicitly build two-point quantum correlation functions, which capture the specific, superior correlations of quantum systems at finite temperature, and which are directly accessible to experiments when correlating measurable properties. When nonvanishing, these correlation functions rule out a precise form of separability of the equilibrium state. In particular, we show numerically that quantum correlation functions generically exhibit a finite quantum coherence length, dictating the characteristic distance over which degrees of freedom cannot be considered as separable. This coherence length is completely disconnected from the correlation length of the system—as it remains finite even when the correlation length of the system diverges at finite temperature—and it unveils the unique spatial structure of quantum correlations.
Directly Measuring the Degree of Quantum Coherence using Interference Fringes
NASA Astrophysics Data System (ADS)
Wang, Yi-Tao; Tang, Jian-Shun; Wei, Zhi-Yuan; Yu, Shang; Ke, Zhi-Jin; Xu, Xiao-Ye; Li, Chuan-Feng; Guo, Guang-Can
2017-01-01
Quantum coherence is the most distinguished feature of quantum mechanics. It lies at the heart of the quantum-information technologies as the fundamental resource and is also related to other quantum resources, including entanglement. It plays a critical role in various fields, even in biology. Nevertheless, the rigorous and systematic resource-theoretic framework of coherence has just been developed recently, and several coherence measures are proposed. Experimentally, the usual method to measure coherence is to perform state tomography and use mathematical expressions. Here, we alternatively develop a method to measure coherence directly using its most essential behavior—the interference fringes. The ancilla states are mixed into the target state with various ratios, and the minimal ratio that makes the interference fringes of the "mixed state" vanish is taken as the quantity of coherence. We also use the witness observable to witness coherence, and the optimal witness constitutes another direct method to measure coherence. For comparison, we perform tomography and calculate l1 norm of coherence, which coincides with the results of the other two methods in our situation. Our methods are explicit and robust, providing a nice alternative to the tomographic technique.
Quantum walk coherences on a dynamical percolation graph.
Elster, Fabian; Barkhofen, Sonja; Nitsche, Thomas; Novotný, Jaroslav; Gábris, Aurél; Jex, Igor; Silberhorn, Christine
2015-08-27
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.
Coherent and incoherent charge transport in linear triple quantum dots.
Contreras-Pulido, L Debora; Bruderer, Martin
2017-03-15
One of the fundamental questions in quantum transport is how charge transfer through complex nanostructures is influenced by quantum coherence. We address this issue for linear triple quantum dots by comparing a Lindblad density matrix description with a Pauli rate equation approach and analyze the corresponding zero-frequency counting statistics of charge transfer. The impact of decaying coherences of the density matrix due to dephasing is also studied. Our findings reveal that the sensitivity to coherence shown by shot noise and skewness, in particular in the limit of large coupling to the drain reservoir, can be used to unambiguously evidence coherent processes involved in charge transport across triple quantum dots. Our analytical results are obtained by using the characteristic polynomial approach to full counting statistics.
Quantum superchemistry in an output coupler of coherent matter waves
Jing, H.; Cheng, J.
2006-12-15
We investigate the quantum superchemistry or Bose-enhanced atom-molecule conversions in a coherent output coupler of matter waves, as a simple generalization of the two-color photoassociation. The stimulated effects of molecular output step and atomic revivals are exhibited by steering the rf output couplings. The quantum noise-induced molecular damping occurs near a total conversion in a levitation trap. This suggests a feasible two-trap scheme to make a stable coherent molecular beam.
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.
Broadspectrum InGaAs/InP Quantum Well Infrared Photodetector via Quantum Well Intermixing
NASA Technical Reports Server (NTRS)
Sengupta, D.; Chang, Y. C.; Stillman, G.
1998-01-01
We have demonstrated red shifting and broadening of the wavelength response of a bound-to-continuum ultra-thin p-type InGaAs/InP quantum well infrared photodetector (QWIP) after growth via quantum well intermixing.
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.
Decoherence of multiple quantum coherences generated from a dipolar ordered state
NASA Astrophysics Data System (ADS)
González, C. E.; Segnorile, H. H.; Zamar, R. C.
2011-01-01
Starting from the hypothesis that the decay of coherent signals observed in H1 NMR experiments is driven by quantum interference, irreversible decoherence, and nonidealities in the experiment, we design an experiment to isolate and identify the irreversible attenuation of multiple-quantum coherences toward quasiequilibrium states of dipolar order in nematic liquid crystals (LCs). The experiment combines the well-known “magic sandwich” pulse sequence with preparation of dipolar ordered states and encoding of multiple-quantum coherences. The spin system composed of the dipole-coupled protons of a LC molecule provides an example of a small cluster of strongly interacting spins. We study decoherence rates under a sequence that reverses time evolution with the secular dipolar Hamiltonian to compensate coherent evolution of a closed quantum system. In this way, the time scale is made evident where irreversible decoherence takes place, providing insight into the nature of the processes responsible for the attainment of quasiequilibrium. The behavior of single- and double-quantum-coherence amplitudes with reversal time is interpreted as evidence of the quantum character (as opposed to stochastic character) of the processes that drive irreversible decoherence. The experimental method proposed is useful for probing the action of the environment on materials with quantum information processing potential.
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.
Coherent quantum depletion of an interacting atom condensate.
Kira, M
2015-03-13
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.
Coherently driven semiconductor quantum dot at a telecommunication wavelength.
Takagi, Hiroyuki; Nakaoka, Toshihiro; Watanabe, Katsuyuki; Kumagai, Naoto; Arakawa, Yasuhiko
2008-09-01
We proposed and demonstrate use of optical driving pulses at a telecommunication wavelength for exciton-based quantum gate operation. The exciton in a self-assembled quantum dot is coherently manipulated at 1.3 microm through Rabi oscillation. The telecom-band exciton-qubit system incorporates standard optical fibers and fiber optic devices. The coherent manipulation of the two-level system compatible with flexible and stable fiber network paves the way toward practical optical implementation of quantum information processing devices.
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.
Coherent spin-exchange via a quantum mediator
NASA Astrophysics Data System (ADS)
Baart, Timothy Alexander; Fujita, Takafumi; Reichl, Christian; Wegscheider, Werner; Vandersypen, Lieven Mark Koenraad
2017-01-01
Coherent interactions at a distance provide a powerful tool for quantum simulation and computation. The most common approach to realize an effective long-distance coupling 'on-chip' is to use a quantum mediator, as has been demonstrated for superconducting qubits and trapped ions. For quantum dot arrays, which combine a high degree of tunability with extremely long coherence times, the experimental demonstration of the time evolution of coherent spin-spin coupling via an intermediary system remains an important outstanding goal. Here, we use a linear triple-quantum-dot array to demonstrate a coherent time evolution of two interacting distant spins via a quantum mediator. The two outer dots are occupied with a single electron spin each, and the spins experience a superexchange interaction through the empty middle dot, which acts as mediator. Using single-shot spin readout, we measure the coherent time evolution of the spin states on the outer dots and observe a characteristic dependence of the exchange frequency as a function of the detuning between the middle and outer dots. This approach may provide a new route for scaling up spin qubit circuits using quantum dots, and aid in the simulation of materials and molecules with non-nearest-neighbour couplings such as MnO (ref. 27), high-temperature superconductors and DNA. The same superexchange concept can also be applied in cold atom experiments.
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.
Velizhanin, Kirill A; Piryatinski, Andrei
2011-05-12
Employing the interband exciton scattering model, we have derived a closed set of equations determining the 2D double-quantum coherence signal sensitive to the interband Coulomb interactions (i.e., many-body Coulomb interactions leading to the couplings between exciton and biexciton bands) in semiconductor nanostructures such as nanocrystals, quantum wires, wells, and carbon nanotubes. Our general analysis of 2D double-quantum coherence resonances has demonstrated that the interband Coulomb interactions lead to new cross-peaks whose appearance can be interpreted as a result of exciton and biexciton state mixing. The presence of the strongly coupled resonant states and weakly coupled background of off-resonant states can significantly simplify cross-peak analysis by eliminating the congested background spectrum. Our simulations of the 2D double-quantum coherence signal in PbSe NCs have validated this approach.
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
Persistent Inter-Excitonic Quantum Coherence in CdSe Quantum Dots
Caram, Justin R.; Zheng, Haibin; Dahlberg, Peter D.; Rolczynski, Brian S.; Griffin, Graham B.; Fidler, Andrew F.; Dolzhnikov, Dmitriy S.; Talapin, Dmitri V.; Engel, Gregory S.
2014-01-01
The creation and manipulation of quantum superpositions is a fundamental goal for the development of materials with novel optoelectronic properties. In this letter, we report persistent (~80 fs lifetime) quantum coherence between the 1S and 1P excitonic states in zinc-blende colloidal CdSe quantum dots at room temperature, measured using Two-Dimensional Electronic Spectroscopy. We demonstrate that this quantum coherence manifests as an intradot phenomenon, the frequency of which depends on the size of the dot excited within the ensemble of QDs. We model the lifetime of the coherence and demonstrate that correlated interexcitonic fluctuations preserve relative phase between excitonic states. These observations suggest an avenue for engineering long-lived interexcitonic quantum coherence in colloidal quantum dots. PMID:24719679
Persistent Inter-Excitonic Quantum Coherence in CdSe Quantum Dots.
Caram, Justin R; Zheng, Haibin; Dahlberg, Peter D; Rolczynski, Brian S; Griffin, Graham B; Fidler, Andrew F; Dolzhnikov, Dmitriy S; Talapin, Dmitri V; Engel, Gregory S
2014-01-02
The creation and manipulation of quantum superpositions is a fundamental goal for the development of materials with novel optoelectronic properties. In this letter, we report persistent (~80 fs lifetime) quantum coherence between the 1S and 1P excitonic states in zinc-blende colloidal CdSe quantum dots at room temperature, measured using Two-Dimensional Electronic Spectroscopy. We demonstrate that this quantum coherence manifests as an intradot phenomenon, the frequency of which depends on the size of the dot excited within the ensemble of QDs. We model the lifetime of the coherence and demonstrate that correlated interexcitonic fluctuations preserve relative phase between excitonic states. These observations suggest an avenue for engineering long-lived interexcitonic quantum coherence in colloidal quantum dots.
Ultrabroad stimulated emission from quantum well laser
Wang, Huolei; Zhou, Xuliang; Yu, Hongyan; Mi, Junping; Wang, Jiaqi; Bian, Jing; Wang, Wei; Pan, Jiaoqing; Ding, Ying; Chen, Weixi
2014-06-23
Observation of ultrabroad stimulated emission from a simplex quantum well based laser at the center wavelength of 1.06 μm is reported. With increased injection current, spectrum as broad as 38 nm and a pulsed output power of ∼50 mW have been measured. The experiments show evidence of an unexplored broad emission regime in the InGaAs/GaAs quantum well material system, which still needs theoretical modeling and further analysis.
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.
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
NASA Astrophysics Data System (ADS)
Yukalov, V. I.; Yukalova, E. P.
2014-03-01
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.
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 mirages formed by coherent projection of electronic structure
Manoharan; Lutz; Eigler
2000-02-03
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.
Strong Analog Classical Simulation of Coherent Quantum Dynamics
NASA Astrophysics Data System (ADS)
Wang, Dong-Sheng
2017-02-01
A strong analog classical simulation of general quantum evolution is proposed, which serves as a novel scheme in quantum computation and simulation. The scheme employs the approach of geometric quantum mechanics and quantum informational technique of quantum tomography, which applies broadly to cases of mixed states, nonunitary evolution, and infinite dimensional systems. The simulation provides an intriguing classical picture to probe quantum phenomena, namely, a coherent quantum dynamics can be viewed as a globally constrained classical Hamiltonian dynamics of a collection of coupled particles or strings. Efficiency analysis reveals a fundamental difference between the locality in real space and locality in Hilbert space, the latter enables efficient strong analog classical simulations. Examples are also studied to highlight the differences and gaps among various simulation methods. Funding support from NSERC of Canada and a research fellowship at Department of Physics and Astronomy, University of British Columbia are acknowledged
Generalized coherent and intelligent states for exact solvable quantum systems
NASA Astrophysics Data System (ADS)
El Kinani, A. H.; Daoud, M.
2002-02-01
The so-called Gazeau-Klauder and Perelomov coherent states are introduced for an arbitrary quantum system. We give also the general framework to construct the generalized intelligent states which minimize the Robertson-Schrödinger uncertainty relation. As illustration, the Pöschl-Teller potentials of trigonometric type will be chosen. We show the advantage of the analytical representations of Gazeau-Klauder and Perelomov coherent states in obtaining the generalized intelligent states in analytical way.
Continuous-variable quantum network coding for coherent states
NASA Astrophysics Data System (ADS)
Shang, Tao; Li, Ke; Liu, Jian-wei
2017-04-01
As far as the spectral characteristic of quantum information is concerned, the existing quantum network coding schemes can be looked on as the discrete-variable quantum network coding schemes. Considering the practical advantage of continuous variables, in this paper, we explore two feasible continuous-variable quantum network coding (CVQNC) schemes. Basic operations and CVQNC schemes are both provided. The first scheme is based on Gaussian cloning and ADD/SUB operators and can transmit two coherent states across with a fidelity of 1/2, while the second scheme utilizes continuous-variable quantum teleportation and can transmit two coherent states perfectly. By encoding classical information on quantum states, quantum network coding schemes can be utilized to transmit classical information. Scheme analysis shows that compared with the discrete-variable paradigms, the proposed CVQNC schemes provide better network throughput from the viewpoint of classical information transmission. By modulating the amplitude and phase quadratures of coherent states with classical characters, the first scheme and the second scheme can transmit 4{log _2}N and 2{log _2}N bits of information by a single network use, respectively.
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.
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
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.
Advantages of coherent feedback for cooling quantum oscillators.
Hamerly, Ryan; Mabuchi, Hideo
2012-10-26
We model the cooling of open optical and optomechanical resonators via optical feedback in the linear quadratic Gaussian setting of stochastic control theory. We show that coherent feedback control schemes, in which the resonator is embedded in an interferometer to achieve all-optical feedback, can outperform the best possible linear quadratic Gaussian measurement-based schemes in the quantum regime of low steady-state excitation number. Such performance gains are attributed to the coherent controller's ability to process noncommuting output field quadratures simultaneously without loss of fidelity, and may provide important clues for the design of coherent feedback schemes for more general problems of nonlinear and robust control.
Quantum Process Tomography Quantifies Coherence Transfer Dynamics in Vibrational Exciton
Chuntonov, Lev; Ma, Jianqiang
2013-01-01
Quantum coherence has been a subject of great interest in many scientific disciplines. However, detailed characterization of the quantum coherence in molecular systems, especially its transfer and relaxation mechanisms, still remains a major challenge. The difficulties arise in part because the spectroscopic signatures of the coherence transfer are typically overwhelmed by other excitation relaxation processes. We use quantum process tomography (QPT) via two-dimensional infrared spectroscopy to quantify the rate of the elusive coherence transfer between two vibrational exciton states. QPT retrieves the dynamics of the dissipative quantum system directly from the experimental observables. It thus serves as an experimental alternative to theoretical models of the system-bath interaction, and can be used to validate these theories. Our results for coupled carbonyl groups of a diketone molecule in chloroform, used as a benchmark system, reveal the non-secular nature of the interaction between the exciton and the Markovian bath and open the door for the systematic studies of the dissipative quantum systems dynamics in detail. PMID:24079417
NASA Astrophysics Data System (ADS)
Yahiaoui, S. A.; Bentaiba, M.
2012-11-01
In the context of the factorization method, we investigate the pseudo-Hermitian coherent states and their Hermitian counterpart coherent states under the generalized quantum condition in the framework of a position-dependent mass. By considering a specific modification in the superpotential, suitable annihilation and creation operators are constructed in order to reproduce the Hermitian counterpart Hamiltonian in the factorized form. We show that by means of these ladder operators, we can construct a wide range of exactly solvable potentials as well as their accompanying coherent states. Alternatively, we explore the relationship between the pseudo-Hermitian Hamiltonian and its Hermitian counterparts, obtained from a similarity transformation, to construct the associated pseudo-Hermitian coherent states. These latter preserve the structure of Perelomov’s states and minimize the generalized position-momentum uncertainty principle. This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical devoted to ‘Quantum physics with non-Hermitian operators’.
Coherent pulse position modulation quantum cipher
NASA Astrophysics Data System (ADS)
Sohma, Masaki; Hirota, Osamu
2014-12-01
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.
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.
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.
Photonic crystal slab quantum well infrared photodetector
NASA Astrophysics Data System (ADS)
Kalchmair, S.; Detz, H.; Cole, G. D.; Andrews, A. M.; Klang, P.; Nobile, M.; Gansch, R.; Ostermaier, C.; Schrenk, W.; Strasser, G.
2011-01-01
In this letter we present a quantum well infrared photodetector (QWIP), which is fabricated as a photonic crystal slab (PCS). With the PCS it is possible to enhance the absorption efficiency by increasing photon lifetime in the detector active region. To understand the optical properties of the device we simulate the PCS photonic band structure, which differs significantly from a real two-dimensional photonic crystal. By fabricating a PCS-QWIP with 100x less quantum well doping, compared to a standard QWIP, we are able to see strong absorption enhancement and sharp resonance peaks up to temperatures of 170 K.
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.
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.
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
Average subentropy, coherence and entanglement of random mixed quantum states
NASA Astrophysics Data System (ADS)
Zhang, Lin; Singh, Uttam; Pati, Arun K.
2017-02-01
Compact expressions for the average subentropy and coherence are obtained for random mixed states that are generated via various probability measures. Surprisingly, our results show that the average subentropy of random mixed states approaches the maximum value of the subentropy which is attained for the maximally mixed state as we increase the dimension. In the special case of the random mixed states sampled from the induced measure via partial tracing of random bipartite pure states, we establish the typicality of the relative entropy of coherence for random mixed states invoking the concentration of measure phenomenon. Our results also indicate that mixed quantum states are less useful compared to pure quantum states in higher dimension when we extract quantum coherence as a resource. This is because of the fact that average coherence of random mixed states is bounded uniformly, however, the average coherence of random pure states increases with the increasing dimension. As an important application, we establish the typicality of relative entropy of entanglement and distillable entanglement for a specific class of random bipartite mixed states. In particular, most of the random states in this specific class have relative entropy of entanglement and distillable entanglement equal to some fixed number (to within an arbitrary small error), thereby hugely reducing the complexity of computation of these entanglement measures for this specific class of mixed states.
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.
Coherence depletion in the Grover quantum search algorithm
NASA Astrophysics Data System (ADS)
Shi, Hai-Long; Liu, Si-Yuan; Wang, Xiao-Hui; Yang, Wen-Li; Yang, Zhan-Ying; Fan, Heng
2017-03-01
We investigate the role of quantum coherence depletion (QCD) in the Grover search algorithm (GA) by using several typical measures of quantum coherence and quantum correlations. By using the relative entropy of coherence measure (Cr), we show that the success probability depends on the QCD. The same phenomenon is also found by using the l1 norm of coherence measure (Cl1).In the limit case, the cost performance is defined to characterize the behavior about QCD in enhancing the success probability of GA, which is only related to the number of searcher items and the scale of the database, regardless of using Cr or Cl 1. In the generalized Grover search algorithm (GGA), the QCD for a class of states increases with the required optimal measurement time. In comparison, the quantification of other quantum correlations in GA, such as pairwise entanglement, multipartite entanglement, pairwise discord, and genuine multipartite discord, cannot be directly related to the success probability or the optimal measurement time. Additionally, we do not detect pairwise nonlocality or genuine tripartite nonlocality in GA since Clauser-Horne-Shimony-Holt inequality and Svetlichny's inequality are not violated.
Recombination Dynamics in Quantum Well Semiconductor Structures
NASA Astrophysics Data System (ADS)
Fouquet, Julie Elizabeth
Time-resolved and time-integrated photoluminescence as a function of excitation energy density have been observed in order to study recombination dynamics in GaAs/Al(,x)Ga(,1 -x)As quantum well structures. The study of room temperature photoluminescence from the molecular beam epitaxy (MBE) -grown multiple quantum well structure and photoluminescence peak energy as a function of tem- perature shows that room temperature recombination at excitation densities above the low 10('16) cm('-3) level is due to free carriers, not excitons. This is the first study of time-resolved photoluminescence of impurities in quantum wells; data taken at different emission wave- lengths at low temperatures shows that the impurity-related states at photon energies lower than the free exciton peaks luminesce much more slowly than the free exciton states. Results from a similar structure grown by metal -organic chemical vapor deposition (MOCVD) are explained by saturation of traps. An unusual increase in decay rate observed tens of nanoseconds after excitation is probably due to carriers falling out of the trap states. Since this is the first study of time-resolved photoluminescence of MOCVD-grown quantum well structures, this unusual behavior may be realted to the MOCVD growth process. Further investigations indi- cate that the traps are not active at low temperatures; they become active at approximately 150 K. The traps are probably associated with the (hetero)interfaces rather than the bulk Al(,x)Ga(,1-x)As material. The 34 K photoluminescence spectrum of this sample revealed a peak shifted down by approximately 36 meV from the main peak. Time-resolved and time-integrated photoluminescence results here show that this peak is not a stimulated phonon emission sideband, but rather is an due to an acceptor impurity, probably carbon. Photo- luminescence for excitation above and below the barrier bandgap shows that carriers are efficiently collected in the wells in both single and multiple
Optical injection enables coherence resonance in quantum-dot lasers
NASA Astrophysics Data System (ADS)
Ziemann, D.; Aust, R.; Lingnau, B.; Schöll, E.; Lüdge, K.
2013-07-01
We demonstrate that optically injected semiconductor quantum-dot lasers operated in the frequency-locked regime exhibit the counterintuitive effect of coherence resonance, i.e., the regularity of noise-induced spiking is a non-monotonic function of the spontaneous emission noise, and it is optimally correlated at a non-zero value of the noise intensity. We uncover the mechanism of coherence resonance from a microscopically based model of the quantum-dot laser structure, and show that it is related to excitability under optical injection and to a saddle-node infinite period (SNIPER) bifurcation occurring for small injection strength at the border of the frequency locking regime. By a model reduction we argue that the phenomenon of coherence resonance is generic for a wide class of optically injected lasers.
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.
Long-lived quantum coherence in photosynthetic complexes at physiological temperature
Panitchayangkoon, Gitt; Hayes, Dugan; Fransted, Kelly A.; Caram, Justin R.; Harel, Elad; Wen, Jianzhong; Blankenship, Robert E.; Engel, Gregory S.
2010-01-01
Photosynthetic antenna complexes capture and concentrate solar radiation by transferring the excitation to the reaction center that stores energy from the photon in chemical bonds. This process occurs with near-perfect quantum efficiency. Recent experiments at cryogenic temperatures have revealed that coherent energy transfer—a wave-like transfer mechanism—occurs in many photosynthetic pigment-protein complexes. Using the Fenna–Matthews–Olson antenna complex (FMO) as a model system, theoretical studies incorporating both incoherent and coherent transfer as well as thermal dephasing predict that environmentally assisted quantum transfer efficiency peaks near physiological temperature; these studies also show that this mechanism simultaneously improves the robustness of the energy transfer process. This theory requires long-lived quantum coherence at room temperature, which never has been observed in FMO. Here we present evidence that quantum coherence survives in FMO at physiological temperature for at least 300 fs, long enough to impact biological energy transport. These data prove that the wave-like energy transfer process discovered at 77 K is directly relevant to biological function. Microscopically, we attribute this long coherence lifetime to correlated motions within the protein matrix encapsulating the chromophores, and we find that the degree of protection afforded by the protein appears constant between 77 K and 277 K. The protein shapes the energy landscape and mediates an efficient energy transfer despite thermal fluctuations. PMID:20615985
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.
NASA Astrophysics Data System (ADS)
Tian, Si-Cong; Wan, Ren-Gang; Wang, Li-Jie; Shu, Shi-Li; Tong, Cun-Zhu; Wang, Li-Jun
2016-12-01
A scheme is proposed for coherent population transfer and creation of coherent superposition states assisted by one time-dependent tunneling pulse and one time-independent tunneling pulse in triple quantum dots. Time-dependent tunneling, which is similar to the Stokes laser pulse used in traditional stimulated Raman adiabatic passage, can lead to complete population transfer from the ground state to the indirect exciton states. Time-independent tunneling can also create double dark states, resulting in the distribution of the population and arbitrary coherent superposition states. Such a scheme can also be extended to multiple quantum dots assisted by one time-dependent tunneling pulse and more time-independent tunneling pulses.
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
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).
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.
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.
Quantum Coherence between Two Atoms beyond Q=1015
NASA Astrophysics Data System (ADS)
Chou, C. W.; Hume, D. B.; Thorpe, M. J.; Wineland, D. J.; Rosenband, T.
2011-04-01
We place two atoms in quantum superposition states and observe coherent phase evolution for 3.4×1015 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 Al+27 ions with fractional uncertainty 3.7-0.8+1.0×10-16/τ/s. Two measures of the Q factor are reported: The Q factor derived from quantum coherence is 3.4-1.1+2.4×1016, and the spectroscopic Q factor for a Ramsey time of 3 s is 6.7×1015. We demonstrate a method to detect the individual quantum states of two Al+ ions in a Mg+-Al+-Al+ linear ion chain without spatially resolving the ions.
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.
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.
Carrier cooling in colloidal quantum wells.
Pelton, Matthew; Ithurria, Sandrine; Schaller, Richard D; Dolzhnikov, Dmitriy S; Talapin, Dmitri V
2012-12-12
It has recently become possible to chemically synthesize atomically flat semiconductor nanoplatelets with monolayer-precision control over the platelet thickness. It has been suggested that these platelets are quantum wells; that is, carriers in these platelets are confined in one dimension but are free to move in the other two dimensions. Here, we report time-resolved photoluminescence and transient-absorption measurements of carrier relaxation that confirm the quantum-well nature of these nanomaterials. Excitation of the nanoplatelets by an intense laser pulse results in the formation of a high-temperature carrier population that cools back down to ambient temperature on the time scale of several picoseconds. The rapid carrier cooling indicates that the platelets are well-suited for optoelectronic applications such as lasers and modulators.
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.
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.
Microwave Switching in Amorphous-Carbon Quantum Wells
NASA Astrophysics Data System (ADS)
Bhattacharyya, Somnath; Gomez Rojas, Luis; Silva, S. Ravi. P.
2007-03-01
Demonstration of long phase coherence length showing resonant tunnelling and fast switching in amorphous carbon quantum well structures has recently been established [1]. Here we show a bias controlled reversible switching of the complex impedance by transmitting a microwave signal up to 110GHz through amorphous carbon resonant tunnel diodes. By employing a coplanar waveguide technique and through the analysis of the return loss (S11) microwave enhanced mobility greater than 30cm^2(Vs)-1 in the delocalized regime of (filamentary) conduction in these devices is demonstrated. Also a switching behaviour at about 85GHz can also be observed. We suggest a new model for the microscopic origin of the increased mobility and show routes to achieve longer coherence lengths. In addition microwave conductance of carbon quantum wells parallel to their plane and across a channel length larger than 100 nm determines the momentum scattering time of electrons in carbon. These results exhibit a potential for pure amorphous carbon-based fast memory devices. [1] S. Bhattacharyya, S.J. Henley, E. Mendoza, L. Gomez Rojas, J. Allam and S.R.P. Silva, Nature Mater. 5, 19 (2006).
NASA Astrophysics Data System (ADS)
Erenso, Daniel Bekele
We have studied quantum statistical properties including squeezing and entanglement, and conditional measurements in a two level atom, semiconductor quantum well and quantum dots interacting with light and a degenerate parametric oscillator. For a two-level atom in a coherently driven cavity and damped by a broad-band squeezed vacuum, we have studied atomic inversion, fluorescent spectrum, and the intensity correlations of the transmitted and fluorescent photons in the bad cavity limit using the Fokker-Planck equation. For semiconductor quantum well with a single exciton mode in a microcavity driven by squeezed vacuum in the low exciton density regime, we have studied the intensity, spectrum, and intensity correlations for the fluorescent light by solving the quantum Langevin equations. We have also derived an expression for the Q-function of the field and using this function we have studied the intracavity photon number distribution and the quadrature fluctuations for the field. For identical semiconductor quantum dots (QDs) interacting with a quantized cavity field, using quantum mechanical as well as semiclassical treatment, we have investigated the time evolution of the states of the QDs, the photon number distribution and the quadratures variances for the cavity field. Specifically, we consider two QDs initially prepared in a Bell and three QDs in a GHZ entangled excitonic states and the field in coherent, squeezed coherent, squeezed vacuum or thermal states at the initial time. By considering the light from a degenerate parametric oscillator, we have also discussed conditional measurements as probes of quantum dynamics and show that they provide novel ways to characterize quantum fluctuations.
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
Cavity-based architecture to preserve quantum coherence and entanglement.
Man, Zhong-Xiao; Xia, Yun-Jie; Lo Franco, Rosario
2015-09-09
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.
Cavity-based architecture to preserve quantum coherence and entanglement
NASA Astrophysics Data System (ADS)
Man, Zhong-Xiao; Xia, Yun-Jie; Lo Franco, Rosario
2015-09-01
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.
Hot Electron Energy Relaxation in Quantum Wells
NASA Astrophysics Data System (ADS)
Yang, Chia-Hung
We present experimental results on hot electron relaxation in doped bulk GaAs and quantum wells. Using steady state photoluminescence we measured the electron -LO phonon scattering time for thermalized hot electrons in quantum wells. The results are in good agreement with our theoretical calculation of electron-LO phonon interaction in two dimensional systems. Within random phase approximation, the emitted LO phonons may couple to two dimensional plasmons. Both the screening and phonon reabsorption properties can be drastically changed as a function of electron density, temperature and phonon lifetime. Theoretical energy relaxation rates, including dynamical screening and phonon reabsorption effects, will be presented. For hot electrons with energies well above the LO phonon energy, we developed a two-beam, lock-in technique to measure the energy-resolved cooling rate. In the case of quantum wells, hot electrons relax at a constant rate. For heavily doped bulk GaAs, the relaxation rate is inversely proportional to electron kinetic energy. The new method demonstrates itself as a valuable way to study the fast initial relaxation which would otherwise need femtosecond pulse laser techniques.
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.
Amplifying and freezing of quantum coherence using weak measurement and quantum measurement reversal
NASA Astrophysics Data System (ADS)
Yang, Lian-Wu; Xia, Yun-Jie
2016-11-01
We analyze universal conditions where the l 1 norm and relative entropy of coherence are amplified and frozen under identical bit-flip channels; that is, using pre-measurements (quantum weak measurements or quantum measurement reversals) on the systems before undergoing local bit-flip channels. With the option of quantum weak measurements or quantum measurement reversals, the measurement strength and the success probability are all determined by the initial state of the quantum system. Project supported by the National Natural Science Foundation of China (Grant Nos. 11204156, 61178012, 11304179, and 11247240) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20133705110001).
Luminescence Rings in Quantum Well Structures
NASA Astrophysics Data System (ADS)
Denev, S.; Liu, Y.; Snoke, D.; Rapaport, R.; Chen, Gang; Simon, S.; Pfeiffer, L.; West, K.
2004-03-01
An unique ring-shaped luminescence pattern with size of hundreds of microns to millimeters has been observed in GaAs-based quantum well structures excited by weak laser light [1]. The ring persists up to high temperatures, and its size can be manipulated by the laser power, electric field, external stress or magnetic field. We give a detailed description of the effect and discuss various proposed explanations including an optical effect, diffusion and excitonic superfluidity. A realistic explanation based on a simple, nonlinear model of charge separation is proposed, and numerical results from the model are compared to the experiment. This work has been supported by the National Science Foundation under grant No. DMR-0102457 and by the Department of Energy under grant No. DE-FG0299ER45780 [1] D. Snoke, S. Denev, Y. Liu, L. Pfeiffer, and K. West, "Long-range Transport in Excitonic Dark States in Coupled Quantum Wells," Nature 418, 754 (2002).
Excitonic susceptibility in near triangular quantum wells
NASA Astrophysics Data System (ADS)
Anitha, A.; Arulmozhi, M.
2017-03-01
Diamagnetic susceptibility and binding energy of an exciton in a near triangular quantum well, with potential profile proportional to |z|2/3 composed of GaAs/Ga1- x Al x As and ZnO/Zn1- x Mg x O are calculated as a function of the wellwidth and concentration of Al and Mg respectively varying the magnetic field applied along growth direction (i.e. z-axis). Diamagnetic susceptibility of light hole exciton and heavy hole exciton, shows inverse behaviors in the two materials below 20 nm wellwidth and the binding energy of both excitons increases, as the magnetic field increases. The results obtained, are compared with those of quantum wells with varied potential profiles and the experimental results reported in the literature.
Coherence length of photons from a single quantum system
Jelezko, F.; Volkmer, A.; Popa, I.; Wrachtrup, J.; Rebane, K.K.
2003-04-01
We present a methodology that allows recording the coherence length of photons emitted by a single quantum system in a solid. The feasibility of this approach is experimentally demonstrated by measuring the self-interference of photons from the zero-phonon line emission of a single nitrogen-vacancy defect in diamond at 1.6 K. The first-order correlation function has been recorded and analyzed in terms of a single exponential decay time. A coherence time of {approx}5 ps has been obtained, which is in good agreement with the corresponding spectral line width and demonstrates the feasibility of the Fourier-transform spectroscopy with single photons.
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.
Coherent Quantum Control of Multidimensional Vibrational Spectroscopy
2006-05-01
double- sided Feynman diagrams. The results of stochastic models of spectral diffusion where the bath dynamics were independent of the state of the...Liouville space, and the calculation only involves forward propagation in real time and is represented by the double sided Feynman diagrams commonly used for...expressed in terms of multipoint quantum correlation functions and represented by double-sided Feynman diagrams, in close formal analogy with nonlinear
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 effect in HgTe quantum wells
NASA Astrophysics Data System (ADS)
Wang, Jianhui
2016-02-01
We study the possibility of fractional quantum Hall effects in HgTe quantum wells using exact diagonalization. Our results show that Laughlin states, the Moore-Read state, and the Read-Rezayi Z3 state can all be supported. However, near the level crossing point (of the single-particle spectrum) the gap can be destroyed by Landau level mixing, and the Moore-Read state and the Read-Rezayi state dominate over their respective competing states only for wide wells. For smaller well widths the Moore-Read state crosses over to the composite fermion Fermi sea, while the Read-Rezayi state loses its dominance over the hierarchy state.
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.
Average coherence and its typicality for random mixed quantum states
NASA Astrophysics Data System (ADS)
Zhang, Lin
2017-04-01
The Wishart ensemble is a useful and important random matrix model used in diverse fields. By realizing induced random mixed quantum states as a Wishart ensemble with fixed unit trace, using matrix integral technique we give a fast track to the average coherence for random mixed quantum states induced via partial-tracing of the Haar-distributed bipartite pure states. As a direct consequence of this result, we get a compact formula for the average subentropy of random mixed states. These compact formulae extend our previous work.
Quantum Well States in Magnetic Nanostructures
NASA Astrophysics Data System (ADS)
Qiu, Z. Q.
2000-03-01
Quantum Well (QW) states in magnetic nanostructures play an important role in many phenomena such as the oscillatory interlayer coupling in giant magnetoresistance (GMR) multilayers. Photoemission provides the most direct measurement of QW states in k-space. In this talk, I will report our recent results on QW states obtained at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory. The high brightness and fine spot size of photon beam at beamline 7 of ALS allow the performance of photoemission experiment on double wedged samples. First, the nature of QW states in metallic thin films will be discussed. Using one monolayer Ni as a probe, we show that the amplitude of the QW wavefunction is described by an envelope function. Second, quantum interference between two QWs will be discussed. Finally, we demonstrate the interconnection between the QW states and the oscillatory interlayer coupling in magnetic multilayers.
Glauber theory and the quantum coherence of curvature inhomogeneities
NASA Astrophysics Data System (ADS)
Giovannini, Massimo
2017-02-01
The curvature inhomogeneities are systematically scrutinized in the framework of the Glauber approach. The amplified quantum fluctuations of the scalar and tensor modes of the geometry are shown to be first-order coherent while the interference of the corresponding intensities is larger than in the case of Bose–Einstein correlations. After showing that the degree of second-order coherence does not suffice to characterize unambiguously the curvature inhomogeneities, we argue that direct analyses of the degrees of third- and fourth-order coherence are necessary to discriminate between different correlated states and to infer more reliably the statistical properties of the large-scale fluctuations. We speculate that the moments of the multiplicity distributions of the relic phonons might be observationally accessible thanks to new generations of instruments able to count the single photons of the Cosmic Microwave Background in the THz region.
Bosonization, coherent states and semiclassical quantum Hall skyrmions.
Dutta, Sreedhar B; Shankar, R
2008-07-09
We bosonize (2+1)-dimensional fermionic theory using coherent states. The gauge-invariant subspace of boson-Chern-Simons Hilbert space is mapped to fermionic Hilbert space. This subspace is then equipped with a coherent state basis. These coherent states are labelled by a dynamic spinor field. The label manifold could be assigned a physical meaning in terms of density and spin density. A path-integral representation of the evolution operator in terms of these physical variables is given. The corresponding classical theory when restricted to LLL is described by spin fluctuations alone and is found to be the NLSM with Hopf term. The formalism developed here is suitable to study quantum Hall skyrmions semiclassically and/or beyond the hydrodynamic limit. The effects of Landau level mixing or the presence of slowly varying external fields can also be easily incorporated.
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.
Quantum state transfer in double-quantum-well devices
NASA Technical Reports Server (NTRS)
Jakumeit, Jurgen; Tutt, Marcel; Pavlidis, Dimitris
1994-01-01
A Monte Carlo simulation of double-quantum-well (DQW) devices is presented in view of analyzing the quantum state transfer (QST) effect. Different structures, based on the AlGaAs/GaAs system, were simulated at 77 and 300 K and optimized in terms of electron transfer and device speed. The analysis revealed the dominant role of the impurity scattering for the QST. Different approaches were used for the optimization of QST devices and basic physical limitations were found in the electron transfer between the QWs. The maximum transfer of electrons from a high to a low mobility well was at best 20%. Negative differential resistance is hampered by the almost linear rather than threshold dependent relation of electron transfer on electric field. By optimizing the doping profile the operation frequency limit could be extended to 260 GHz.
Quantum coherence selective 2D Raman–2D electronic spectroscopy
Spencer, Austin P.; Hutson, William O.; Harel, Elad
2017-01-01
Electronic and vibrational correlations report on the dynamics and structure of molecular species, yet revealing these correlations experimentally has proved extremely challenging. Here, we demonstrate a method that probes correlations between states within the vibrational and electronic manifold with quantum coherence selectivity. Specifically, we measure a fully coherent four-dimensional spectrum which simultaneously encodes vibrational–vibrational, electronic–vibrational and electronic–electronic interactions. By combining near-impulsive resonant and non-resonant excitation, the desired fifth-order signal of a complex organic molecule in solution is measured free of unwanted lower-order contamination. A critical feature of this method is electronic and vibrational frequency resolution, enabling isolation and assignment of individual quantum coherence pathways. The vibronic structure of the system is then revealed within an otherwise broad and featureless 2D electronic spectrum. This method is suited for studying elusive quantum effects in which electronic transitions strongly couple to phonons and vibrations, such as energy transfer in photosynthetic pigment–protein complexes. PMID:28281541
Quantum coherence selective 2D Raman-2D electronic spectroscopy
NASA Astrophysics Data System (ADS)
Spencer, Austin P.; Hutson, William O.; Harel, Elad
2017-03-01
Electronic and vibrational correlations report on the dynamics and structure of molecular species, yet revealing these correlations experimentally has proved extremely challenging. Here, we demonstrate a method that probes correlations between states within the vibrational and electronic manifold with quantum coherence selectivity. Specifically, we measure a fully coherent four-dimensional spectrum which simultaneously encodes vibrational-vibrational, electronic-vibrational and electronic-electronic interactions. By combining near-impulsive resonant and non-resonant excitation, the desired fifth-order signal of a complex organic molecule in solution is measured free of unwanted lower-order contamination. A critical feature of this method is electronic and vibrational frequency resolution, enabling isolation and assignment of individual quantum coherence pathways. The vibronic structure of the system is then revealed within an otherwise broad and featureless 2D electronic spectrum. This method is suited for studying elusive quantum effects in which electronic transitions strongly couple to phonons and vibrations, such as energy transfer in photosynthetic pigment-protein complexes.
Quantum coherence selective 2D Raman-2D electronic spectroscopy.
Spencer, Austin P; Hutson, William O; Harel, Elad
2017-03-10
Electronic and vibrational correlations report on the dynamics and structure of molecular species, yet revealing these correlations experimentally has proved extremely challenging. Here, we demonstrate a method that probes correlations between states within the vibrational and electronic manifold with quantum coherence selectivity. Specifically, we measure a fully coherent four-dimensional spectrum which simultaneously encodes vibrational-vibrational, electronic-vibrational and electronic-electronic interactions. By combining near-impulsive resonant and non-resonant excitation, the desired fifth-order signal of a complex organic molecule in solution is measured free of unwanted lower-order contamination. A critical feature of this method is electronic and vibrational frequency resolution, enabling isolation and assignment of individual quantum coherence pathways. The vibronic structure of the system is then revealed within an otherwise broad and featureless 2D electronic spectrum. This method is suited for studying elusive quantum effects in which electronic transitions strongly couple to phonons and vibrations, such as energy transfer in photosynthetic pigment-protein complexes.
Quantum-well lasers for direct solar photopumping
NASA Astrophysics Data System (ADS)
Unnikrishnan, Sreenath; Anderson, Neal G.
1993-09-01
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.
Coherent quantum transport features in carbon superlattice structures.
McIntosh, R; Henley, S J; Silva, S R P; Bhattacharyya, S
2016-10-19
Whilst resonant transmission is well understood and can be fully harnessed for crystalline superlattices, a complete picture has not yet emerged for disordered superlattices. It has proven difficult to tune resonant transmission in disordered diamond-like carbon (DLC) superlattices as conventional models are not equipped to incorporate significant structural disorder. In this work, we present concurrent experimental and theoretical analysis which addresses resonant transmission in DLC superlattices. Devices were fabricated by growing alternate layers of DLC with different percentages of sp(3) hybridized carbon.Coherent quantum transport effects were demonstrated in these structurally disordered DLC superlattices through distinct current modulation with negative differential resistance (NDR) in the current-voltage (I-V) measurements. A model was developed using tight-binding calculations assuming a random variation of the hopping integral to simulate structural (bond-length) disorder. Calculations of the I-V characteristics compliment the interpretation of the measurements and illustrate that while DLC superlattice structures are unlike their classical counterparts, the near-field structural order will help with the confinement of quantised states. The present model provides an empirical guide for tailoring the properties of future devices, giving rise to much hope that carbon electronics operating at high frequencies over large areas can now be developed.
Coherent quantum transport features in carbon superlattice structures
McIntosh, R.; Henley, S. J.; Silva, S. R. P.; Bhattacharyya, S.
2016-01-01
Whilst resonant transmission is well understood and can be fully harnessed for crystalline superlattices, a complete picture has not yet emerged for disordered superlattices. It has proven difficult to tune resonant transmission in disordered diamond-like carbon (DLC) superlattices as conventional models are not equipped to incorporate significant structural disorder. In this work, we present concurrent experimental and theoretical analysis which addresses resonant transmission in DLC superlattices. Devices were fabricated by growing alternate layers of DLC with different percentages of sp3 hybridized carbon.Coherent quantum transport effects were demonstrated in these structurally disordered DLC superlattices through distinct current modulation with negative differential resistance (NDR) in the current-voltage (I-V) measurements. A model was developed using tight-binding calculations assuming a random variation of the hopping integral to simulate structural (bond-length) disorder. Calculations of the I-V characteristics compliment the interpretation of the measurements and illustrate that while DLC superlattice structures are unlike their classical counterparts, the near-field structural order will help with the confinement of quantised states. The present model provides an empirical guide for tailoring the properties of future devices, giving rise to much hope that carbon electronics operating at high frequencies over large areas can now be developed. PMID:27759047
Coherent quantum transport features in carbon superlattice structures
NASA Astrophysics Data System (ADS)
McIntosh, R.; Henley, S. J.; Silva, S. R. P.; Bhattacharyya, S.
2016-10-01
Whilst resonant transmission is well understood and can be fully harnessed for crystalline superlattices, a complete picture has not yet emerged for disordered superlattices. It has proven difficult to tune resonant transmission in disordered diamond-like carbon (DLC) superlattices as conventional models are not equipped to incorporate significant structural disorder. In this work, we present concurrent experimental and theoretical analysis which addresses resonant transmission in DLC superlattices. Devices were fabricated by growing alternate layers of DLC with different percentages of sp3 hybridized carbon.Coherent quantum transport effects were demonstrated in these structurally disordered DLC superlattices through distinct current modulation with negative differential resistance (NDR) in the current-voltage (I-V) measurements. A model was developed using tight-binding calculations assuming a random variation of the hopping integral to simulate structural (bond-length) disorder. Calculations of the I-V characteristics compliment the interpretation of the measurements and illustrate that while DLC superlattice structures are unlike their classical counterparts, the near-field structural order will help with the confinement of quantised states. The present model provides an empirical guide for tailoring the properties of future devices, giving rise to much hope that carbon electronics operating at high frequencies over large areas can now be developed.
Sadeghi, S M
2014-09-01
When a hybrid system consisting of a semiconductor quantum dot and a metallic nanoparticle interacts with a laser field, the plasmonic field of the metallic nanoparticle can be normalized by the quantum coherence generated in the quantum dot. In this Letter, we study the states of polarization of such a coherent-plasmonic field and demonstrate how these states can reveal unique aspects of the collective molecular properties of the hybrid system formed via coherent exciton-plasmon coupling. We show that transition between the molecular states of this system can lead to ultrafast polarization dynamics, including sudden reversal of the sense of variations of the plasmonic field and formation of circular and elliptical polarization.
Quantum Well Infrared Photodetectors (QWIPs) for Astronomy
NASA Technical Reports Server (NTRS)
Gunapala, S. D.; Bandara, S. V.; Liu, J. K.; Luong, E.; Bock, J. J.; Ressler, M. E.; Werner, M. W.
1998-01-01
In recent years, many research groups in the world have demonstrated large format Quantum Well Infrared Photodetector (QWIP) focal plane arrays for various thermal imaging applications. QWIPs as opposed to conventional low bandgap infrared detectors, are limited by thermionic dark current and not tunneling currents down to 30K or less. As a result the performance of QWIPs can be substantially improved (orders of magnitude) by cooling from 70K to 30K. Cooling does not induce any nonuniformity or 1/f noise in QWIP focal plane arrays. In this paper, we discuss the development of highly uniform long- wavelength QWIPs for astronomical applications.
Can quantum coherent solar cells break detailed balance?
Kirk, Alexander P.
2015-07-21
Carefully engineered coherent quantum states have been proposed as a design attribute that is hypothesized to enable solar photovoltaic cells to break the detailed balance (or radiative) limit of power conversion efficiency by possibly causing radiative recombination to be suppressed. However, in full compliance with the principles of statistical mechanics and the laws of thermodynamics, specially prepared coherent quantum states do not allow a solar photovoltaic cell—a quantum threshold energy conversion device—to exceed the detailed balance limit of power conversion efficiency. At the condition given by steady-state open circuit operation with zero nonradiative recombination, the photon absorption rate (or carrier photogeneration rate) must balance the photon emission rate (or carrier radiative recombination rate) thus ensuring that detailed balance prevails. Quantum state transitions, entropy-generating hot carrier relaxation, and photon absorption and emission rate balancing are employed holistically and self-consistently along with calculations of current density, voltage, and power conversion efficiency to explain why detailed balance may not be violated in solar photovoltaic cells.
Quantum Coherence and Random Fields at Mesoscopic Scales
Rosenbaum, Thomas F.
2016-03-01
We seek to explore and exploit model, disordered and geometrically frustrated magnets where coherent spin clusters stably detach themselves from their surroundings, leading to extreme sensitivity to finite frequency excitations and the ability to encode information. Global changes in either the spin concentration or the quantum tunneling probability via the application of an external magnetic field can tune the relative weights of quantum entanglement and random field effects on the mesoscopic scale. These same parameters can be harnessed to manipulate domain wall dynamics in the ferromagnetic state, with technological possibilities for magnetic information storage. Finally, extensions from quantum ferromagnets to antiferromagnets promise new insights into the physics of quantum fluctuations and effective dimensional reduction. A combination of ac susceptometry, dc magnetometry, noise measurements, hole burning, non-linear Fano experiments, and neutron diffraction as functions of temperature, magnetic field, frequency, excitation amplitude, dipole concentration, and disorder address issues of stability, overlap, coherence, and control. We have been especially interested in probing the evolution of the local order in the progression from spin liquid to spin glass to long-range-ordered magnet.
Quantum coherence in Mn-based single molecule magnets
NASA Astrophysics Data System (ADS)
Abeywardana, C.; Cho, F. H.; Mowson, A.; Christou, G.; Takahashi, S.
2015-03-01
As spin systems in solids, single-molecule magnets (SMMs) form a unique class of materials that have a high-spin, and their spin state and interaction can be easily tuned by changing peripheral organic ligands and solvate molecules. In addition, it has been shown that an individual or a small ensemble of SMMs can be transferred to surface with retention of their magnetic behavior. SMM is therefore a promising system for fundamental quantum science and for applications to dense and efficient quantum memory, computing, and molecular spintronics devices. In spite of diverse interests on quantum properties in SMMs, decoherence properties that ultimately limit such behaviors have not been understood yet. Until now, coherent manipulation of spin states in SMMs has been experimentally demonstrated only in a few SMMs. In this presentation, we investigate quantum coherence in Mn-based SMMs using a high-frequency pulsed EPR technique, which has a significant advantage to quench the spin decoherence due to electron spins.
Observation of Time-Invariant Coherence in a Nuclear Magnetic Resonance Quantum Simulator.
Silva, Isabela A; Souza, Alexandre M; Bromley, Thomas R; Cianciaruso, Marco; Marx, Raimund; Sarthour, Roberto S; Oliveira, Ivan S; Lo Franco, Rosario; Glaser, Steffen J; deAzevedo, Eduardo R; Soares-Pinto, Diogo O; Adesso, Gerardo
2016-10-14
The ability to live in coherent superpositions is a signature trait of quantum systems and constitutes an irreplaceable resource for quantum-enhanced technologies. However, decoherence effects usually destroy quantum superpositions. It was recently predicted that, in a composite quantum system exposed to dephasing noise, quantum coherence in a transversal reference basis can stay protected for an indefinite time. This can occur for a class of quantum states independently of the measure used to quantify coherence, and it requires no control on the system during the dynamics. Here, such an invariant coherence phenomenon is observed experimentally in two different setups based on nuclear magnetic resonance at room temperature, realizing an effective quantum simulator of two- and four-qubit spin systems. Our study further reveals a novel interplay between coherence and various forms of correlations, and it highlights the natural resilience of quantum effects in complex systems.
Observation of Time-Invariant Coherence in a Nuclear Magnetic Resonance Quantum Simulator
NASA Astrophysics Data System (ADS)
Silva, Isabela A.; Souza, Alexandre M.; Bromley, Thomas R.; Cianciaruso, Marco; Marx, Raimund; Sarthour, Roberto S.; Oliveira, Ivan S.; Lo Franco, Rosario; Glaser, Steffen J.; deAzevedo, Eduardo R.; Soares-Pinto, Diogo O.; Adesso, Gerardo
2016-10-01
The ability to live in coherent superpositions is a signature trait of quantum systems and constitutes an irreplaceable resource for quantum-enhanced technologies. However, decoherence effects usually destroy quantum superpositions. It was recently predicted that, in a composite quantum system exposed to dephasing noise, quantum coherence in a transversal reference basis can stay protected for an indefinite time. This can occur for a class of quantum states independently of the measure used to quantify coherence, and it requires no control on the system during the dynamics. Here, such an invariant coherence phenomenon is observed experimentally in two different setups based on nuclear magnetic resonance at room temperature, realizing an effective quantum simulator of two- and four-qubit spin systems. Our study further reveals a novel interplay between coherence and various forms of correlations, and it highlights the natural resilience of quantum effects in complex systems.
Coherence-Driven Topological Transition in Quantum Metamaterials.
Jha, Pankaj K; Mrejen, Michael; Kim, Jeongmin; Wu, Chihhui; Wang, Yuan; Rostovtsev, Yuri V; Zhang, Xiang
2016-04-22
We introduce and theoretically demonstrate a quantum metamaterial made of dense ultracold neutral atoms loaded into an inherently defect-free artificial crystal of light, immune to well-known critical challenges inevitable in conventional solid-state platforms. We demonstrate an all-optical control, on ultrafast time scales, over the photonic topological transition of the isofrequency contour from an open to closed topology at the same frequency. This atomic lattice quantum metamaterial enables a dynamic manipulation of the decay rate branching ratio of a probe quantum emitter by more than an order of magnitude. Our proposal may lead to practically lossless, tunable, and topologically reconfigurable quantum metamaterials, for single or few-photon-level applications as varied as quantum sensing, quantum information processing, and quantum simulations using metamaterials.
NASA Astrophysics Data System (ADS)
Castro, E.; Gómez, R.; Ladera, C. L.; Zambrano, A.
2013-11-01
Among many applications quantum weak measurements have been shown to be important in exploring fundamental physics issues, such as the experimental violation of the Heisenberg uncertainty relation and the Hardy paradox, and have also technological implications in quantum optics, quantum metrology and quantum communications, where the precision of the measurement is as important as the precision of quantum state preparation. The theory of weak measurement can be formulated using the pre-and post-selected quantum systems, as well as using the weak measurement operator formalism. In this work, we study the quantum discord (QD) of quasi-Werner mixed states based on bipartite entangled coherent states using the weak measurements operator, instead of the projective measurement operators. We then compare the quantum discord for both kinds of measurement operators, in terms of the entanglement quality, the latter being measured using the concept of concurrence. It's found greater quantum correlations using the weak measurement operators.
NASA Astrophysics Data System (ADS)
Daoud, M.; Ahl Laamara, R.
2012-07-01
We give the explicit expressions of the pairwise quantum correlations present in superpositions of multipartite coherent states. A special attention is devoted to the evaluation of the geometric quantum discord. The dynamics of quantum correlations under a dephasing channel is analyzed. A comparison of geometric measure of quantum discord with that of concurrence shows that quantum discord in multipartite coherent states is more resilient to dissipative environments than is quantum entanglement. To illustrate our results, we consider some special superpositions of Weyl-Heisenberg, SU(2) and SU(1,1) coherent states which interpolate between Werner and Greenberger-Horne-Zeilinger states.
Bose-Einstein condensation of dipolar excitons in quantum wells
NASA Astrophysics Data System (ADS)
Timofeev, V. B.; Gorbunov, A. V.
2009-02-01
The experiments on Bose-Einstein condensation (BEC) of dipolar (spatially-indirect) excitons in the lateral traps in GaAs/AlGaAs Schottky-diode heterostructures with double and single quantum wells are presented. The condensed part of dipolar excitons under detection in the far zone is placed in k-space in the range which is almost two orders of magnitude less than thermal exciton wave vector. BEC occurs spontaneously in a reservoir of thermalized excitons. Luminescence images of Bose-condensate of dipolar excitons exhibit along perimeter of circular trap axially symmetrical spatial structures of equidistant bright spots which strongly depend on excitation power and temperature. By means of two-beam interference experiments with the use of cw and pulsed photoexcitation it was found that the state of dipolar exciton Bose-condensate is spatially coherent and the whole patterned luminescence configuration in real space is described by a common wave function.
Zhang, Zhedong; Wang, Jin
2015-04-02
Recently, the quantum nature in the energy transport in solar cells and light-harvesting complexes has attracted much attention as being triggered by the experimental observations. We model the light-harvesting complex (i.e., PEB50 dimer) as a quantum heat engine (QHE) and study the effect of the undamped intramolecule vibrational modes on the coherent energy-transfer process and quantum transport. We find that the exciton-vibration interaction has nontrivial contribution to the promotion of quantum yield as well as transport properties of the QHE at steady state by enhancing the quantum coherence quantified by entanglement entropy. The perfect quantum yield over 90% has been obtained, with the exciton-vibration coupling. We attribute these improvements to the renormalization of the electronic couplings effectively induced by exciton-vibration interaction and the subsequent delocalization of excitons. Finally, we demonstrate that the thermal relaxation and dephasing can help the excitation energy transfer in the PEB50 dimer.
Manipulating coherence resonance in a quantum dot semiconductor laser via electrical pumping.
Otto, Christian; Lingnau, Benjamin; Schöll, Eckehard; Lüdge, Kathy
2014-06-02
Excitability and coherence resonance are studied in a semiconductor quantum dot laser under short optical self-feedback. For low pump levels, these are observed close to a homoclinic bifurcation, which is in correspondence with earlier observations in quantum well lasers. However, for high pump levels, we find excitability close to a boundary crisis of a chaotic attractor. We demonstrate that in contrast to the homoclinic bifurcation the crisis and thus the excitable regime is highly sensitive to the pump current. The excitability threshold increases with the pump current, which permits to adjust the sensitivity of the excitable unit to noise as well as to shift the optimal noise strength, at which maximum coherence is observed. The shift adds up to more than one order of magnitude, which strongly facilitates experimental realizations.
NASA Astrophysics Data System (ADS)
Li, Jun; Lu, Dawei; Luo, Zhihuang; Laflamme, Raymond; Peng, Xinhua; Du, Jiangfeng
2016-07-01
Precisely characterizing and controlling realistic quantum systems under noises is a challenging frontier in quantum sciences and technologies. In developing reliable controls for open quantum systems, one is often confronted with the problem of the lack of knowledge on the system controllability. The purpose of this paper is to give a numerical approach to this problem, that is, to approximately compute the reachable set of states for coherently controlled quantum Markovian systems. The approximation consists of setting both upper and lower bounds for system's reachable region of states. Furthermore, we apply our reachability analysis to the control of the relaxation dynamics of a two-qubit nuclear magnetic resonance spin system. We implement some experimental tasks of quantum state engineering in this open system at a near optimal performance in view of purity: e.g., increasing polarization and preparing pseudopure states. These results demonstrate the usefulness of our theory and show interesting and promising applications of environment-assisted quantum dynamics.
Corrugated Quantum Well Infrared Photodetectors and Arrays
NASA Technical Reports Server (NTRS)
Choi, K. K.; Chen, C. J.; Rohkinson, L. P.; Das, N. C.; Jhabvala, M.
1999-01-01
Quantum well infrared photodetectors (QWIPs) have many advantages in infrared detection, mainly due to the mature Ill-V material technology. The employment of the corrugation structure further advances the technology by providing a simple, yet efficient light-coupling scheme. A C-QWIP enjoys the same flexibility as a detector with intrinsic normal incident absorption. In this paper, we will discuss the utilities of C-QWIPs in different applications, including two-color detection and polarization-sensitive detection. Besides practical applications, C-QWIPs are also useful in detector characterization. They can be used for measuring the absorption coefficient of light propagating parallel to the layers under bias and providing information on the energy resolved photoconductive gain. These two quantities have never been measured before. Based on the corrugation design, we have made several modifications that further improve the detector sensitivity without increasing its complexity. Other than the C-QWIP structure, we also continue searching for other sensitive detector architectures. In a quantum grid infrared photodetector, 3-dimensional electron confinement can be achieved, with which the detector is able to absorb light in all directions. At the same time, the photoconductive gain can also be improved. We further improve the design using a blazed structure. All the experimental results are supported by a rigorous electromagnetic modal transmission-line theory developed especially for these types of structures. Preliminary thermal imaging using C-QWIP FPAs validates the advantages of the present approach.
NASA Astrophysics Data System (ADS)
Hatef, Ali; Sadeghi, S. M.; Singh, Mahi R.
2012-05-01
It is known that surface-plasmon resonances of metallic nanoparticles can significantly enhance the field experienced by semiconductor quantum dots. In this paper we show that, when quantum dots are in the vicinity of metallic nanoparticles and interact with coherent light sources (laser fields), coherent exciton-plasmon coupling (quantum coherence effects) can increase the amount of the plasmonic field enhancement significantly. We also study how the coherent molecular resonances generated by such a coupling process are influenced by the self-renormalization of the plasmonic fields and the structural parameters of the systems, particularly the size and shape of the metallic nanoparticle. The renormalization process happens via mutual impacts of the radiative decay rate of excitons and the coherent exciton-plasmon coupling on each other. Our results highlight the conditions where the molecular resonances become very sharp, offering optical switching processes with high extinction ratio and wide ranging device applications.
Survival of coherence for open quantum systems in thermal baths
NASA Astrophysics Data System (ADS)
Giraldi, Filippo; Petruccione, Francesco
2013-10-01
The loss of coherence in a general open quantum system interacting with a bosonic environment is analyzed. The reservoir is initially in a thermal state. The reduced dynamics is described by a non-Markovian time-local master equation. We consider spectral densities that are sub- or super-Ohmic at low frequencies and arbitrarily shaped at high frequencies. In the super-Ohmic regime, for noninteger frequency powers larger than 2, long time survival of coherence appears. In the latter regime, at vanishing temperature, the asymptotic amount of surviving coherence is stabilized to its initial value, up to a phase factor, by properly increasing the bandwidth and decreasing the low-frequency profile of the spectral density. For noninteger positive frequency powers less than 2, stretched exponential-like decoherence is found over long times. The relaxations to the asymptotic configurations become arbitrarily slow by approaching the frequency power 2 of the super-Ohmic regime. The same dependence on temperature, spectral density, and scale frequency appears for purity and concurrence of two qubits and coherence of a qubit.
Spin-orbit interaction in multiple quantum wells
Hao, Ya-Fei
2015-01-07
In this paper, we investigate how the structure of multiple quantum wells affects spin-orbit interactions. To increase the interface-related Rashba spin splitting and the strength of the interface-related Rashba spin-orbit interaction, we designed three kinds of multiple quantum wells. We demonstrate that the structure of the multiple quantum wells strongly affected the interface-related Rashba spin-orbit interaction, increasing the interface-related Rashba spin splitting to up to 26% larger in multiple quantum wells than in a stepped quantum well. We also show that the cubic Dresselhaus spin-orbit interaction similarly influenced the spin relaxation time of multiple quantum wells and that of a stepped quantum well. The increase in the interface-related Rashba spin splitting originates from the relationship between interface-related Rashba spin splitting and electron probability density. Our results suggest that multiple quantum wells can be good candidates for spintronic devices.
Coexistence of coherent and incoherent tunneling in asymmetric double-well potentials
Carjan, N.; Grigorescu, M.; Strottman, D.
1995-12-01
Double-well potentials are widely used to model phenomena in physics and chemistry. The system is assumed to be formed in a metastable state, its dynamical evolution providing the clues for the interpretation of the experimental data. Quantum mechanics predicts coherent oscillations of probability between wells if the double-well potential is nearly symmetric and irreversible exponential decay if the final well has an infinite width. For very asymmetric double-well potentials, these two extreme behaviors are expected to coexist. The purpose of the present paper is to investigate this coexistence and its evolution as a function of the width of (or density of states in) the second well. In this sense, increasing the density of states can be regarded as a mechanism for coherence breakdown. The dynamical evolution of the metastable state can be simulated by solving numerically the time-dependent Schroedinger equation (TDSE). This approach is general, intuitive, and gives access to time scales and dynamical effects. It also allows the inclusion of phenomenological dissipation.
Coherence properties and quantum state transportation in an optical conveyor belt.
Kuhr, S; Alt, W; Schrader, D; Dotsenko, I; Miroshnychenko, Y; Rosenfeld, W; Khudaverdyan, M; Gomer, V; Rauschenbeutel, A; Meschede, D
2003-11-21
We have prepared and detected quantum coherences of trapped cesium atoms with long dephasing times. Controlled transport by an "optical conveyor belt" over macroscopic distances preserves the atomic coherence with slight reduction of coherence time. The limiting dephasing effects are experimentally identified, and we present an analytical model of the reversible and irreversible dephasing mechanisms. Our experimental methods are applicable at the single-atom level. Coherent quantum bit operations along with quantum state transport open the route towards a "quantum shift register" of individual neutral atoms.
Conversion of type of quantum well structure
NASA Technical Reports Server (NTRS)
Ning, Cun-Zheng (Inventor)
2007-01-01
A method for converting a Type 2 quantum well semiconductor material to a Type 1 material. A second layer of undoped material is placed between first and third layers of selectively doped material, which are separated from the second layer by undoped layers having small widths. Doping profiles are chosen so that a first electrical potential increment across a first layer-second layer interface is equal to a first selected value and/or a second electrical potential increment across a second layer-third layer interface is equal to a second selected value. The semiconductor structure thus produced is useful as a laser material and as an incident light detector material in various wavelength regions, such as a mid-infrared region.
Conversion of Type of Quantum Well Structure
NASA Technical Reports Server (NTRS)
Ning, Cun-Zheng (Inventor)
2007-01-01
A method for converting a Type 2 quantum well semiconductor material to a Type 1 material. A second layer of undoped material is placed between first and third layers of selectively doped material, which are separated from the second layer by undoped layers having small widths. Doping profiles are chosen so that a first electrical potential increment across a first layer-second layer interface is equal to a first selected value and/or a second electrical potential increment across a second layer-third layer interface is equal to a second selected value. The semiconductor structure thus produced is useful as a laser material and as an incident light detector material in various wavelength regions, such as a mid-infrared region.
Magnetic quantum coherence effect in Ni4 molecular transistors.
González, Gabriel; Leuenberger, Michael N
2014-07-09
We present a theoretical study of electron transport in Ni4 molecular transistors in the presence of Zeeman spin splitting and magnetic quantum coherence (MQC). The Zeeman interaction is extended along the leads which produces gaps in the energy spectrum which allow electron transport with spin polarized along a certain direction. We show that the coherent states in resonance with the spin up or down states in the leads induces an effective coupling between localized spin states and continuum spin states in the single molecule magnet and leads, respectively. We investigate the conductance at zero temperature as a function of the applied bias and magnetic field by means of the Landauer formula, and show that the MQC is responsible for the appearence of resonances. Accordingly, we name them MQC resonances.
NASA Astrophysics Data System (ADS)
Shen, Jian Qi; Zeng, Rui Xi
2017-02-01
Quantum-dot-molecular phase coherence (and the relevant quantum-interference-switchable optical response) can be utilized to control electromagnetic wave propagation via a gate voltage, since quantum-dot molecules can exhibit an effect of quantum coherence (phase coherence) when quantum-dot-molecular discrete multilevel transitions are driven by an electromagnetic wave. Interdot tunneling of carriers (electrons and holes) controlled by the gate voltage can lead to destructive quantum interference in a quantum-dot molecule that is coupled to an incident electromagnetic wave, and gives rise to a quantum coherence effect (e.g., electromagnetically induced transparency, EIT) in a quantum-dot-molecule dielectric film. The tunable on- and off-resonance tunneling effect of an incident electromagnetic wave (probe field) through such a quantum-coherent quantum-dot-molecule dielectric film is investigated. It is found that a high gate voltage can lead to the EIT phenomenon of the quantum-dot-molecular systems. Under the condition of on-resonance light tunneling through the present quantum-dot-molecule dielectric film, the probe field should propagate without loss if the probe frequency detuning is zero. Such an effect caused by both EIT and resonant tunneling, which is sensitive to the gate voltage, can be utilized for designing devices such as photonic switching, transistors, and logic gates.
NASA Astrophysics Data System (ADS)
Kato, Kentaro; Hirota, Osamu
2005-08-01
The quadrature amplitude modulation (QAM) signal of coherent state of light is applied to the quantum stream cipher by Y-00 protocol. We first discuss on the performance of the square-root measurement (SRM) for the QAM signals in comparison with the optimum receiver. It is shown that the quantum stream cipher with the QAM signals is designed by using the SRM, taking account of the ciphertext-only attack and the known/chosen plain attack. Furthermore, the modification of the quantum stream cipher with the QAM signals is considered.
Requirement of optical coherence for continuous-variable quantum teleportation.
Rudolph, T; Sanders, B C
2001-08-13
We show that the sender and the receiver each require coherent devices in order to achieve unconditional continuous variable quantum teleportation (CVQT), and this requirement cannot be achieved with conventional laser sources, linear optics, ideal photon detectors, and perfect Fock state sources. The appearance of successful CVQT in recent experiments is due to interpreting the measurement record fallaciously in terms of one preferred ensemble (or decomposition) of the correct density matrix describing the state. Our analysis is unrelated to technical problems such as laser phase drift or finite squeezing bandwidth.
Coherent long-range thermoelectrics in nonadiabatic driven quantum systems
NASA Astrophysics Data System (ADS)
Gallego-Marcos, F.; Platero, G.
2017-02-01
We investigate direct energy and heat transfer between two distant sites of a triple quantum dot connected to reservoirs, where one of the edge dots is driven by an ac-gate voltage. We theoretically propose how to implement heat and cooling engines mediated by long-range photoassisted transport. Additionally, we propose a simple setup to heat up coherently the two reservoirs symmetrically and a mechanism to store energy in the closed system. The present proposals can be experimentally implemented and easily controlled by tuning the external parameters.
NASA Astrophysics Data System (ADS)
Sadeghi, S. M.
2012-11-01
We study quantum coherence effects in single nanoparticle systems consisting of a semiconductor quantum dot and a metallic nanoparticle in the presence of the ultra-short dephasing times of the quantum dots. The results suggest that coherent exciton-plasmon coupling can sustain the collective molecular resonances (plasmonic meta-resonances) of these systems at about room temperature. We investigate quantum optical properties of the quantum dots under this condition, demonstrating formation of ultranarrow gain and absorption spectral lines. These results are discussed in terms of plasmonic normalization of coherent population oscillation and the collective states of the nanoparticle systems.
Quantum coherence in momentum space of light-matter condensates
NASA Astrophysics Data System (ADS)
Antón, C.; Tosi, G.; Martín, M. D.; Hatzopoulos, Z.; Konstantinidis, G.; Eldridge, P. S.; Savvidis, P. G.; Tejedor, C.; Viña, L.
2014-08-01
We show that the use of momentum-space optical interferometry, which avoids any spatial overlap between two parts of a macroscopic quantum state, presents a unique way to study coherence phenomena in polariton condensates. In this way, we address the longstanding question in quantum mechanics: "Do two components of a condensate, which have never seen each other, possess a definitive phase?" [P. W. Anderson, Basic Notions of Condensed Matter Physics (Benjamin Cummings, Menlo Park, CA, 1984)]. A positive answer to this question is experimentally obtained here for light-matter condensates, created under precise symmetry conditions, in semiconductor microcavities, taking advantage of the direct relation between the angle of emission and the in-plane momentum of polaritons.
QAM quantum stream cipher using digital coherent optical transmission.
Nakazawa, Masataka; Yoshida, Masato; Hirooka, Toshihiko; Kasai, Keisuke
2014-02-24
A Quantum Stream Cipher (QSC) using Quadrature Amplitude Modulation (QAM) is presented to greatly increase the secure degree compared with ASK or PSK/QSC. We propose encoding multi-bit data in one symbol with a multi-bit basis state, resulting in QAM/QSC, which employs amplitude and phase encryption of the light beam simultaneously. A 16 QAM/QSC experiment at 10 Gbit/s was successfully carried out over 160 km using a digital coherent optical transmission technique, where 16 QAM data were encrypted in a constellation with 32 × 32~4096 × 4096 symbols. We show experimentally that the Number of Masked Signals (NMS) in the quantum noise Γ(QAM) for QAM/QSC becomes a square multiple larger than Γ(ASK) for ASK/QSC. Γ(QAM) exceeds 10,000. This result indicates that the QSC technique is more robust against eavesdroppers than ASK or PSK/QSC.
Quantum Optics Theory of Electronic Noise in Coherent Conductors.
Grimsmo, Arne L; Qassemi, Farzad; Reulet, Bertrand; Blais, Alexandre
2016-01-29
We consider the electromagnetic field generated by a coherent conductor in which electron transport is described quantum mechanically. We obtain an input-output relation linking the quantum current in the conductor to the measured electromagnetic field. This allows us to compute the outcome of measurements on the field in terms of the statistical properties of the current. We moreover show how under ac bias the conductor acts as a tunable medium for the field, allowing for the generation of single- and two-mode squeezing through fermionic reservoir engineering. These results explain the recently observed squeezing using normal tunnel junctions [G. Gasse et al., Phys. Rev. Lett. 111, 136601 (2013); J.-C. Forgues et al., Phys. Rev. Lett. 114, 130403 (2015)].
Quantum transport in a two-level quantum dot driven by coherent and stochastic fields
NASA Astrophysics Data System (ADS)
Ke, Sha-Sha; Miao, Ling-E.; Guo, Zhen; Guo, Yong; Zhang, Huai-Wu; Lü, Hai-Feng
2016-12-01
We study theoretically the current and shot noise properties flowing through a two-level quantum dot driven by a strong coherent field and a weak stochastic field. The interaction x(t) between the quantum dot and the stochastic field is assumed to be a Gaussian-Markovian random process with zero mean value and correlation function < x (t) x (t ‧) > = Dκe - κ | t - t ‧ | , where D and κ are the strength and bandwidth of the stochastic field, respectively. It is found that the stochastic field could enhance the resonant effect between the quantum dot and the coherent field, and generate new resonant points. At the resonant points, the state population difference between two levels is suppressed and the current is considerably enhanced. The zero-frequency shot noise of the current varies dramatically between sub- and super-Poissonian characteristics by tuning the stochastic field appropriately.
Quantum cryptography using coherent states: Randomized encryption and key generation
NASA Astrophysics Data System (ADS)
Corndorf, Eric
objectives of key generation and direct data-encryption, a new quantum cryptographic principle is demonstrated wherein keyed coherent-state signal sets are employed. Taking advantage of the fundamental and irreducible quantum-measurement noise of coherent states, these schemes do not require the users to measure the influence of an attacker. Experimental key-generation and data encryption schemes based on these techniques, which are compatible with today's WDM fiber-optic telecommunications infrastructure, are implemented and analyzed.
Bonderson, Parsa; Lutchyn, Roman M
2011-04-01
We propose computing bus devices that enable quantum information to be coherently transferred between topological and conventional qubits. We describe a concrete realization of such a topological quantum bus acting between a topological qubit in a Majorana wire network and a conventional semiconductor double quantum dot qubit. Specifically, this device measures the joint (fermion) parity of these two different qubits by using the Aharonov-Casher effect in conjunction with an ancilliary superconducting flux qubit that facilitates the measurement. Such a parity measurement, together with the ability to apply Hadamard gates to the two qubits, allows one to produce states in which the topological and conventional qubits are maximally entangled and to teleport quantum states between the topological and conventional quantum systems.
NASA Astrophysics Data System (ADS)
Bonderson, Parsa; Lutchyn, Roman M.
2011-04-01
We propose computing bus devices that enable quantum information to be coherently transferred between topological and conventional qubits. We describe a concrete realization of such a topological quantum bus acting between a topological qubit in a Majorana wire network and a conventional semiconductor double quantum dot qubit. Specifically, this device measures the joint (fermion) parity of these two different qubits by using the Aharonov-Casher effect in conjunction with an ancilliary superconducting flux qubit that facilitates the measurement. Such a parity measurement, together with the ability to apply Hadamard gates to the two qubits, allows one to produce states in which the topological and conventional qubits are maximally entangled and to teleport quantum states between the topological and conventional quantum systems.
Environment-induced quantum coherence spreading of a qubit
NASA Astrophysics Data System (ADS)
Pozzobom, Mauro B.; Maziero, Jonas
2017-02-01
We make a thorough study of the spreading of quantum coherence (QC), as quantified by the l1-norm QC, when a qubit (a two-level quantum system) is subjected to noise quantum channels commonly appearing in quantum information science. We notice that QC is generally not conserved and that even incoherent initial states can lead to transitory system-environment QC. We show that for the amplitude damping channel the evolved total QC can be written as the sum of local and non-local parts, with the last one being equal to entanglement. On the other hand, for the phase damping channel (PDC) entanglement does not account for all non-local QC, with the gap between them depending on time and also on the qubit's initial state. Besides these issues, the possibility and conditions for time invariance of QC are regarded in the case of bit, phase, and bit-phase flip channels. Here we reveal the qualitative dynamical inequivalence between these channels and the PDC and show that the creation of system-environment entanglement does not necessarily imply the destruction of the qubit's QC. We also investigate the resources needed for non-local QC creation, showing that while the PDC requires initial coherence of the qubit, for some other channels non-zero population of the excited state (i.e., energy) is sufficient. Related to that, considering the depolarizing channel we notice the qubit's ability to act as a catalyst for the creation of joint QC and entanglement, without need for nonzero initial QC or excited state population.
Chitambar, Eric; Gour, Gilad
2016-07-15
Considerable work has recently been directed toward developing resource theories of quantum coherence. In this Letter, we establish a criterion of physical consistency for any resource theory. This criterion requires that all free operations in a given resource theory be implementable by a unitary evolution and projective measurement that are both free operations in an extended resource theory. We show that all currently proposed basis-dependent theories of coherence fail to satisfy this criterion. We further characterize the physically consistent resource theory of coherence and find its operational power to be quite limited. After relaxing the condition of physical consistency, we introduce the class of dephasing-covariant incoherent operations as a natural generalization of the physically consistent operations. Necessary and sufficient conditions are derived for the convertibility of qubit states using dephasing-covariant operations, and we show that these conditions also hold for other well-known classes of incoherent operations.
Zero Quantum Coherence in a Series of Covalent Spin-Correlated Radical Pairs.
Nelson, Jordan N; Krzyaniak, Matthew D; Horwitz, Noah E; Rugg, Brandon K; Phelan, Brian T; Wasielewski, Michael R
2017-03-14
Photoinitiated subnanosecond electron transfer within covalently linked electron donor-acceptor molecules can result in the formation of a spin-correlated radical pair (SCRP) with a well-defined initial singlet spin configuration. Subsequent coherent mixing between the SCRP singlet and triplet ms = 0 spin states, the so-called zero quantum coherence (ZQC), is of potential interest in quantum information processing applications because the ZQC can be probed using pulse electron paramagnetic resonance (pulse-EPR) techniques. Here, pulse-EPR spectroscopy is utilized to examine the ZQC oscillation frequencies and ZQC dephasing in three structurally well-defined D-A systems. While transitions between the singlet and triplet ms = 0 spin states are formally forbidden (Δms = 0), they can be addressed using specific microwave pulse turning angles to map information from the ZQC onto observable single quantum coherences. In addition, by using structural variations to tune the singlet-triplet energy gap, the ZQC frequencies determined for this series of molecules indicate a stronger dependence on the electronic g-factor than on electron-nuclear hyperfine interactions.
Intersubband optical transients in multi-quantum-well structures
NASA Astrophysics Data System (ADS)
Luc, F.; Rosencher, E.; Bois, Ph.
1993-05-01
We show that optical transients due to the intersubband photoionization of the electrons from quantum wells may be observed by inserting a multi-quantum-well structure in the space-charge layer of a Schottky diode. This method provides a direct measurement of the photoionization cross section of a quantum well. The escape probability of the photoexcited electron from the quantum well can thus be unambiguously deduced. Its variation with the electric field may be described by a simple model based on the statistical fluctuation of the quantum-well width.
Coherence-enhanced efficiency of feedback-driven quantum engines
NASA Astrophysics Data System (ADS)
Brandner, Kay; Bauer, Michael; Schmid, Michael T.; Seifert, Udo
2015-06-01
A genuine feature of projective quantum measurements is that they inevitably alter the mean energy of the observed system if the measured quantity does not commute with the Hamiltonian. Compared to the classical case, Jacobs proved that this additional energetic cost leads to a stronger bound on the work extractable after a single measurement from a system initially in thermal equilibrium (2009 Phys. Rev. A 80 012322). Here, we extend this bound to a large class of feedback-driven quantum engines operating periodically and in finite time. The bound thus implies a natural definition for the efficiency of information to work conversion in such devices. For a simple model consisting of a laser-driven two-level system, we maximize the efficiency with respect to the observable whose measurement is used to control the feedback operations. We find that the optimal observable typically does not commute with the Hamiltonian and hence would not be available in a classical two level system. This result reveals that periodic feedback engines operating in the quantum realm can exploit quantum coherences to enhance efficiency.
Electron Spin Relaxation and Coherence Times in Si/SiGe Quantum Dots
NASA Astrophysics Data System (ADS)
Jock, R. M.; He, Jianhua; Tyryshkin, A. M.; Lyon, S. A.; Lee, C.-H.; Huang, S.-H.; Liu, C. W.
2013-03-01
Single electron spin states in Si/SiGe quantum dots have shown promise as qubits for quantum information processing. Recently, electron spins in gated Si/SiGe quantum dots have displayed relaxation (T1) and coherence (T2) times of 250 μs at 350mK. The experiments used conventional X-band (10 GHz) pulsed Electron Spin Resonance (pESR) on a large area (3.5 x 20 mm2) , double gated, undoped Si/SiGe heterostructure, which was patterned with 2 x 108 quantum dots using e-beam lithography. Dots with 150 nm radii and 700 nm period are induced in a natural Si quantum well by the gates. Smaller dots are expected to reduce the effects of nearly degenerate valley states and spin-orbit coupling on the electron spin coherence. However, the small number of spins makes signal recovery extremely challenging. We have implemented a broadband cryogenic HEMT low-noise-amplifier and a high-speed single-pole double-throw switch operating at liquid helium temperatures. The switch and preamp have improved our signal to noise by an order of magnitude, allowing for smaller samples and shorter measurement times. We will describe these improvements and the data they have enabled. supported by the ARO
Theory of ultrafast optical manipulation of electron spins in quantum wells
NASA Astrophysics Data System (ADS)
Jin, Jinshuang; Li, Xin-Qi
2005-12-01
Based on a multiparticle-state stimulated Raman adiabatic passage approach, a comprehensive theoretical study of the ultrafast optical manipulation of electron spins in quantum wells is presented. In addition to corroborating experimental findings [Gupta et al., Science 292, 2458 (2001)], we improve the expression for the optical-pulse-induced effective magnetic field, in comparison with the one obtained via the conventional single-particle ac Stark shift. Further study of the effect of hole-spin relaxation reveals that, while the coherent optical manipulation of electron spin in undoped quantum wells would deteriorate in the presence of relatively fast hole-spin relaxation, the coherent control in doped systems can be quite robust against decoherence. The implications of the present results on quantum dots will also be discussed.
Tunable quantum beam splitters for coherent manipulation of a solid-state tripartite qubit system.
Sun, Guozhu; Wen, Xueda; Mao, Bo; Chen, Jian; Yu, Yang; Wu, Peiheng; Han, Siyuan
2010-08-10
Coherent control of quantum states is at the heart of implementing solid-state quantum processors and testing quantum mechanics at the macroscopic level. Despite significant progress made in recent years in controlling single- and bi-partite quantum systems, coherent control of quantum wave function in multipartite systems involving artificial solid-state qubits has been hampered due to the relatively short decoherence time and lack of precise control methods. Here we report the creation and coherent manipulation of quantum states in a tripartite quantum system, which is formed by a superconducting qubit coupled to two microscopic two-level systems (TLSs). The avoided crossings in the system's energy-level spectrum due to the qubit-TLS interaction act as tunable quantum beam splitters of wave functions. Our result shows that the Landau-Zener-Stückelberg interference has great potential in precise control of the quantum states in the tripartite system.
The quantum coherent mechanism for singlet fission: experiment and theory.
Chan, Wai-Lun; Berkelbach, Timothy C; Provorse, Makenzie R; Monahan, Nicholas R; Tritsch, John R; Hybertsen, Mark S; Reichman, David R; Gao, Jiali; Zhu, X-Y
2013-06-18
The absorption of one photon by a semiconductor material usually creates one electron-hole pair. However, this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron-hole pairs. This process, where a singlet exciton transforms to two triplet excitons, can have quantum yields as high as 200%. Singlet fission may be useful to solar cell technologies to increase the power conversion efficiency beyond the so-called Shockley-Queisser limit. Through time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene, our lab has recently provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state (also called a correlated triplet pair). More importantly, we found that population of the multiexciton state rises at the same time as the singlet state on the ultrafast time scale upon photoexcitation. This observation does not fit with the traditional view of singlet fission involving the incoherent conversion of a singlet to a triplet pair. However, it provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately after optical excitation. In this Account, we review key experimental findings from TR-2PPE experiments and present a theoretical analysis of the quantum coherent mechanism based on electronic structural and density matrix calculations for crystalline tetracene lattices. Using multistate density functional theory, we find that the direct electronic coupling between singlet and multiexciton states is too weak to explain the experimental observation. Instead, indirect coupling via charge transfer intermediate states is two orders of magnitude stronger, and dominates the dynamics for ultrafast multiexciton formation. Density matrix calculation for the crystalline
Coherent states, quantum gravity, and the Born- Oppenheimer approximation. II. Compact Lie groups
NASA Astrophysics Data System (ADS)
Stottmeister, Alexander; Thiemann, Thomas
2016-07-01
In this article, the second of three, we discuss and develop the basis of a Weyl quantisation for compact Lie groups aiming at loop quantum gravity-type models. This Weyl quantisation may serve as the main mathematical tool to implement the program of space adiabatic perturbation theory in such models. As we already argued in our first article, space adiabatic perturbation theory offers an ideal framework to overcome the obstacles that hinder the direct implementation of the conventional Born-Oppenheimer approach in the canonical formulation of loop quantum gravity. Additionally, we conjecture the existence of a new form of the Segal-Bargmann-Hall "coherent state" transform for compact Lie groups G, which we prove for G = U(1)n and support by numerical evidence for G = SU(2). The reason for conjoining this conjecture with the main topic of this article originates in the observation that the coherent state transform can be used as a basic building block of a coherent state quantisation (Berezin quantisation) for compact Lie groups G. But, as Weyl and Berezin quantisation for ℝ2d are intimately related by heat kernel evolution, it is natural to ask whether a similar connection exists for compact Lie groups as well. Moreover, since the formulation of space adiabatic perturbation theory requires a (deformation) quantisation as minimal input, we analyse the question to what extent the coherent state quantisation, defined by the Segal-Bargmann-Hall transform, can serve as basis of the former.
NASA Astrophysics Data System (ADS)
Eftekhari, F.; Tavassoly, M. K.
In this paper, we will present a general formalism for constructing the nonlinear charge coherent states which in special case lead to the standard charge coherent states. The suQ(1, 1) algebra as a nonlinear deformed algebra realization of the introduced states is established. In addition, the corresponding even and odd nonlinear charge coherent states have also been introduced. The formalism has the potentiality to be applied to systems either with known "nonlinearity function" f(n) or solvable quantum system with known "discrete nondegenerate spectrum" en. As some physical appearances, a few known physical systems in the two mentioned categories have been considered. Finally, since the construction of nonclassical states is a central topic of quantum optics, nonclassical features and quantum statistical properties of the introduced states have been investigated by evaluating single- and two-mode squeezing, su(1, 1)-squeezing, Mandel parameter and antibunching effect (via g-correlation function) as well as some of their generalized forms we have introduced in the present paper.
NASA Astrophysics Data System (ADS)
Opanchuk, B.; Rosales-Zárate, L.; Teh, R. Y.; Reid, M. D.
2016-12-01
We examine how to signify and quantify the mesoscopic quantum coherence of approximate two-mode NOON states and spin-squeezed two-mode Bose-Einstein condensates (BEC). We identify two criteria that verify a nonzero quantum coherence between states with quantum number different by n . These criteria negate certain mixtures of quantum states, thereby signifying a generalized n -scopic Schrödinger cat-type paradox. The first criterion is the correlation ≠0 (here a ̂ and b ̂ are the boson operators for each mode). The correlation manifests as interference fringes in n -particle detection probabilities and is also measurable via quadrature phase amplitude and spin-squeezing measurements. Measurement of enables a quantification of the overall n th order quantum coherence, thus providing an avenue for high efficiency verification of high-fidelity photonic NOON states. The second criterion is based on a quantification of the measurable spin-squeezing parameter ξN. We apply the criteria to theoretical models of NOON states in lossy interferometers and double-well trapped BECs. By analyzing existing BEC experiments, we demonstrate generalized atomic "kitten" states and atomic quantum coherence with n ⪆10 atoms.
NASA Astrophysics Data System (ADS)
Kraft, Manuel; Hein, Sven M.; Lehnert, Judith; Schöll, Eckehard; Hughes, Stephen; Knorr, Andreas
2016-08-01
Quantum coherent feedback control is a measurement-free control method fully preserving quantum coherence. In this paper we show how time-delayed quantum coherent feedback can be used to control the degree of squeezing in the output field of a cavity containing a degenerate parametric oscillator. We focus on the specific situation of Pyragas-type feedback control where time-delayed signals are fed back directly into the quantum system. Our results show how time-delayed feedback can enhance or decrease the degree of squeezing as a function of time delay and feedback strength.
Zhou, Zong-Quan; Huelga, Susana F; Li, Chuan-Feng; Guo, Guang-Can
2015-09-11
We discuss the use of inequalities of the Leggett-Garg type (LGtI) to witness quantum coherence and present the first experimental violation of this type of inequalities using a light-matter interfaced system. By separately benchmarking the Markovian character of the evolution and the translational invariance of the conditional probabilities, the observed violation of a LGtI is attributed to the quantum coherent character of the process. These results provide a general method to benchmark "quantumness" when the absence of memory effects can be independently certified and confirm the persistence of quantum coherent features within systems of increasing complexity.
Transport through quantum wells and superlattices on topological insulator surfaces.
Song, J-T; Li, Y-X; Sun, Q-F
2014-05-07
We investigate electron transmission coefficients through quantum wells and quantum superlattices on topological insulator surfaces. The quantum well or superlattice is not constituted by general electronic potential barriers but by Fermi velocity barriers which originate in the different topological insulator surfaces. It is found that electron resonant modes can be renormalized by quantum wells and more clearly by quantum superlattices. The depth and width of a quantum well and superlattice, the incident angle of an electron, and the Fermi energy can be used to effectively tune the electron resonant modes. In particular, the number N of periodic structures that constitute a superlattice can further strengthen these regulating effects. These results suggest that a device could be developed to select and regulate electron propagation modes on topological insulator surfaces. Finally, we also study the conductance and the Fano factor through quantum wells and quantum superlattices. In contrast to what has been reported before, the suppression factors of 0.4 in the conductance and 0.85 in the Fano factor are observed in a quantum well, while the transport for a quantum superlattice shows strong oscillating behavior at low energy and reaches the same saturated values as in the case of a quantum well at sufficiently large energies.
Long Spin Relaxation and Coherence Times of Electrons In Gated Si/SiGe Quantum Dots
NASA Astrophysics Data System (ADS)
He, Jianhua; Tyryshkin, A. M.; Lyon, S. A.; Lee, C.-H.; Huang, S.-H.; Liu, C. W.
2012-02-01
Single electron spin states in semiconductor quantum dots are promising candidate qubits. We report the measurement of 250 μs relaxation (T1) and coherence (T2) times of electron spins in gated Si/SiGe quantum dots at 350 mK. The experiments used conventional X-band (10 GHz) pulsed electron spin resonance (pESR), on a large area (3.5 x 20 mm^2) dual-gate undoped high mobility Si/SiGe heterostructure sample, which was patterned with 2 x 10^8 quantum dots using e-beam lithography. Dots having 150 nm radii with a 700 nm period are induced in a natural Si quantum well by the gates. The measured T1 and T2 at 350 mK are much longer than those of free 2D electrons, for which we measured T1 to be 10 μs and T2 to be 6.5 μs in this gated sample. The results provide direct proof that the effects of a fluctuating Rashba field have been greatly suppressed by confining the electrons in quantum dots. From 0.35 K to 0.8 K, T1 of the electron spins in the quantum dots shows little temperature dependence, while their T2 decreased to about 150 μs at 0.8 K. The measured 350 mK spin coherence time is 10 times longer than previously reported for any silicon 2D electron-based structures, including electron spins confined in ``natural quantum dots'' formed by potential disorder at the Si/SiO2ootnotetextS. Shankar et al., Phys. Rev. B 82, 195323 (2010) or Si/SiGe interface, where the decoherence appears to be controlled by spin exchange.
NASA Astrophysics Data System (ADS)
Liu, Jian-Heng; Tu, Matisse Wei-Yuan; Zhang, Wei-Min
2016-07-01
By considering a nanoscale Aharonov-Bohm (AB) interferometer consisting of a laterally coupled double dot coupled to the source and drain electrodes, we investigate the AB phase dependence of the bonding and antibonding states and the transport currents via the bonding and antibonding state channels. The relations of the AB phase dependence between the quantum states and the associated transport current components are analyzed, which provides useful information for the reconstruction of quantum states through the measurement of the transport current in such systems. We also obtain the validity of the experimental analysis [given in T. Hatano et al., Phys. Rev. Lett. 106, 076801 (2011), 10.1103/PhysRevLett.106.076801] that bonding state currents in different energy configurations are almost the same. With the coherent properties in the quantum dot states as well as in the transport currents, we also provide a way to manipulate the bonding and antibonding states through the AB magnetic flux.
Quantum-well lasers for direct solar photopumping
NASA Astrophysics Data System (ADS)
Unnikrishnan, Sreenath; Anderson, Neal G.
1993-09-01
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 AlGaAs, 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 (or solar magnification requirements) depend upon material and structure parameters, design optimum structures for solar-pumped operation, and identify design trade offs. Our results suggest that laser action should be possible in properly designed structures at readily achievable solar concentrations (103-104 suns under air-mass-zero conditions), and that optimum designs for solar-pumped SCQWH lasers differ significantly from those for analogous current injection devices.
Coherent state excitons in anisotropic quantum dots: classical and quantum correlations
NASA Astrophysics Data System (ADS)
Thilagam, A.
2012-06-01
We investigate the entanglement dynamics of excitons confined in two adjacent quantum dots, which we describe through their algebraic properties using \\mathfrak {su}(1,1) in the z-direction. We use two explicit forms of coherent states: the Perelomov and Barut-Girardello states to represent the electronic component of the excitonic state in the z-direction. Our results show that in a coherent state basis, the concurrence of an excitonic-qubit pair shows subtle variations which are dependent on the algebraic parameters of the \\mathfrak {su}(1,1) group. These variations also appear in the classical and quantum correlations, present in the qubit-qubit density matrix that is associated with the purely Förster-coupled excitonic-qubit pair. A brief discussion of a plausible experimental technique of detecting excitonic coherent states is provided. This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical devoted to ‘Coherent states: mathematical and physical aspects’.
Protecting quantum coherence of two-level atoms from vacuum fluctuations of electromagnetic field
Liu, Xiaobao; Tian, Zehua; Wang, Jieci; Jing, Jiliang
2016-03-15
In the framework of open quantum systems, we study the dynamics of a static polarizable two-level atom interacting with a bath of fluctuating vacuum electromagnetic field and explore under which conditions the coherence of the open quantum system is unaffected by the environment. For both a single-qubit and two-qubit systems, we find that the quantum coherence cannot be protected from noise when the atom interacts with a non-boundary electromagnetic field. However, with the presence of a boundary, the dynamical conditions for the insusceptible of quantum coherence are fulfilled only when the atom is close to the boundary and is transversely polarizable. Otherwise, the quantum coherence can only be protected in some degree in other polarizable direction. -- Highlights: •We study the dynamics of a two-level atom interacting with a bath of fluctuating vacuum electromagnetic field. •For both a single and two-qubit systems, the quantum coherence cannot be protected from noise without a boundary. •The insusceptible of the quantum coherence can be fulfilled only when the atom is close to the boundary and is transversely polarizable. •Otherwise, the quantum coherence can only be protected in some degree in other polarizable direction.
Guiding effect of quantum wells in semiconductor lasers
Aleshkin, V Ya; Dikareva, Natalia V; Dubinov, A A; Zvonkov, B N; Karzanova, Maria V; Kudryavtsev, K E; Nekorkin, S M; Yablonskii, A N
2013-05-31
The guiding effect of InGaAs quantum wells in GaAs- and InP-based semiconductor lasers has been studied theoretically and experimentally. The results demonstrate that such waveguides can be effectively used in laser structures with a large refractive index difference between the quantum well material and semiconductor matrix and a large number of quantum wells (e.g. in InP-based structures). (semiconductor lasers. physics and technology)
Nagy, Andrea; Prokhorenko, Valentyn; Miller, R J Dwayne
2006-10-01
The issue of quantum effects in biological functions reduces to determining the relevant length and/or time scales over which phase relationships (coherence) in the wave properties of matter are conserved and lead to observable interference effects. Recent advances in femtosecond laser-based two-dimensional spectroscopy and coherent control have made it possible to directly determine the relevant timescales of quantum coherence in biological systems and even manipulate such effects, respectively, and also provide direct information on the interactions between the different degrees of freedom (electronic and nuclear) with sufficient time resolution to catch the very chemical processes driving biological functions in action. The picture that is emerging is that there are primary events in biological processes that occur on timescales commensurate with quantum coherence effects.
Coherent quantum squeezing due to the phase space noncommutativity
NASA Astrophysics Data System (ADS)
Bernardini, Alex E.; Mizrahi, Salomon S.
2015-06-01
The effects of general noncommutativity of operators on producing deformed coherent squeezed states is examined in phase space. A two-dimensional noncommutative (NC) quantum system supported by a deformed mathematical structure, similar to that of Hadamard billiard, is obtained and the components behaviour is monitored in time. It is assumed that the independent degrees of freedom are two free 1D harmonic oscillators (HOs), so the system Hamiltonian does not contain interaction terms. Through the NC deformation parameterized by a Seiberg-Witten transform on the original canonical variables, one gets the standard commutation relations for the new ones, such that the obtained, new, Hamiltonian represents two interacting 1D HOs. By admitting that one HO is inverted relatively to the other, we show that their effective interaction induces a squeezing dynamics for initial coherent states imaged in the phase space. A suitable pattern of logarithmic spirals is obtained and some relevant properties are discussed in terms of Wigner functions, which are essential to put in evidence the effects of the noncommutativity.
Killoran, N.; Huelga, S. F.; Plenio, M. B.
2015-10-21
Recent evidence suggests that quantum effects may have functional importance in biological light-harvesting systems. Along with delocalized electronic excitations, it is now suspected that quantum coherent interactions with certain near-resonant vibrations may contribute to light-harvesting performance. However, the actual quantum advantage offered by such coherent vibrational interactions has not yet been established. We investigate a quantum design principle, whereby coherent exchange of single energy quanta between electronic and vibrational degrees of freedom can enhance a light-harvesting system’s power above what is possible by thermal mechanisms alone. We present a prototype quantum heat engine which cleanly illustrates this quantum design principle and quantifies its quantum advantage using thermodynamic measures of performance. We also demonstrate the principle’s relevance in parameter regimes connected to natural light-harvesting structures.
Killoran, N; Huelga, S F; Plenio, M B
2015-10-21
Recent evidence suggests that quantum effects may have functional importance in biological light-harvesting systems. Along with delocalized electronic excitations, it is now suspected that quantum coherent interactions with certain near-resonant vibrations may contribute to light-harvesting performance. However, the actual quantum advantage offered by such coherent vibrational interactions has not yet been established. We investigate a quantum design principle, whereby coherent exchange of single energy quanta between electronic and vibrational degrees of freedom can enhance a light-harvesting system's power above what is possible by thermal mechanisms alone. We present a prototype quantum heat engine which cleanly illustrates this quantum design principle and quantifies its quantum advantage using thermodynamic measures of performance. We also demonstrate the principle's relevance in parameter regimes connected to natural light-harvesting structures.
Quantum entropy and uncertainty for two-mode squeezed, coherent and intelligent spin states
NASA Technical Reports Server (NTRS)
Aragone, C.; Mundarain, D.
1993-01-01
We compute the quantum entropy for monomode and two-mode systems set in squeezed states. Thereafter, the quantum entropy is also calculated for angular momentum algebra when the system is either in a coherent or in an intelligent spin state. These values are compared with the corresponding values of the respective uncertainties. In general, quantum entropies and uncertainties have the same minimum and maximum points. However, for coherent and intelligent spin states, it is found that some minima for the quantum entropy turn out to be uncertainty maxima. We feel that the quantum entropy we use provides the right answer, since it is given in an essentially unique way.
NASA Astrophysics Data System (ADS)
Sadeghi, S. M.
2015-08-01
When a quantum dot is in the vicinity of a metallic nanoparticle and is driven by a laser field, quantum coherence can renormalize the plasmon field of the metallic nanoparticle, forming a coherent-plasmonic field (CP field). We demonstrate that for a given form of variation of this laser field with time, the CP field around the metallic nanoparticle can offer different forms of ultrafast field dynamics, depending on the location. In other words, we show the coherent exciton-plasmon coupling in such a system allows it to act as coherent nanoantenna capable of generation position-dependent coherent-plasmonic dynamics, designating each location around the metallic nanoparticle with characteristic time-position coordinates. These investigations are carried out by demonstrating that the coherent dynamics responsible for these effects can persist in the presence of the ultrafast polarization dephasing of the quantum dots. This highlights the prospect of generation and preservation of quantum coherence effects in hybrid quantum dot-metallic nanoparticle systems at elevated temperatures. Therefore, even when the decoherence times of the quantum dots are of the order of several hundreds of femtoseconds, as observed at room temperature, such coherent dynamics can remain quite distinct and observable.
Elucidation of the timescales and origins of quantum electronic coherence in LHCII
NASA Astrophysics Data System (ADS)
Schlau-Cohen, Gabriela S.; Ishizaki, Akihito; Calhoun, Tessa R.; Ginsberg, Naomi S.; Ballottari, Matteo; Bassi, Roberto; Fleming, Graham R.
2012-05-01
Photosynthetic organisms harvest sunlight with near unity quantum efficiency. The complexity of the electronic structure and energy transfer pathways within networks of photosynthetic pigment-protein complexes often obscures the mechanisms behind the efficient light-absorption-to-charge conversion process. Recent experiments, particularly using two-dimensional spectroscopy, have detected long-lived quantum coherence, which theory suggests may contribute to the effectiveness of photosynthetic energy transfer. Here, we present a new, direct method to access coherence signals: a coherence-specific polarization sequence, which isolates the excitonic coherence features from the population signals that usually dominate two-dimensional spectra. With this polarization sequence, we elucidate coherent dynamics and determine the overall measurable lifetime of excitonic coherence in the major light-harvesting complex of photosystem II. Coherence decays on two distinct timescales of 47 fs and ~800 fs. We present theoretical calculations to show that these two timescales are from weakly and moderately strongly coupled pigments, respectively.
Tunable Bandwidth Quantum Well Infrared Photo Detector (TB-QWIP)
2003-12-01
conduction band called the band gap of the material. In a semiconductor the band gap is the minimum energy necessary for an electron to transfer from the...the optical energy from a heated object, instead of relying directly on the transfer of heat energy (like thermal detectors do). A quantum well can...to achieve electronically tunable bandwidth quantum well infrared photo detectors (Choi K. K. 1), or tunable bandwidth quantum dot infrared photo
Tian, Si-Cong; Wan, Ren-Gang; Wang, Chun-Liang; Shu, Shi-Li; Wang, Li-Jie; Tong, Chun-Zhu
2016-12-01
We propose a scheme for creation and transfer of coherence among ground state and indirect exciton states of triple quantum dots via the technique of stimulated Raman adiabatic passage. Compared with the traditional stimulated Raman adiabatic passage, the Stokes laser pulse is replaced by the tunneling pulse, which can be controlled by the externally applied voltages. By varying the amplitudes and sequences of the pump and tunneling pulses, a complete coherence transfer or an equal coherence distribution among multiple states can be obtained. The investigations can provide further insight for the experimental development of controllable coherence transfer in semiconductor structure and may have potential applications in quantum information processing.
Quantum detection and ranging using exciton-plasmon coupling in coherent nanoantennas
NASA Astrophysics Data System (ADS)
Sadeghi, S. M.; Hatef, A.; Meunier, Michel
2013-05-01
We utilize interaction of a laser field with a quantum dot-metallic nanoshell system to investigate nanoscale detection and ranging using quantum coherence. We demonstrate that the nanoshell in this system can act as a coherent nanoantenna capable of designating each position in its range with unique space-time field coordinates. This shows that coherent exciton-plasmon coupling in such a system allows the electric field of this antenna generates position-dependent dynamics in molecules and nanostructures in its vicinity, allowing their remote detection. The results are obtained considering the ultrafast polarization dephasing of the quantum dot at elevated temperatures.
Quantum Zeno switch for single-photon coherent transport
Zhou Lan; Yang, S.; Liu Yuxi; Sun, C. P.; Nori, Franco
2009-12-15
Using a dynamical quantum Zeno effect, we propose a general approach to control the coupling between a two-level system (TLS) and its surroundings, by modulating the energy-level spacing of the TLS with a high-frequency signal. We show that the TLS-surroundings interaction can be turned off when the ratio between the amplitude and the frequency of the modulating field is adjusted to be a zero of a Bessel function. The quantum Zeno effect of the TLS can also be observed by the vanishing of the photon reflection at these zeros. Based on these results, we propose a quantum switch to control the transport of a single photon in a one-dimensional waveguide. Our analytical results agree well with numerical results using Floquet theory.
Screening effects and Friedel oscillations in quantum-well nanostructures
Kovalev, V. M.; Chaplik, A. V.
2008-11-15
The screening of the Coulomb interaction is studied with regard to Friedel oscillations in multicomponent electron plasma structure. A double quantum well (QW) and a superlattice are considered. The groundstate energy of a donor (exciton) in a double quantum well is calculated by a variational method as a function of the population of subbands.
NASA Technical Reports Server (NTRS)
Yuen, H. P.; Shapiro, J. H.
1978-01-01
To determine the ultimate performance limitations imposed by quantum effects, it is also essential to consider optimum quantum-state generation. Certain 'generalized' coherent states of the radiation field possess novel quantum noise characteristics that offer the potential for greatly improved optical communications. These states have been called two-photon coherent states because they can be generated, in principle, by stimulated two-photon processes. The use of two-photon coherent state (TCS) radiation in free-space optical communications is considered. A simple theory of quantum state propagation is developed. The theory provides the basis for representing the free-space channel in a quantum-mechanical form convenient for communication analysis. The new theory is applied to TCS radiation.
Li, Hai; Zou, Jian; Yu, Wen-Li; Xu, Bao-Ming; Li, Jun-Gang; Shao, Bin
2014-05-01
We consider a model of an optical cavity with a nonequilibrium reservoir consisting of a beam of identical two-level atom pairs (TLAPs) in the general X state. We find that coherence of multiparticle nonequilibrium reservoir plays a central role on the potential work capability of the cavity. We show that no matter whether there are quantum correlations in each TLAP (including quantum entanglement and quantum discord) or not, the coherence of the TLAPs has an effect on the work capability of the cavity. Additionally, constructive and destructive interferences could be induced to influence the work capability of the cavity by adjusting only the relative phase, with which quantum correlations have nothing to do. In this paper, the coherence of the reservoir, rather than the quantum correlations, effectively reflecting the effects of the reservoir on the system's work capability is demonstrated clearly.
Quantum Coherent Atomic Tunneling between Two Trapped Bose-Einstein Condensates
Smerzi, A.; Fantoni, S.; Giovanazzi, S.
1997-12-01
We study the coherent atomic tunneling between two zero-temperature Bose-Einstein condensates (BEC) confined in a double-well magnetic trap. Two Gross-Pitaevskii equations for the self-interacting BEC amplitudes, coupled by a transfer matrix element, describe the dynamics in terms of the interwell phase difference and population imbalance. In addition to the anharmonic generalization of the familiar ac Josephson effect and plasma oscillations occurring in superconductor junctions, the nonlinear BEC tunneling dynamics sustains a self-maintained population imbalance: a novel {open_quotes}macroscopic quantum self-trapping{close_quotes} effect. {copyright} {ital 1997} {ital The American Physical Society}
Quantum coherent optical phase modulation in an ultrafast transmission electron microscope.
Feist, Armin; Echternkamp, Katharina E; Schauss, Jakob; Yalunin, Sergey V; Schäfer, Sascha; Ropers, Claus
2015-05-14
Coherent manipulation of quantum systems with light is expected to be a cornerstone of future information and communication technology, including quantum computation and cryptography. The transfer of an optical phase onto a quantum wavefunction is a defining aspect of coherent interactions and forms the basis of quantum state preparation, synchronization and metrology. Light-phase-modulated electron states near atoms and molecules are essential for the techniques of attosecond science, including the generation of extreme-ultraviolet pulses and orbital tomography. In contrast, the quantum-coherent phase-modulation of energetic free-electron beams has not been demonstrated, although it promises direct access to ultrafast imaging and spectroscopy with tailored electron pulses on the attosecond scale. Here we demonstrate the coherent quantum state manipulation of free-electron populations in an electron microscope beam. We employ the interaction of ultrashort electron pulses with optical near-fields to induce Rabi oscillations in the populations of electron momentum states, observed as a function of the optical driving field. Excellent agreement with the scaling of an equal-Rabi multilevel quantum ladder is obtained, representing the observation of a light-driven 'quantum walk' coherently reshaping electron density in momentum space. We note that, after the interaction, the optically generated superposition of momentum states evolves into a train of attosecond electron pulses. Our results reveal the potential of quantum control for the precision structuring of electron densities, with possible applications ranging from ultrafast electron spectroscopy and microscopy to accelerator science and free-electron lasers.
Quantum coherent optical phase modulation in an ultrafast transmission electron microscope
NASA Astrophysics Data System (ADS)
Feist, Armin; Echternkamp, Katharina E.; Schauss, Jakob; Yalunin, Sergey V.; Schäfer, Sascha; Ropers, Claus
2015-05-01
Coherent manipulation of quantum systems with light is expected to be a cornerstone of future information and communication technology, including quantum computation and cryptography. The transfer of an optical phase onto a quantum wavefunction is a defining aspect of coherent interactions and forms the basis of quantum state preparation, synchronization and metrology. Light-phase-modulated electron states near atoms and molecules are essential for the techniques of attosecond science, including the generation of extreme-ultraviolet pulses and orbital tomography. In contrast, the quantum-coherent phase-modulation of energetic free-electron beams has not been demonstrated, although it promises direct access to ultrafast imaging and spectroscopy with tailored electron pulses on the attosecond scale. Here we demonstrate the coherent quantum state manipulation of free-electron populations in an electron microscope beam. We employ the interaction of ultrashort electron pulses with optical near-fields to induce Rabi oscillations in the populations of electron momentum states, observed as a function of the optical driving field. Excellent agreement with the scaling of an equal-Rabi multilevel quantum ladder is obtained, representing the observation of a light-driven `quantum walk' coherently reshaping electron density in momentum space. We note that, after the interaction, the optically generated superposition of momentum states evolves into a train of attosecond electron pulses. Our results reveal the potential of quantum control for the precision structuring of electron densities, with possible applications ranging from ultrafast electron spectroscopy and microscopy to accelerator science and free-electron lasers.
Gate fidelity and coherence of an electron spin in an Si/SiGe quantum dot with micromagnet
Kawakami, Erika; Jullien, Thibaut; Scarlino, Pasquale; Ward, Daniel R.; Savage, Donald E.; Lagally, Max G.; Dobrovitski, Viatcheslav V.; Friesen, Mark; Coppersmith, Susan N.; Eriksson, Mark A.; Vandersypen, Lieven M. K.
2016-01-01
The gate fidelity and the coherence time of a quantum bit (qubit) are important benchmarks for quantum computation. We construct a qubit using a single electron spin in an Si/SiGe quantum dot and control it electrically via an artificial spin-orbit field from a micromagnet. We measure an average single-qubit gate fidelity of ∼99% using randomized benchmarking, which is consistent with dephasing from the slowly evolving nuclear spins in the substrate. The coherence time measured using dynamical decoupling extends up to ∼400 μs for 128 decoupling pulses, with no sign of saturation. We find evidence that the coherence time is limited by noise in the 10-kHz to 1-MHz range, possibly because charge noise affects the spin via the micromagnet gradient. This work shows that an electron spin in an Si/SiGe quantum dot is a good candidate for quantum information processing as well as for a quantum memory, even without isotopic purification. PMID:27698123
Gate fidelity and coherence of an electron spin in an Si/SiGe quantum dot with micromagnet
Kawakami, Erika; Jullien, Thibaut; Scarlino, Pasquale; Ward, Daniel R.; Savage, Donald E.; Lagally, Max G.; Dobrovitski, Viatcheslav V.; Friesen, Mark; Coppersmith, Susan N.; Eriksson, Mark A.; Vandersypen, Lieven M. K.
2016-10-03
The gate fidelity and the coherence time of a quantum bit (qubit) are important benchmarks for quantum computation. We construct a qubit using a single electron spin in an Si/SiGe quantum dot and control it electrically via an artificial spin-orbit field from a micromagnet. We measure an average single-qubit gate fidelity of ~99% using randomized benchmarking, which is consistent with dephasing from the slowly evolving nuclear spins in the substrate. The coherence time measured using dynamical decoupling extends up to ~400 μs for 128 decoupling pulses, with no sign of saturation. We find evidence that the coherence time is limited by noise in the 10-kHz to 1-MHz range, possibly because charge noise affects the spin via the micromagnet gradient. Furthermore, this work shows that an electron spin in an Si/SiGe quantum dot is a good candidate for quantum information processing as well as for a quantum memory, even without isotopic purification.
Gate fidelity and coherence of an electron spin in an Si/SiGe quantum dot with micromagnet
Kawakami, Erika; Jullien, Thibaut; Scarlino, Pasquale; ...
2016-10-03
The gate fidelity and the coherence time of a quantum bit (qubit) are important benchmarks for quantum computation. We construct a qubit using a single electron spin in an Si/SiGe quantum dot and control it electrically via an artificial spin-orbit field from a micromagnet. We measure an average single-qubit gate fidelity of ~99% using randomized benchmarking, which is consistent with dephasing from the slowly evolving nuclear spins in the substrate. The coherence time measured using dynamical decoupling extends up to ~400 μs for 128 decoupling pulses, with no sign of saturation. We find evidence that the coherence time is limitedmore » by noise in the 10-kHz to 1-MHz range, possibly because charge noise affects the spin via the micromagnet gradient. Furthermore, this work shows that an electron spin in an Si/SiGe quantum dot is a good candidate for quantum information processing as well as for a quantum memory, even without isotopic purification.« less
Complete Coherent Control of a Quantum Dot Strongly Coupled to a Nanocavity
Dory, Constantin; Fischer, Kevin A.; Müller, Kai; Lagoudakis, Konstantinos G.; Sarmiento, Tomas; Rundquist, Armand; Zhang, Jingyuan L.; Kelaita, Yousif; Vučković, Jelena
2016-01-01
Strongly coupled quantum dot-cavity systems provide a non-linear configuration of hybridized light-matter states with promising quantum-optical applications. Here, we investigate the coherent interaction between strong laser pulses and quantum dot-cavity polaritons. Resonant excitation of polaritonic states and their interaction with phonons allow us to observe coherent Rabi oscillations and Ramsey fringes. Furthermore, we demonstrate complete coherent control of a quantum dot-photonic crystal cavity based quantum-bit. By controlling the excitation power and phase in a two-pulse excitation scheme we achieve access to the full Bloch sphere. Quantum-optical simulations are in good agreement with our experiments and provide insight into the decoherence mechanisms. PMID:27112420
Generation of Bi-partite Polarization Correlation using Coherent States for Quantum Communication
NASA Astrophysics Data System (ADS)
Bollen, Viktor; Meng Sua, Yong; Fook Lee, Kim
2010-03-01
We present a novel scheme to generate bi-partite polarization correlation using coherent states for quantum communication. The scheme can be used for entanglement based quantum cryptography, where the bi-partite correlation will be protected by quantum noise. We perform experimental measurement on two independent coherent states with low mean photon numbers. A coherent state with polarization H is mixed with another coherent state with polarization V through a beam splitter. Polarization correlation is manipulated by using a quarter wave plate and a linear polarizer at each output of the beam splitter. The product signal obtained from the output modes contains bi-partite correlation and other noise terms. We obtain the bi-partite correlation function by employing mean-value measurement based on Stapp's formulation on the product signal, where the noise term is then averaged to zero due to randomness of quantum phase noise. The bi-partite correlation obtained by using two coherent states is quantum correlation because coherent states with low mean photon numbers are involved and the correlations are protected by randomness of quantum noise as inherited by mean photon number fluctuation and its associated phase fluctuation. Preparations for four types of coherent-state polarization correlation functions are also outlined.
Optical Two-Dimensional Spectroscopy of Disordered Semiconductor Quantum Wells and Quantum Dots
Cundiff, Steven T.
2016-05-03
This final report describes the activities undertaken under grant "Optical Two-Dimensional Spectroscopy of Disordered Semiconductor Quantum Wells and Quantum Dots". The goal of this program was to implement optical 2-dimensional Fourier transform spectroscopy and apply it to electronic excitations, including excitons, in semiconductors. Specifically of interest are quantum wells that exhibit disorder due to well width fluctuations and quantum dots. In both cases, 2-D spectroscopy will provide information regarding coupling among excitonic localization sites.
Jeong, Byeong Guk; Park, Young-Shin; Chang, Jun Hyuk; Cho, Ikjun; Kim, Jai Kyeong; Kim, Heesuk; Char, Kookheon; Cho, Jinhan; Klimov, Victor I; Park, Philip; Lee, Doh C; Bae, Wan Ki
2016-10-02
Thick inorganic shell endows colloidal nanocrystals (NCs) with enhanced photochemical stability and suppression of photoluminescence intermittency (also known as blinking). However, the progress of using thick-shell heterostructure NCs in applications has been limited, due to low photoluminescence quantum yield (PL QY 60%) at room temperature. Here, we demonstrate thick-shell NCs with CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) geometry that exhibit near-unity PL QY at room temperature and suppression of blinking. In SQW NCs, the lattice mismatch is diminished between the emissive CdSe layer and the surrounding CdS layers as a result of coherent strain, which suppresses the formation of misfit defects and consequently permits ~ 100% PL QY for SQW NCs with thick CdS shell (≥ 5 nm). High PL QY of thick-shell SQW NCs are preserved even in concentrated dispersion and in film under thermal stress, which makes them promising candidates for applications in solid-state lightings and luminescent solar concentrators.
On-chip coherent conversion of photonic quantum entanglement between different degrees of freedom
Feng, Lan-Tian; Zhang, Ming; Zhou, Zhi-Yuan; Li, Ming; Xiong, Xiao; Yu, Le; Shi, Bao-Sen; Guo, Guo-Ping; Dai, Dao-Xin; Ren, Xi-Feng; Guo, Guang-Can
2016-01-01
In the quantum world, a single particle can have various degrees of freedom to encode quantum information. Controlling multiple degrees of freedom simultaneously is necessary to describe a particle fully and, therefore, to use it more efficiently. Here we introduce the transverse waveguide-mode degree of freedom to quantum photonic integrated circuits, and demonstrate the coherent conversion of a photonic quantum state between path, polarization and transverse waveguide-mode degrees of freedom on a single chip. The preservation of quantum coherence in these conversion processes is proven by single-photon and two-photon quantum interference using a fibre beam splitter or on-chip beam splitters. These results provide us with the ability to control and convert multiple degrees of freedom of photons for quantum photonic integrated circuit-based quantum information process. PMID:27321821
On-chip coherent conversion of photonic quantum entanglement between different degrees of freedom.
Feng, Lan-Tian; Zhang, Ming; Zhou, Zhi-Yuan; Li, Ming; Xiong, Xiao; Yu, Le; Shi, Bao-Sen; Guo, Guo-Ping; Dai, Dao-Xin; Ren, Xi-Feng; Guo, Guang-Can
2016-06-20
In the quantum world, a single particle can have various degrees of freedom to encode quantum information. Controlling multiple degrees of freedom simultaneously is necessary to describe a particle fully and, therefore, to use it more efficiently. Here we introduce the transverse waveguide-mode degree of freedom to quantum photonic integrated circuits, and demonstrate the coherent conversion of a photonic quantum state between path, polarization and transverse waveguide-mode degrees of freedom on a single chip. The preservation of quantum coherence in these conversion processes is proven by single-photon and two-photon quantum interference using a fibre beam splitter or on-chip beam splitters. These results provide us with the ability to control and convert multiple degrees of freedom of photons for quantum photonic integrated circuit-based quantum information process.
On-chip coherent conversion of photonic quantum entanglement between different degrees of freedom
NASA Astrophysics Data System (ADS)
Feng, Lan-Tian; Zhang, Ming; Zhou, Zhi-Yuan; Li, Ming; Xiong, Xiao; Yu, Le; Shi, Bao-Sen; Guo, Guo-Ping; Dai, Dao-Xin; Ren, Xi-Feng; Guo, Guang-Can
2016-06-01
In the quantum world, a single particle can have various degrees of freedom to encode quantum information. Controlling multiple degrees of freedom simultaneously is necessary to describe a particle fully and, therefore, to use it more efficiently. Here we introduce the transverse waveguide-mode degree of freedom to quantum photonic integrated circuits, and demonstrate the coherent conversion of a photonic quantum state between path, polarization and transverse waveguide-mode degrees of freedom on a single chip. The preservation of quantum coherence in these conversion processes is proven by single-photon and two-photon quantum interference using a fibre beam splitter or on-chip beam splitters. These results provide us with the ability to control and convert multiple degrees of freedom of photons for quantum photonic integrated circuit-based quantum information process.
Photogalvanic Effects in HgTe Quantum Wells
NASA Astrophysics Data System (ADS)
Wittmann, B.; Danilov, S. N.; Kwon, Z. D.; Mikhailov, N. N.; Dvoretsky, S. A.; Ravash, R.; Prettl, W.; Ganichev, S. D.
We report on the observation of the terahertz radiation induced circular (CPGE) and linear (LPGE) photogalvanic effects in HgTe quantum wells. The current response is well described by the phenomenological theory of CPGE and LPGE.
Ćwikliński, Piotr; Studziński, Michał; Horodecki, Michał; Oppenheim, Jonathan
2015-11-20
The second law of thermodynamics places a limitation into which states a system can evolve into. For systems in contact with a heat bath, it can be combined with the law of energy conservation, and it says that a system can only evolve into another if the free energy goes down. Recently, it's been shown that there are actually many second laws, and that it is only for large macroscopic systems that they all become equivalent to the ordinary one. These additional second laws also hold for quantum systems, and are, in fact, often more relevant in this regime. They place a restriction on how the probabilities of energy levels can evolve. Here, we consider additional restrictions on how the coherences between energy levels can evolve. Coherences can only go down, and we provide a set of restrictions which limit the extent to which they can be maintained. We find that coherences over energy levels must decay at rates that are suitably adapted to the transition rates between energy levels. We show that the limitations are matched in the case of a single qubit, in which case we obtain the full characterization of state-to-state transformations. For higher dimensions, we conjecture that more severe constraints exist. We also introduce a new class of thermodynamical operations which allow for greater manipulation of coherences and study its power with respect to a class of operations known as thermal operations.
NASA Astrophysics Data System (ADS)
Yang, Yuxiang; Chiribella, Giulio; Adesso, Gerardo
2014-10-01
Quantum technology promises revolutionary advantages in information processing and transmission compared to classical technology; however, determining which specific resources are needed to surpass the capabilities of classical machines often remains a nontrivial problem. To address such a problem, one first needs to establish the best classical solutions, which set benchmarks that must be beaten by any implementation claiming to harness quantum features for an enhanced performance. Here we introduce and develop a self-contained formalism to obtain the ultimate, generally probabilistic benchmarks for quantum information protocols including teleportation and approximate cloning, with arbitrary ensembles of input states generated by a group action, so-called Gilmore-Perelomov coherent states. This allows us to construct explicit fidelity thresholds for the transmission of multimode Gaussian and non-Gaussian states of continuous-variable systems, as well as qubit and qudit pure states drawn according to nonuniform distributions on the Bloch hypersphere, which accurately model the current laboratory facilities. The performance of deterministic classical procedures such as square-root measurement strategies is further compared with the optimal probabilistic benchmarks, and the state-of-the-art performance of experimental quantum implementations against our newly derived thresholds is discussed. This work provides a comprehensive collection of directly useful criteria for the reliable certification of quantum communication technologies.
Relations Between Narrative Coherence, Identity, and Psychological Well-being in Emerging Adulthood
Waters, Theodore E. A.; Fivush, Robyn
2014-01-01
Objective The hypothesis that the ability to construct a coherent account of personal experience is reflective, or predictive, of psychological adjustment cuts across numerous domains of psychological science. It has been argued that coherent accounts of identity are especially adaptive. We tested these hypotheses by examining relations between narrative coherence of personally significant autobiographical memories and three psychological well-being components (Purpose and Meaning; Positive Self View; Positive Relationships). We also examined the potential moderation of the relations between coherence and well-being by assessing the identity content of each narrative. Method We collected two autobiographical narratives of personally significant events from 103 undergraduate students and coded them for coherence and identity content. Two additional narratives about generic/recurring events were also collected and coded for coherence. Results We confirmed the prediction that constructing coherent autobiographical narratives is related to psychological well-being. Further, we found that this relation was moderated by the narratives’ relevance to identity and that this moderation held after controlling for narrative ability more generally (i.e. coherence of generic/recurring events). Conclusion These data lend strong support to the coherent narrative identity hypothesis and the prediction that unique events are a critical feature of identity construction in emerging adulthood. PMID:25110125
Relations Between Narrative Coherence, Identity, and Psychological Well-Being in Emerging Adulthood.
Waters, Theodore E A; Fivush, Robyn
2015-08-01
The hypothesis that the ability to construct a coherent account of personal experience is reflective, or predictive, of psychological adjustment cuts across numerous domains of psychological science. It has been argued that coherent accounts of identity are especially adaptive. We tested these hypotheses by examining relations between narrative coherence of personally significant autobiographical memories and three psychological well-being components (i.e., purpose and meaning, positive self-view, positive relationships). We also examined the potential moderation of the relations between coherence and well-being by assessing the identity content of each narrative. We collected two autobiographical narratives of personally significant events from 103 undergraduate students and coded them for coherence and identity content. Two additional narratives about generic/recurring events were also collected and coded for coherence. We confirmed the prediction that constructing coherent autobiographical narratives is related to psychological well-being. Further, we found that this relation was moderated by the narratives' relevance to identity and that this moderation held after controlling for narrative ability more generally (i.e., coherence of generic/recurring events). These data lend strong support to the coherent narrative identity hypothesis and the prediction that unique events are a critical feature of identity construction in emerging adulthood.
Chemical compass model for avian magnetoreception as a quantum coherent device.
Cai, Jianming; Plenio, Martin B
2013-12-06
It is known that more than 50 species use the Earth's magnetic field for orientation and navigation. Intensive studies, particularly behavior experiments with birds, provide support for a chemical compass based on magnetically sensitive free radical reactions as a source of this sense. However, the fundamental question of how quantum coherence plays an essential role in such a chemical compass model of avian magnetoreception yet remains controversial. Here, we show that the essence of the chemical compass model can be understood in analogy to a quantum interferometer exploiting global quantum coherence rather than any subsystem coherence. Within the framework of quantum metrology, we quantify global quantum coherence and correlate it with the function of chemical magnetoreception. Our results allow us to understand and predict how various factors can affect the performance of a chemical compass from the unique perspective of quantum coherence assisted metrology. This represents a crucial step to affirm a direct connection between quantum coherence and the function of a chemical compass.
Long-lived quantum coherence and non-Markovianity of photosynthetic complexes
NASA Astrophysics Data System (ADS)
Chen, Hong-Bin; Lien, Jiun-Yi; Hwang, Chi-Chuan; Chen, Yueh-Nan
2014-04-01
Long-lived quantum coherence in photosynthetic pigment-protein complexes has recently been reported at physiological temperature. It has been pointed out that the discrete vibrational modes may be responsible for the long-lived coherence. Here, we propose an analytical non-Markovian model to explain the origin of the long-lived coherence in pigment-protein complexes. We show that the memory effect of the discrete vibrational modes produces a long oscillating tail in the coherence. We further use the recently proposed measure to quantify the non-Markovianity of the system and find out the prolonged coherence is highly correlated to it.
Generation of acoustic terahertz waves in hybrid InGaN/GaN quantum wells
NASA Astrophysics Data System (ADS)
Mahat, Meg; Llopis, Antonia; Choi, Tae Youl; Periera, Sergio; Watson, Ian; Neogi, Arup
2015-03-01
We have carried out differential transmission measurements on InGaN/ GaN quantum wells with Au nanoparticles inserted inside V-pits with high filling fraction. We have observed acoustic wave packets generated with multiple THz frequencies as 0.12 THz from GaN buffer layer, 0.22 THz from Au-InGaN multiple quantum wells region, 0.07 THz from sapphire substrate, and 0.17 THz mixed signals from the sample. These THz wave packets are observed as a result of generation of coherent acoustic phonons propagating in hybrid Au-InGaN quantum wells. The study of these acoustic THz wave generation is crucial for the imaging of nanostructures.
Wei, Wei; Dai, Ying; Niu, Chengwang; Huang, Baibiao
2015-01-01
In-plane transition-metal dichalcogenides (TMDs) quantum wells have been studied on the basis of first-principles density functional calculations to reveal how to control the electronic structures and the properties. In collection of quantum confinement, strain and intrinsic electric field, TMD quantum wells offer a diverse of exciting new physics. The band gap can be continuously reduced ascribed to the potential drop over the embedded TMD and the strain substantially affects the band gap nature. The true type-II alignment forms due to the coherent lattice and strong interface coupling suggesting the effective separation and collection of excitons. Interestingly, two-dimensional quantum wells of in-plane TMD can enrich the photoluminescence properties of TMD materials. The intrinsic electric polarization enhances the spin-orbital coupling and demonstrates the possibility to achieve topological insulator state and valleytronics in TMD quantum wells. In-plane TMD quantum wells have opened up new possibilities of applications in next-generation devices at nanoscale. PMID:26616013
Coherent confinement of plasmonic field in quantum dot-metallic nanoparticle molecules
NASA Astrophysics Data System (ADS)
Sadeghi, S. M.; Hatef, A.; Fortin-Deschenes, Simon; Meunier, Michel
2013-05-01
Interaction of a hybrid system consisting of a semiconductor quantum dot and a metallic nanoparticle (MNP) with a laser beam can replace the intrinsic plasmonic field of the MNP with a coherently normalized field (coherent-plasmonic or CP field). In this paper we show how quantum coherence effects in such a hybrid system can form a coherent barrier (quantum cage) that spatially confines the CP field. This allows us to coherently control the modal volume of this field, making it significantly smaller or larger than that of the intrinsic plasmonic field of the MNP. We investigate the spatial profiles of the CP field and discuss how the field barrier depends on the collective states of the hybrid system.
Coherent confinement of plasmonic field in quantum dot-metallic nanoparticle molecules.
Sadeghi, S M; Hatef, A; Fortin-Deschenes, Simon; Meunier, Michel
2013-05-24
Interaction of a hybrid system consisting of a semiconductor quantum dot and a metallic nanoparticle (MNP) with a laser beam can replace the intrinsic plasmonic field of the MNP with a coherently normalized field (coherent-plasmonic or CP field). In this paper we show how quantum coherence effects in such a hybrid system can form a coherent barrier (quantum cage) that spatially confines the CP field. This allows us to coherently control the modal volume of this field, making it significantly smaller or larger than that of the intrinsic plasmonic field of the MNP. We investigate the spatial profiles of the CP field and discuss how the field barrier depends on the collective states of the hybrid system.
A coherence preservation control strategy in cavity QED based on classical quantum feedback.
Li, Ming; Chen, Wei; Gao, Junli
2013-01-01
For eliminating the unexpected decoherence effect in cavity quantum electrodynamics (cavity QED), the transfer function of Rabi oscillation is derived theoretically using optical Bloch equations. In particular, the decoherence in cavity QED from the atomic spontaneous emission is especially considered. A feedback control strategy is proposed to preserve the coherence through Rabi oscillation stabilization. In the scheme, a classical quantum feedback channel for the quantum information acquisition is constructed via the quantum tomography technology, and a compensation system based on the root locus theory is put forward to suppress the atomic spontaneous emission and the associated decoherence. The simulation results have proved its effectiveness and superiority for the coherence preservation.
A Coherence Preservation Control Strategy in Cavity QED Based on Classical Quantum Feedback
Chen, Wei; Gao, Junli
2013-01-01
For eliminating the unexpected decoherence effect in cavity quantum electrodynamics (cavity QED), the transfer function of Rabi oscillation is derived theoretically using optical Bloch equations. In particular, the decoherence in cavity QED from the atomic spontaneous emission is especially considered. A feedback control strategy is proposed to preserve the coherence through Rabi oscillation stabilization. In the scheme, a classical quantum feedback channel for the quantum information acquisition is constructed via the quantum tomography technology, and a compensation system based on the root locus theory is put forward to suppress the atomic spontaneous emission and the associated decoherence. The simulation results have proved its effectiveness and superiority for the coherence preservation. PMID:23781154
Quantum confined Stark effect in Gaussian quantum wells: A tight-binding study
Ramírez-Morales, A.; Martínez-Orozco, J. C.; Rodríguez-Vargas, I.
2014-05-15
The main characteristics of the quantum confined Stark effect (QCSE) are studied theoretically in quantum wells of Gaussian profile. The semi-empirical tight-binding model and the Green function formalism are applied in the numerical calculations. A comparison of the QCSE in quantum wells with different kinds of confining potential is presented.
Electronic Enhancement of the Exciton Coherence Time in Charged Quantum Dots
Moody, G.; McDonald, C.; Feldman, A.; Harvey, T.; Mirin, R. P.; Silverman, K. L.
2016-01-01
Minimizing decoherence due to coupling of a quantum system to its fluctuating environment is at the forefront of quantum information and photonics research. Nature sets the ultimate limit, however, given by the strength of the system’s coupling to the electromagnetic field. Here, we establish the ability to electronically control this coupling and enhance the optical coherence time of the charged exciton transition in quantum dots embedded in a photonic waveguide. By manipulating the electronic wavefunctions through an applied lateral electric field, we increase the coherence time from ~ 1.4 ns to ~ 2.7 ns. Numerical calculations reveal that longer coherence arises from the separation of charge carriers by up to ~ 6 nm, which leads to a 30% weaker transition dipole moment. The ability to electronically control the coherence time opens new avenues for quantum communication and novel coupling schemes between distant qubits. PMID:26849614
Large-alphabet quantum key distribution with two-mode coherently correlated beams
NASA Astrophysics Data System (ADS)
Usenko, Vladyslav C.; Lev, Bohdan I.
2005-12-01
The large-alphabet quantum cryptography protocol based on the two-mode coherently correlated multi-photon beams is proposed. The alphabet extension for the protocol is shown to result in the increase of the QKD effectiveness and security.
Quantum Coherent Three-Terminal Thermoelectrics: Maximum Efficiency at Given Power Output
NASA Astrophysics Data System (ADS)
Whitney, Robert
2016-05-01
We consider the nonlinear scattering theory for three-terminal thermoelectric devices, used for power generation or refrigeration. Such a system is a quantum phase-coherent version of a thermocouple, and the theory applies to systems in which interactions can be treated at a mean-field level. We consider an arbitrary three-terminal system in any external magnetic field, including systems with broken time-reversal symmetry, such as chiral thermoelectrics, as well as systems in which the magnetic field plays no role. We show that the upper bound on efficiency at given power output is of quantum origin and is stricter than Carnot's bound. The bound is exactly the same as previously found for two-terminal devices, and can be achieved by three-terminal systems with or without broken time-reversal symmetry. Thus the bound appears to be universal for two-terminal and three-terminal (chiral and non-chiral) thermoelectrics.
Polaron mass of charge carriers in semiconductor quantum wells
Maslov, A. Yu. Proshina, O. V.
2015-10-15
A theory of the interaction of charge carriers with optical phonons in a quantum well is developed with consideration for interface optical phonons. The dependence of the polaron effective mass on the quantum-well dimensions and dielectric characteristics of barriers is analyzed in detail. It is shown that, in narrow quantum wells, a quasi-two-dimensional polaron can be formed. In this case, however, the interaction parameters are defined by the charge-carrier effective mass in the quantum well and by the frequencies of interface optical phonons. If barriers are made of a nonpolar material, the polaron effective mass depends on the quantum-well width. As the quantum-well width is increased, a new mechanism of enhancement of the electron–phonon interaction develops. The mechanism is implemented, if the optical phonon energy is equal to the energy of one of the electronic transitions. This condition yields an unsteady dependence of the polaron effective mass on the quantum-well width.
Quantum secure direct communication of digital and analog signals using continuum coherent states
NASA Astrophysics Data System (ADS)
Guerra, Antônio Geovan de Araújo Holanda; Rios, Francisco Franklin Sousa; Ramos, Rubens Viana
2016-11-01
In this work, we present optical schemes for secure direct quantum communication of digital and analog signals using continuum coherent states and frequency-dependent phase modulation. The main advantages of the proposed schemes are that they do not use entangled states and they can be implemented with today technology. The theory of quantum interference of continuum coherent state is described, and the optical setups for secure direct communication are presented and their securities are discussed.
Students' Conceptual Difficulties in Quantum Mechanics: Potential Well Problems
ERIC Educational Resources Information Center
Ozcan, Ozgur; Didis, Nilufer; Tasar, Mehmet Fatih
2009-01-01
In this study, students' conceptual difficulties about some basic concepts in quantum mechanics like one-dimensional potential well problems and probability density of tunneling particles were identified. For this aim, a multiple choice instrument named Quantum Mechanics Conceptual Test has been developed by one of the researchers of this study…
NASA Astrophysics Data System (ADS)
Jalali, H. R.; Tavassoly, M. K.
2013-07-01
In this paper the factorization method is used in order to obtain the eigenvalues and eigenfunctions of a quantum particle confined in a one-dimensional infinite well. The output results from the mentioned approach allow us to explore an appropriate new pair of raising and lowering operators corresponding to the physical system under consideration. From the symmetrical considerations, the connection between the obtained ladder operators with su(1,1) Lie algebra is explicitly established. Next, after the construction of Barut-Girardello and Gilmore-Perelomov representations of coherent states associated with the considered system, some of their important properties like the resolution of the identity including a few nonclassical features are illustrated in detail. Finally, a theoretical scheme for generation of the Gilmore-Perelomov type of coherent state via a generalized Jaynes-Cummings model is proposed.
Piezo-Phototronic Effect in a Quantum Well Structure.
Huang, Xin; Du, Chunhua; Zhou, Yongli; Jiang, Chunyan; Pu, Xiong; Liu, Wei; Hu, Weiguo; Chen, Hong; Wang, Zhong Lin
2016-05-24
With enhancements in the performance of optoelectronic devices, the field of piezo-phototronics has attracted much attention, and several theoretical works have been reported based on semiclassical models. At present, the feature size of optoelectronic devices are rapidly shrinking toward several tens of nanometers, which results in the quantum confinement effect. Starting from the basic piezoelectricity equation, Schrödinger equation, Poisson equation, and Fermi's golden rule, a self-consistent theoretical model is proposed to study the piezo-phototronic effect in the framework of perturbation theory in quantum mechanics. The validity and universality of this model are well-proven with photoluminescence measurements in a single GaN/InGaN quantum well and multiple GaN/InGaN quantum wells. This study provides important insight into the working principle of nanoscale piezo-phototronic devices as well as guidance for the future device design.
Preparing and preserving the double quantum coherence in NV- centers in Diamond at low fields
NASA Astrophysics Data System (ADS)
Moussa, Osama; Hincks, Ian; Cory, David G.
2014-12-01
We present and demonstrate a simple idea to excite and preserve the double-quantum-coherence (DQC) in the ground state of the electron spin of the Nitrogen-vacancy (NV) color center in diamond. We measure the coherence time of the DQC and compare it to the single quantum coherence time, both, in a Ramsey fringe experiment and under a Hahn echo sequence. We also demonstrate a robust pulse sequence based on the DANTE pulse sequence for selectively isolating the signal from the electron transitions conditional on the state of the always-present Nitrogen spin.
Optimal discrimination of M coherent states with a small quantum computer
Silva, Marcus P. da; Guha, Saikat; Dutton, Zachary
2014-12-04
The ability to distinguish between coherent states optimally plays in important role in the efficient usage of quantum resources for classical communication and sensing applications. While it has been known since the early 1970’s how to optimally distinguish between two coherent states, generalizations to larger sets of coherent states have so far failed to reach optimality. In this work we outline how optimality can be achieved by using a small quantum computer, building on recent proposals for optimal qubit state discrimination with multiple copies.
Coherent manipulation of quantum spin states in a single molecular nanomagnet
NASA Astrophysics Data System (ADS)
Wernsdorfer, Wolfgang
The endeavour of quantum electronics is driven by one of the most ambitious technological goals of today's scientists: the realization of an operational quantum computer (http://qurope.eu). We started to address this goal by the new research field of molecular quantum spintronics. The building blocks are magnetic molecules, i.e. well-defined spin qubits. We will discuss this still largely unexplored field and present our first results: For example, using a molecular spin-transistor, we achieved the electronic read-out of the nuclear spin of an individual metal atom embedded in an SMM. We could show very long spin lifetimes (>10 s). Using the hyperfine Stark effect, which transforms electric fields into local effective magnetic fields, we could not only tune the resonance frequency by several MHz, but also perform coherent quantum manipulations on a single nuclear qubit faster than a μs by means of electrical fields only, establishing the individual addressability of identical nuclear qubits. Using three different microwave frequencies, we could implement a simple four-level Grover algorithm. S. Thiele, F. Balestro, R. Ballou, S. Klyatskaya, M. Ruben, W. Wernsdorfer, Science 344, 1135 (2014).
GaAs/InAs Multi Quantum Well Solar Cell
2012-12-01
4. TITLE AND SUBTITLE GaAs /InAs MULTI QUANTUM WELL SOLAR CELL 5. FUNDING NUMBERS 6. AUTHOR(S) Evangelos Koletsios 7. PERFORMING ORGANIZATION NAME... GaAs /InAs MULTI QUANTUM WELL SOLAR CELL Evangelos Koletsios Lieutenant, Hellenic Navy B.S., Hellenic Naval Academy, 2001 Submitted in...similar, such as a GaAs crystal, which is mainly used in solar cells and it is described as a direct bandgap semiconductor. On the other hand, for
Quantum displacement receiver for M-ary phase-shift-keyed coherent states
Izumi, Shuro; Takeoka, Masahiro; Fujiwara, Mikio; Sasaki, Masahide; Pozza, Nicola Dalla; Assalini, Antonio
2014-12-04
We propose quantum receivers for 3- and 4-ary phase-shift-keyed (PSK) coherent state signals to overcome the standard quantum limit (SQL). Our receiver, consisting of a displacement operation and on-off detectors with or without feedforward, provides an error probability performance beyond the SQL. We show feedforward operations can tolerate the requirement for the detector specifications.
Quantum displacement receiver for M-ary phase-shift-keyed coherent states
NASA Astrophysics Data System (ADS)
Izumi, Shuro; Takeoka, Masahiro; Fujiwara, Mikio; Pozza, Nicola Dalla; Assalini, Antonio; Ema, Kazuhiro; Sasaki, Masahide
2014-12-01
We propose quantum receivers for 3- and 4-ary phase-shift-keyed (PSK) coherent state signals to overcome the standard quantum limit (SQL). Our receiver, consisting of a displacement operation and on-off detectors with or without feedforward, provides an error probability performance beyond the SQL. We show feedforward operations can tolerate the requirement for the detector specifications.
NASA Astrophysics Data System (ADS)
Teitelboim, Ayelet; Oron, Dan
2016-09-01
Upconversion (UC) is a nonlinear process in which two, or more, long wavelength photons are converted to a shorter wavelength photon. This process is based on sequential absorption of two or more photons, involving metastable, long lived intermediate energy states, thus is not restricted to upconversion of coherent laser radiation as a non-coherent process. Hence, requirements for UC processes are long lived excited states, a ladder like arrangement of energy levels and a mechanism inhibiting cooling of the hot charge carrier. UC holds great promise for bioimaging, enabling spatially resolved imaging in a scattering specimen and for photovoltaic devices as a mean to surpass the Shockley-Queisser efficiency limit. Here, we present a novel luminescence upconversion nano-system based on colloidal semiconductor double quantum dots, consisting of a NIR-emitting component and a visible emitting component separated by a tunneling barrier in a spherical onion-like geometry. These dual near-infrared and visible emitting core/shell/shell PbSe/CdSe/CdS nanocrystals are shown to upconvert a broad range of NIR wavelengths to visible emission at room temperature, covering a spectral range where there are practically no alternative upconversion systems. The synthesis is a three-step process, which enables versatility and tunability of both the visible emission color and the NIR absorption edge. Using this method one can achieve a range of desired upconverted emission peak positions with a suitable NIR band gap. The physical mechanism for upconversion in this structure, as well as possible extensions and improvements will be discussed. 1 (1) Teitelboim, A.; Oron, D. ACS Nano 2015, acsnano.5b05329.
Effects of Electric Fields on Quantum Well Intersubband Transitions
NASA Astrophysics Data System (ADS)
Harwit, Alex
A new technique is described to calculate the exact eigenstates of a quantum well superlattice of Gallium Arsenide/Aluminum Gallium Arsenide (GaAs/AlGaAs) in a perpendicular electric field. In the model the sloping potential of the conduction band is approximated by a series of small steps. Plane wave states are propagated across the quantum well structure and the quasi-eigenstates and quasi-eigenenergies are found at the transmission resonances of the system. We have used the technique to quantify the tunability of a new infrared modulator utilizing an intra-conduction band transition in the quantum well. Two such quantum well samples were grown by Molecular Beam Epitaxy (MBE). They consisted of 92 and 110 Angstrom GaAs quantum wells separated by AlGaAs barriers. Under the application of a perpendicular electric field, shifts were observed in the quantum well intersubband absorption energies, in good agreement with theoretical calculations. These tunable transitions can be applied to far infrared light modulators.
Kato, Akihito Tanimura, Yoshitaka
2015-08-14
We consider a system consisting of two interacting qubits that are individually coupled to separate heat baths at different temperatures. The quantum effects in heat transport are investigated in a numerically rigorous manner with a hierarchial equations of motion (HEOM) approach for non-perturbative and non-Markovian system-bath coupling cases under non-equilibrium steady-state conditions. For a weak interqubit interaction, the total system is regarded as two individually thermostatted systems, whereas for a strong interqubit interaction, the two-qubit system is regarded as a single system coupled to two baths. The roles of quantum coherence (or entanglement) between the two qubits (q-q coherence) and between the qubit and bath (q-b coherence) are studied through the heat current calculated for various strengths of the system-bath coupling and interqubit coupling for high and low temperatures. The same current is also studied using the time convolutionless (TCL) Redfield equation and using an expression derived from the Fermi golden rule (FGR). We find that the HEOM results exhibit turnover behavior of the heat current as a function of the system-bath coupling strength for all values of the interqubit coupling strength, while the results obtained with the TCL and FGR approaches do not exhibit such behavior, because they do not possess the capability of treating the q-b and q-q coherences. The maximum current is obtained in the case that the q-q coherence and q-b coherence are balanced in such a manner that coherence of the entire heat transport process is realized. We also find that the heat current does not follow Fourier’s law when the temperature difference is very large, due to the non-perturbative system-bath interactions.
Control of coherence transfer via tunneling in quadruple and multiple quantum dots
NASA Astrophysics Data System (ADS)
Tian, Si-Cong; Xing, En-Bo; Wan, Ren-Gang; Wang, Chun-Liang; Wang, Li-Jie; Shu, Shi-Li; Tong, Cun-Zhu; Wang, Li-Jun
2016-12-01
Transfer and manipulation of coherence among the ground state and indirect exciton states via tunneling in quadruple and multiple quantum dots is analyzed. By applying suitable amplitudes and sequences of the pump and tunneling pulses, a complete transfer of coherence or an arbitrary distribution of coherence of multiple states can be realized. The method, which is an adiabatic passage process, is different from previous works on quantum dot molecules in the way that the population can transfer from the ground state to the indirect exciton states without populating the direct exciton state, and thus no spontaneous emission occurs. This investigation can provide further insight to help the experimental development of coherence transfer in semiconductor structures, and may have potential applications in quantum information processing.
Quantum coherent energy transfer over varying pathways in single light-harvesting complexes.
Hildner, Richard; Brinks, Daan; Nieder, Jana B; Cogdell, Richard J; van Hulst, Niek F
2013-06-21
The initial steps of photosynthesis comprise the absorption of sunlight by pigment-protein antenna complexes followed by rapid and highly efficient funneling of excitation energy to a reaction center. In these transport processes, signatures of unexpectedly long-lived coherences have emerged in two-dimensional ensemble spectra of various light-harvesting complexes. Here, we demonstrate ultrafast quantum coherent energy transfer within individual antenna complexes of a purple bacterium under physiological conditions. We find that quantum coherences between electronically coupled energy eigenstates persist at least 400 femtoseconds and that distinct energy-transfer pathways that change with time can be identified in each complex. Our data suggest that long-lived quantum coherence renders energy transfer in photosynthetic systems robust in the presence of disorder, which is a prerequisite for efficient light harvesting.
Description of quantum coherence in thermodynamic processes requires constraints beyond free energy
Lostaglio, Matteo; Jennings, David; Rudolph, Terry
2015-01-01
Recent studies have developed fundamental limitations on nanoscale thermodynamics, in terms of a set of independent free energy relations. Here we show that free energy relations cannot properly describe quantum coherence in thermodynamic processes. By casting time-asymmetry as a quantifiable, fundamental resource of a quantum state, we arrive at an additional, independent set of thermodynamic constraints that naturally extend the existing ones. These asymmetry relations reveal that the traditional Szilárd engine argument does not extend automatically to quantum coherences, but instead only relational coherences in a multipartite scenario can contribute to thermodynamic work. We find that coherence transformations are always irreversible. Our results also reveal additional structural parallels between thermodynamics and the theory of entanglement. PMID:25754774
Modeling and Simulation of Semiconductor Quantum Well Structures and Lasers
NASA Technical Reports Server (NTRS)
Ning, Cun-Zheng; Saini, Subbash (Technical Monitor)
1998-01-01
In this talk I will cover two aspects of modeling and simulation efforts at NASA Ames Research Center. In the quantum well structure simulation, we typically start from the quantum mechanical calculation of the quantum well structures for the confined/and unconfined eigen states and functions. A bandstructure calculation of the k*p type is then performed for the confined valence states. This information is then used to computer the optical gain and refractive index of the quantum well structures by solving the linearized multiband semiconductor Bloch equations with the many-body interactions included. In our laser simulation, we typically solve the envelope equations for the laser field in space-time domain, coupled with a reduced set of material equations using the microscopic calculation of the first step. Finally I will show some examples of both aspects of simulation and modeling.
Quantum teleportation and entanglement swapping of matter qubits with coherent multiphoton states
NASA Astrophysics Data System (ADS)
Torres, J. M.; Bernád, J. Z.; Alber, G.
2014-07-01
Protocols for probabilistic entanglement-assisted quantum teleportation and for entanglement swapping of material qubits are presented. They are based on a protocol for postselective Bell- state projection which is capable of projecting two material qubits onto a Bell state with the help of ancillary coherent multiphoton states and postselection by balanced homodyne photodetection. Provided this photonic postselection is successful, we explore the theoretical possibilities of realizing unit-fidelity quantum teleportation and entanglement swapping with 25% success probability. This photon-assisted Bell projection is generated by coupling almost resonantly the two material qubits to single modes of the radiation field in two separate cavities in a Ramsey-type interaction sequence and by measuring the emerged field states in a balanced homodyne detection scenario. As these quantum protocols require basic tools of quantum state engineering of coherent multiphoton states and balanced homodyne photodetection, they may offer interesting perspectives in particular for current quantum optical applications in quantum information processing.
Fault-tolerant linear optical quantum computing with small-amplitude coherent States.
Lund, A P; Ralph, T C; Haselgrove, H L
2008-01-25
Quantum computing using two coherent states as a qubit basis is a proposed alternative architecture with lower overheads but has been questioned as a practical way of performing quantum computing due to the fragility of diagonal states with large coherent amplitudes. We show that using error correction only small amplitudes (alpha>1.2) are required for fault-tolerant quantum computing. We study fault tolerance under the effects of small amplitudes and loss using a Monte Carlo simulation. The first encoding level resources are orders of magnitude lower than the best single photon scheme.
Control of the probe absorption in coupled quantum wells in two dimensions
NASA Astrophysics Data System (ADS)
Kang, Chengxian; Ma, Yangcheng; Wang, Zhiping; Yu, Benli
2016-06-01
We investigate the probe absorption of a weak probe field in two dimensions (the so-called two-dimensional probe absorption) in an asymmetric two coupled quantum wells. It is found that, due to the joint quantum interference induced by the standing-wave and coherent coupling fields, the probe absorption can be easily controlled via adjusting the system parameters in two dimensions. Most importantly, the pattern of probe absorption can be localized at a particular position and the maximal probability of finding the pattern in one period of the standing-wave fields reaches unity by properly adjusting the system parameters. Thus, our scheme may provide some technological applications in solid-state optoelectronics and quantum information science.
Zhou, Xiaoji; Xu, Xu; Yin, Lan; Liu, W M; Chen, Xuzong
2010-07-19
We propose a new method of detecting quantum coherence of a Bose gas trapped in a one-dimensional optical lattice by measuring the light intensity from Raman scattering in cavity. After pump and displacement process, the intensity or amplitude of scattering light is different for different quantum states of a Bose gas, such as superfluid and Mott-Insulator states. This method can also be useful to detect quantum states of atoms with two components in an optical lattice.
Quantum-coherent coupling of a mechanical oscillator to an optical cavity mode.
Verhagen, E; Deléglise, S; Weis, S; Schliesser, A; Kippenberg, T J
2012-02-01
Optical laser fields have been widely used to achieve quantum control over the motional and internal degrees of freedom of atoms and ions, molecules and atomic gases. A route to controlling the quantum states of macroscopic mechanical oscillators in a similar fashion is to exploit the parametric coupling between optical and mechanical degrees of freedom through radiation pressure in suitably engineered optical cavities. If the optomechanical coupling is 'quantum coherent'--that is, if the coherent coupling rate exceeds both the optical and the mechanical decoherence rate--quantum states are transferred from the optical field to the mechanical oscillator and vice versa. This transfer allows control of the mechanical oscillator state using the wide range of available quantum optical techniques. So far, however, quantum-coherent coupling of micromechanical oscillators has only been achieved using microwave fields at millikelvin temperatures. Optical experiments have not attained this regime owing to the large mechanical decoherence rates and the difficulty of overcoming optical dissipation. Here we achieve quantum-coherent coupling between optical photons and a micromechanical oscillator. Simultaneously, coupling to the cold photon bath cools the mechanical oscillator to an average occupancy of 1.7 ± 0.1 motional quanta. Excitation with weak classical light pulses reveals the exchange of energy between the optical light field and the micromechanical oscillator in the time domain at the level of less than one quantum on average. This optomechanical system establishes an efficient quantum interface between mechanical oscillators and optical photons, which can provide decoherence-free transport of quantum states through optical fibres. Our results offer a route towards the use of mechanical oscillators as quantum transducers or in microwave-to-optical quantum links.
Experimental control of transport resonances in a coherent quantum rocking ratchet
Grossert, Christopher; Leder, Martin; Denisov, Sergey; Hänggi, Peter; Weitz, Martin
2016-01-01
The ratchet phenomenon is a means to get directed transport without net forces. Originally conceived to rectify stochastic motion and describe operational principles of biological motors, the ratchet effect can be used to achieve controllable coherent quantum transport. This transport is an ingredient of several perspective quantum devices including atomic chips. Here we examine coherent transport of ultra-cold atoms in a rocking quantum ratchet. This is realized by loading a rubidium atomic Bose–Einstein condensate into a periodic optical potential subjected to a biharmonic temporal drive. The achieved long-time coherence allows us to resolve resonance enhancement of the atom transport induced by avoided crossings in the Floquet spectrum of the system. By tuning the strength of the temporal modulations, we observe a bifurcation of a single resonance into a doublet. Our measurements reveal the role of interactions among Floquet eigenstates for quantum ratchet transport. PMID:26852803
Photonic quantum well composed of photonic crystal and quasicrystal
NASA Astrophysics Data System (ADS)
Xu, Shaohui; Zhu, Yiping; Wang, Lianwei; Yang, Pingxiong; Chu, Paul K.
2014-02-01
A photonic quantum well structure composed of photonic crystal and Fibonacci quasicrystal is investigated by analyzing the transmission spectra and electric field distributions. The defect band in the photonic well can form confined quantized photonic states that can change in the band-gap of the photonic barriers by varying the thickness ratio of the two stacking layers. The number of confined states can be tuned by adjusting the period of the photonic well. The photons traverse the photonic quantum well by resonance tunneling and the coupling effect leads to the high transmission intensity of the confined photonic states.
Quantum well in a microcavity with injected squeezed vacuum
Erenso, Daniel; Vyas, Reeta; Singh, Surendra
2003-01-01
A quantum well with a single exciton mode in a microcavity driven by squeezed vacuum is studied in the low exciton density regime. By solving the quantum Langevin equations, we study the intensity, spectrum, and intensity correlation function for the fluorescent light. An expression for the Q function of the field inside the cavity is derived from the solutions of the quantum Langevin equations. Using the Q function, the intracavity photon number distribution and the quadrature fluctuations for both the cavity and fluorescent fields are studied. Several interesting and new effects due to squeezed vacuum are found.
Controlled interaction of surface quantum-well electronic states.
Seufert, Knud; Auwärter, Willi; García de Abajo, F J; Ecija, David; Vijayaraghavan, Saranyan; Joshi, Sushobhan; Barth, Johannes V
2013-01-01
We report on the construction of well-defined surface quantum well arrangements by combining self-assembly protocols and molecular manipulation procedures. After the controlled removal of individual porphyrin molecules from dense-packed arrays on Ag(111), the surface state electrons are confined at the bare silver patches. These act as quantum wells that show well-defined unoccupied bound surface states. Scanning tunneling spectroscopy and complementary boundary element method calculations are performed to characterize the interaction between the bound states of adjacent quantum wells and reveal a hybridization of wave functions resulting in bonding and antibonding states. The interwell coupling can be tuned by the deliberate choice of the molecules acting as potential barriers. The fabrication method is shown to be ideally suited to engineer specific configurations as one-dimensional chains or two-dimensional artificial molecules.
Coherent singlet-triplet oscillations in a silicon-based double quantum dot.
Maune, B M; Borselli, M G; Huang, B; Ladd, T D; Deelman, P W; Holabird, K S; Kiselev, A A; Alvarado-Rodriguez, I; Ross, R S; Schmitz, A E; Sokolich, M; Watson, C A; Gyure, M F; Hunter, A T
2012-01-18
Silicon is more than the dominant material in the conventional microelectronics industry: it also has potential as a host material for emerging quantum information technologies. Standard fabrication techniques already allow the isolation of single electron spins in silicon transistor-like devices. Although this is also possible in other materials, silicon-based systems have the advantage of interacting more weakly with nuclear spins. Reducing such interactions is important for the control of spin quantum bits because nuclear fluctuations limit quantum phase coherence, as seen in recent experiments in GaAs-based quantum dots. Advances in reducing nuclear decoherence effects by means of complex control still result in coherence times much shorter than those seen in experiments on large ensembles of impurity-bound electrons in bulk silicon crystals. Here we report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure and show that this system has a nuclei-induced dephasing time of 360 nanoseconds, which is an increase by nearly two orders of magnitude over similar measurements in GaAs-based quantum dots. The degree of phase coherence observed, combined with fast, gated electrical initialization, read-out and control, should motivate future development of silicon-based quantum information processors.
Low-cost DH and quantum well laser array development
NASA Technical Reports Server (NTRS)
Linden, Kurt J.; Geoffroy, Leo M.; Pesarcik, Scott F.; Magee, Carl J.
1989-01-01
The intial results of a program aimed at developing low-cost diode laser arrays for use as solid-state laser pumps are reported. MOCVD is used to demonstrate excellent run-to-run reproducibility in emission wavelength, threshold current density, and quantum efficiency. For this first experimental series, J(th) values of approximately 1310 Amps/sq cm were obtained for broad-area unthinned devices from the growth runs. Differential quantum efficiencies of between 41 percent and 47 percent were measured on the non-facet-coated devices from all four runs. Single quantum well, separate confinement heterostructure lasers fabricated from wafers grown in the same MOCVD reactor exhibited near single-mode emission, with J(th) values of approximately 300 Amps/sq cm. Photoluminescence data confirm quantum well widths of 80 A and 150 A for two different MOCVD growth runs.
Quantum coherent phenomena in superconducting circuits and ultracold atoms
NASA Astrophysics Data System (ADS)
Mitra, Kaushik
This thesis consists of theoretical studies of superconducting qubits, and trapped bosons and fermions at ultracold temperature. In superconducting qubits I analyze the resonant properties and decoherence behavior of dc SQUID phase qubits, in which one junction acts as a phase qubit and the rest of the device provides isolation from dissipation and noise in the bias lead. Typically qubit states in phase qubits are detected by tunneling it to the voltage state. I propose an alternate non-destructive readout mechanism which relies on the difference in the magnetic flux through the SQUID loop due to state of the qubit. I also study decoherence effects in a dc SQUID phase qubit caused by the isolation circuit. When the frequency of the qubit is at least two times larger than the resonance frequency of the isolation circuit, I find that the decoherence time of the qubit is two orders of magnitude larger than the typical ohmic regime, where the frequency of the qubit is much smaller than the resonance frequency of the isolation circuit. This theory is extended to other similar superconducting quantum devices and has been applied to experiments from the group at the University of Maryland. I also demonstrate, theoretically, vacuum Rabi oscillations, analogous to circuit-QED, in superconducting qubits coupled to an environment with resonance. The result obtained gives an exact analytical expression for coherent oscillation of state between the system (the qubit) and the environment with resonance. Next I investigate ultracold atoms in harmonically confined optical lattices. They exhibit a 'wedding cake structure' of alternating Mott shells with different number of bosons per site. In regions between the Mott shells, a superfluid phase emerges at low temperatures which at higher temperatures becomes a normal Bose liquid. Using finite-temperature quantum field theoretic techniques, I find analytically the properties of the superfluid, Bose liquid, and Mott insulating regions
Dot-in-Well Quantum-Dot Infrared Photodetectors
NASA Technical Reports Server (NTRS)
Gunapala, Sarath; Bandara, Sumith; Ting, David; Hill, cory; Liu, John; Mumolo, Jason; Chang, Yia Chung
2008-01-01
Dot-in-well (DWELL) quantum-dot infrared photodetectors (QDIPs) [DWELL-QDIPs] are subjects of research as potentially superior alternatives to prior QDIPs. Heretofore, there has not existed a reliable method for fabricating quantum dots (QDs) having precise, repeatable dimensions. This lack has constituted an obstacle to the development of uniform, high-performance, wavelength-tailorable QDIPs and of focal-plane arrays (FPAs) of such QDIPs. However, techniques for fabricating quantum-well infrared photodetectors (QWIPs) having multiple-quantum- well (MQW) structures are now well established. In the present research on DWELL-QDIPs, the arts of fabrication of QDs and QWIPs are combined with a view toward overcoming the deficiencies of prior QDIPs. The longer-term goal is to develop focal-plane arrays of radiationhard, highly uniform arrays of QDIPs that would exhibit high performance at wavelengths from 8 to 15 m when operated at temperatures between 150 and 200 K. Increasing quantum efficiency is the key to the development of competitive QDIP-based FPAs. Quantum efficiency can be increased by increasing the density of QDs and by enhancing infrared absorption in QD-containing material. QDIPs demonstrated thus far have consisted, variously, of InAs islands on GaAs or InAs islands in InGaAs/GaAs wells. These QDIPs have exhibited low quantum efficiencies because the numbers of QD layers (and, hence, the areal densities of QDs) have been small typically five layers in each QDIP. The number of QD layers in such a device must be thus limited to prevent the aggregation of strain in the InAs/InGaAs/GaAs non-lattice- matched material system. The approach being followed in the DWELL-QDIP research is to embed In- GaAs QDs in GaAs/AlGaAs multi-quantum- well (MQW) structures (see figure). This material system can accommodate a large number of QD layers without excessive lattice-mismatch strain and the associated degradation of photodetection properties. Hence, this material
NASA Astrophysics Data System (ADS)
Ooi, C. H. Raymond; Beadie, Guy; Kattawar, George W.; Reintjes, John F.; Rostovtsev, Yuri; Zubairy, M. Suhail; Scully, Marlan O.
2005-08-01
Backscattered signal of coherent anti-Stokes Raman spectroscopy can be an extremely useful tool for remote identification of airborne particles, provided the signal is sufficiently large. We formulate a semiclassical theory of nonlinear scattering to estimate the number of detectable photons from a bacterial spore at a distance. For the first time, the theory incorporates enhanced quantum coherence via femtosecond pulses and a nonlinear process into the classical scattering problem. Our result shows a large backscattered signal in the far field, using typical parameters of an anthrax spore with maximally prepared vibrational coherence. Using train pulses of 1 kHz of repetition rate each with energy of 10 mJ, we estimate that about 107 photons can be detected by a 1 m diameter detector placed 1 km away from the spore in the backward scattering direction. The result shows the feasibility of developing a real time remote detection of hazardous microparticles in the atmosphere, particularly biopathogenic spores.
Liu, Lei; Li, Yu-Xian; Zhang, Ying-Tao; Liu, Jian-Jun
2014-01-14
The transport properties in graphene-based asymmetric double velocity well (Fermi velocity inside the well less than that outside the well) and electrostatic well structures are investigated using the transfer matrix method. The results show that quantum beats occur in the oscillations of the conductance for asymmetric double velocity wells. The beating effect can also be found in asymmetric double electrostatic wells, but only if the widths of the two wells are different. The beat frequency for the asymmetric double well is exactly equal to the frequency difference between the oscillation rates in two isolated single wells with the same structures as the individual wells in the double well structure. A qualitative interpretation is proposed based on the fact that the resonant levels depend upon the sizes of the quantum wells. The beating behavior can provide a new way to identify the symmetry of double well structures.
Origin of long-lived quantum coherence and excitation dynamics in pigment-protein complexes
NASA Astrophysics Data System (ADS)
Zhang, Zhedong; Wang, Jin
2016-11-01
We explore the mechanism for the long-lived quantum coherence by considering the discrete phonon modes: these vibrational modes effectively weaken the exciton-environment interaction, due to the new composite (polaron) formed by excitons and vibrons. This subsequently demonstrates the role of vibrational coherence which greatly contributes to long-lived feature of the excitonic coherence that has been observed in femtosecond experiments. The estimation of the timescale of coherence elongated by vibrational modes is given in an analytical manner. To test the validity of our theory, we study the pigment-protein complex in detail by exploring the energy transfer and coherence dynamics. The ground-state vibrational coherence generated by incoherent radiations is shown to be long-survived and is demonstrated to be significant in promoting the excitation energy transfer. This is attributed to the nonequilibriumness of the system caused by the detailed-balance-breaking, which funnels the downhill migration of excitons.
Origin of long-lived quantum coherence and excitation dynamics in pigment-protein complexes
Zhang, Zhedong; Wang, Jin
2016-01-01
We explore the mechanism for the long-lived quantum coherence by considering the discrete phonon modes: these vibrational modes effectively weaken the exciton-environment interaction, due to the new composite (polaron) formed by excitons and vibrons. This subsequently demonstrates the role of vibrational coherence which greatly contributes to long-lived feature of the excitonic coherence that has been observed in femtosecond experiments. The estimation of the timescale of coherence elongated by vibrational modes is given in an analytical manner. To test the validity of our theory, we study the pigment-protein complex in detail by exploring the energy transfer and coherence dynamics. The ground-state vibrational coherence generated by incoherent radiations is shown to be long-survived and is demonstrated to be significant in promoting the excitation energy transfer. This is attributed to the nonequilibriumness of the system caused by the detailed-balance-breaking, which funnels the downhill migration of excitons. PMID:27876861
NASA Astrophysics Data System (ADS)
Motazedifard, Ali; Bemani, F.; Naderi, M. H.; Roknizadeh, R.; Vitali, D.
2016-07-01
We propose and analyse a feasible experimental scheme for a quantum force sensor based on the elimination of backaction noise through coherent quantum noise cancellation (CQNC) in a hybrid atom-cavity optomechanical setup assisted with squeezed vacuum injection. The force detector, which allows for a continuous, broadband detection of weak forces well below the standard quantum limit (SQL), is formed by a single optical cavity simultaneously coupled to a mechanical oscillator and to an ensemble of ultracold atoms. The latter acts as a negative-mass oscillator so that atomic noise exactly cancels the backaction noise from the mechanical oscillator due to destructive quantum interference. Squeezed vacuum injection enforces this cancellation and allows sub-SQL sensitivity to be reached in a very wide frequency band, and at much lower input laser powers.
NASA Astrophysics Data System (ADS)
Cooper, Merlin; Slade, Eirion; Karpiński, Michał; Smith, Brian J.
2015-03-01
Conditional quantum optical processes enable a wide range of technologies from generation of highly non-classical states to implementation of quantum logic operations. The process fidelity that can be achieved in a realistic implementation depends on a number of system parameters. Here we experimentally examine Fock state filtration, a canonical example of a broad class of conditional quantum operations acting on a single optical field mode. This operation is based upon interference of the mode to be manipulated with an auxiliary single-photon state at a beam splitter, resulting in the entanglement of the two output modes. A conditional projective measurement onto a single photon state at one output mode heralds the success of the process. This operation, which implements a measurement-induced nonlinearity, is capable of suppressing particular photon-number probability amplitudes of an arbitrary quantum state. We employ coherent-state process tomography to determine the precise operation realized in our experiment, which is mathematically represented by a process tensor. To identify the key sources of experimental imperfection, we develop a realistic model of the process and identify three main contributions that significantly hamper its efficacy. The experimentally reconstructed process tensor is compared with the model, yielding a fidelity better than 0.95. This enables us to identify three key challenges to overcome in realizing a filter with optimal performance—namely the single-photon nature of the auxiliary state, high mode overlap of the optical fields involved, and the need for photon-number-resolving detection when heralding. The results show that the filter does indeed exhibit a non-linear response as a function of input photon number and preserves the phase relation between Fock layers of the output state, providing promise for future applications.
Theoretical examination of quantum coherence in a photosynthetic system at physiological temperature
Ishizaki, Akihito; Fleming, Graham R.
2009-01-01
The observation of long-lived electronic coherence in a photosynthetic pigment–protein complex, the Fenna–Matthews–Olson (FMO) complex, is suggestive that quantum coherence might play a significant role in achieving the remarkable efficiency of photosynthetic electronic energy transfer (EET), although the data were acquired at cryogenic temperature [Engel GS, et al. (2007) Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems. Nature 446:782–786]. In this paper, the spatial and temporal dynamics of EET through the FMO complex at physiological temperature are investigated theoretically. The numerical results reveal that quantum wave-like motion persists for several hundred femtoseconds even at physiological temperature, and suggest that the FMO complex may work as a rectifier for unidirectional energy flow from the peripheral light-harvesting antenna to the reaction center complex by taking advantage of quantum coherence and the energy landscape of pigments tuned by the protein scaffold. A potential role of quantum coherence is to overcome local energetic traps and aid efficient trapping of electronic energy by the pigments facing the reaction center complex. PMID:19815512
Coherent chemical kinetics as quantum walks. I. Reaction operators for radical pairs.
Chia, A; Tan, K C; Pawela, Ł; Kurzyński, P; Paterek, T; Kaszlikowski, D
2016-03-01
Classical chemical kinetics uses rate-equation models to describe how a reaction proceeds in time. Such models are sufficient for describing state transitions in a reaction where coherences between different states do not arise, in other words, a reaction that contains only incoherent transitions. A prominent example of a reaction containing coherent transitions is the radical-pair model. The kinetics of such reactions is defined by the so-called reaction operator that determines the radical-pair state as a function of intermediate transition rates. We argue that the well-known concept of quantum walks from quantum information theory is a natural and apt framework for describing multisite chemical reactions. By composing Kraus maps that act only on two sites at a time, we show how the quantum-walk formalism can be applied to derive a reaction operator for the standard avian radical-pair reaction. Our reaction operator predicts the same recombination dephasing rate as the conventional Haberkorn model, which is consistent with recent experiments [K. Maeda et al., J. Chem. Phys. 139, 234309 (2013)], in contrast to previous work by Jones and Hore [J. A. Jones and P. J. Hore, Chem. Phys. Lett. 488, 90 (2010)]. The standard radical-pair reaction has conventionally been described by either a normalized density operator incorporating both the radical pair and reaction products or a trace-decreasing density operator that considers only the radical pair. We demonstrate a density operator that is both normalized and refers only to radical-pair states. Generalizations to include additional dephasing processes and an arbitrary number of sites are also discussed.
NASA Astrophysics Data System (ADS)
Hutchison, Robert B.; Huntley, James J. A.; Jin, Haoran; Shapiro, Joseph I.
1992-12-01
An investigation into the signal suppression behavior of the paramagnetic shift and relaxation reagents, Dy(P3O10)27- and Gd(P3O10)27-, with regard to their use in the nuclear magnetic resonance spectroscopic study of sodium has been performed. Measurements of T1 and T2 relaxation time constants of sodium in normal saline, Krebs-Henseleit buffer, and human blood serum, as a function of concentration of these reagents showed that, although closely coupled in the saline and K-H buffer environments, in plasma T1 and T2 become decoupled, transverse relaxation dominating in comparison to longitudinal relaxation. Linewidth measurements further suggest that relaxation in the plasma milieu is controlled primarily by inherent T2 relaxation, rather than by field inhomogeneity or diffusion effects. Quantitative single-quantum (1Q) and double-quantum (2Q) intensity measurements, biexponential T2 relaxation measurements, and parametric studies of the preparation time of the 2Q pulse sequence, were obtained in suspensions of bovine serum albumin and human erythrocytes. The observed suppression of sodium 2Q coherence by paramagnetic shift and relaxation reagents was found to exhibit a complex behavior in albumin solutions, involving the biexponential T2 decay to be expected during the preparation time of the 2Q filter pulse sequence, as well as the optimum preparation time for production of the double-quantum coherence itself. The controlling factor for both of these effects is the biexponential amplitude function in the expression for the transverse magnetization observed following application of the 2Q pulse sequence. This in turn is determined entirely by the values for the slow and fast components of biexponential relaxation in sodium, which themselves depend upon the concentration of the macromolecular binding sites for quadrupolar interaction. A similar behavior has been observed in suspensions of human erythrocytes.
Coherent chemical kinetics as quantum walks. I. Reaction operators for radical pairs
NASA Astrophysics Data System (ADS)
Chia, A.; Tan, K. C.; Pawela, Ł.; Kurzyński, P.; Paterek, T.; Kaszlikowski, D.
2016-03-01
Classical chemical kinetics uses rate-equation models to describe how a reaction proceeds in time. Such models are sufficient for describing state transitions in a reaction where coherences between different states do not arise, in other words, a reaction that contains only incoherent transitions. A prominent example of a reaction containing coherent transitions is the radical-pair model. The kinetics of such reactions is defined by the so-called reaction operator that determines the radical-pair state as a function of intermediate transition rates. We argue that the well-known concept of quantum walks from quantum information theory is a natural and apt framework for describing multisite chemical reactions. By composing Kraus maps that act only on two sites at a time, we show how the quantum-walk formalism can be applied to derive a reaction operator for the standard avian radical-pair reaction. Our reaction operator predicts the same recombination dephasing rate as the conventional Haberkorn model, which is consistent with recent experiments [K. Maeda et al., J. Chem. Phys. 139, 234309 (2013), 10.1063/1.4844355], in contrast to previous work by Jones and Hore [J. A. Jones and P. J. Hore, Chem. Phys. Lett. 488, 90 (2010), 10.1016/j.cplett.2010.01.063]. The standard radical-pair reaction has conventionally been described by either a normalized density operator incorporating both the radical pair and reaction products or a trace-decreasing density operator that considers only the radical pair. We demonstrate a density operator that is both normalized and refers only to radical-pair states. Generalizations to include additional dephasing processes and an arbitrary number of sites are also discussed.
Generation of a macroscopic entangled coherent state using quantum memories in circuit QED
Liu, Tong; Su, Qi-Ping; Xiong, Shao-Jie; Liu, Jin-Ming; Yang, Chui-Ping; Nori, Franco
2016-01-01
W-type entangled states can be used as quantum channels for, e.g., quantum teleportation, quantum dense coding, and quantum key distribution. In this work, we propose a way to generate a macroscopic W-type entangled coherent state using quantum memories in circuit QED. The memories considered here are nitrogen-vacancy center ensembles (NVEs), each located in a different cavity. This proposal does not require initially preparing each NVE in a coherent state instead of a ground state, which should significantly reduce its experimental difficulty. For most of the operation time, each cavity remains in a vacuum state, thus decoherence caused by the cavity decay and the unwanted inter-cavity crosstalk are greatly suppressed. Moreover, only one external-cavity coupler qubit is needed, which simplifies the circuit. PMID:27562055
Generation of a macroscopic entangled coherent state using quantum memories in circuit QED.
Liu, Tong; Su, Qi-Ping; Xiong, Shao-Jie; Liu, Jin-Ming; Yang, Chui-Ping; Nori, Franco
2016-08-26
W-type entangled states can be used as quantum channels for, e.g., quantum teleportation, quantum dense coding, and quantum key distribution. In this work, we propose a way to generate a macroscopic W-type entangled coherent state using quantum memories in circuit QED. The memories considered here are nitrogen-vacancy center ensembles (NVEs), each located in a different cavity. This proposal does not require initially preparing each NVE in a coherent state instead of a ground state, which should significantly reduce its experimental difficulty. For most of the operation time, each cavity remains in a vacuum state, thus decoherence caused by the cavity decay and the unwanted inter-cavity crosstalk are greatly suppressed. Moreover, only one external-cavity coupler qubit is needed, which simplifies the circuit.
Generation of a macroscopic entangled coherent state using quantum memories in circuit QED
NASA Astrophysics Data System (ADS)
Liu, Tong; Su, Qi-Ping; Xiong, Shao-Jie; Liu, Jin-Ming; Yang, Chui-Ping; Nori, Franco
2016-08-01
W-type entangled states can be used as quantum channels for, e.g., quantum teleportation, quantum dense coding, and quantum key distribution. In this work, we propose a way to generate a macroscopic W-type entangled coherent state using quantum memories in circuit QED. The memories considered here are nitrogen-vacancy center ensembles (NVEs), each located in a different cavity. This proposal does not require initially preparing each NVE in a coherent state instead of a ground state, which should significantly reduce its experimental difficulty. For most of the operation time, each cavity remains in a vacuum state, thus decoherence caused by the cavity decay and the unwanted inter-cavity crosstalk are greatly suppressed. Moreover, only one external-cavity coupler qubit is needed, which simplifies the circuit.
Fundamental limits to performance of quantum well infrared detectors
NASA Technical Reports Server (NTRS)
Yariv, Amnon; Kinch, Michael; Borenstain, S.; Grave, I.
1990-01-01
Radiometric, density of states (material), and thermal considerations are used to obtain the figure of merit of the quantum-well GaAs/GaAlAs infrared detectors described by Smith et. al. The results are compared with HgCdTe, the present industry standard, as well as with recent experiments at other laboratories.
Chiral quantum optics with V-level atoms and coherent quantum feedback
NASA Astrophysics Data System (ADS)
Guimond, Pierre-Olivier; Pichler, Hannes; Rauschenbeutel, Arno; Zoller, Peter
2016-09-01
We study the dissipative dynamics of an atom in a V-level configuration driven by lasers and coupled to a semi-infinite waveguide. The coupling to the waveguide is chiral, in that each transition interacts only with the modes propagating in a given direction, and this direction is opposite for the two transitions. The waveguide is terminated by a mirror which coherently feeds the photon stream emitted by one transition back to the atom. First, we are interested in the dynamics of the atom in the Markovian limit where the time delay in the feedback is negligible. Specifically, we study the conditions under which the atom evolves towards a pure "dark" stationary state, where the photons emitted by both transitions interfere destructively thanks to the coherent feedback, and the overall emission vanishes. This is a single-atom analog of the quantum dimer, where a pair of laser-driven two-level atoms is coupled to a unidirectional waveguide and dissipates towards a pure entangled dark state. Our setup should be feasible with current state-of-the-art experiments. Second, we extend our study to non-Markovian regimes and investigate the effect of the feedback retardation on the steady state.
Atomistic Analysis of Room Temperature Quantum Coherence in Two-Dimensional CdSe Nanostructures.
Pal, Sougata; Nijjar, Parmeet; Frauenheim, Thomas; Prezhdo, Oleg V
2017-03-02
Recent experiments on CdSe nanoplatelets synthesized with precisely controlled thickness that eliminates ensemble disorder have allowed accurate measurement of quantum coherence at room temperature. Matching exactly the CdSe cores of the experimentally studied particles and considering several defects, we establish the atomistic origins of the loss of coherence between heavy and light hole excitations in two-dimensional CdSe and CdSe/CdZnS core/shell structures. The coherence times obtained using molecular dynamics based on tight-binding density functional theory are in excellent agreement with the measured values. We show that a long coherence time is a consequence of both small fluctuations in the energy gap between the excited state pair, which is much less than thermal energy, and a slow decay of correlation between the energies of the two states. Anionic defects at the core/shell interface have little effect on the coherence lifetime, while cationic defects strongly perturb the electronic structure, destroying the experimentally observed coherence. By coupling to the same phonon modes, the heavy and light holes synchronize their energy fluctuations, facilitating long-lived coherence. We further demonstrate that the electronic excitations are localized close to the surface of these narrow nanoscale systems, and therefore, they couple most strongly to surface acoustic phonons. The established features of electron-phonon coupling and the influence of defects, surfaces, and core/shell interfaces provide important insights into quantum coherence in nanoscale materials in general.
Relaxation of hot excitons in CdZnSe/ZnSe quantum wells and quantum dots
NASA Astrophysics Data System (ADS)
Spiegel, R.; Bacher, G.; Breitwieser, O.; Forchel, A.; Jobst, B.; Hommel, D.; Landwehr, G.
1998-05-01
The relaxation dynamics of hot excitons was studied in (Zn,Cd)Se/ZnSe quantum wells and quantum dots. A fast population of the radiative excitonic ground state occurs for an excitation excess energy corresponding to an integer number of optical phonon energies. This is indicated by a spectrally narrow photoluminescence peak observed immediately after the exciting laser pulse. Spatial diffusion of excitons, controlled by the interaction between excitons and acoustic phonons, causes a distinct linewidth broadening with increasing delay time in quantum wells. In contrast, this process is found to be strongly suppressed in quantum dots.
Phase-Sensitive Coherence and the Classical-Quantum Boundary in Ghost Imaging
NASA Technical Reports Server (NTRS)
Erkmen, Baris I.; Hardy, Nicholas D.; Venkatraman, Dheera; Wong, Franco N. C.; Shapiro, Jeffrey H.
2011-01-01
The theory of partial coherence has a long and storied history in classical statistical optics. the vast majority of this work addresses fields that are statistically stationary in time, hence their complex envelopes only have phase-insensitive correlations. The quantum optics of squeezed-state generation, however, depends on nonlinear interactions producing baseband field operators with phase-insensitive and phase-sensitive correlations. Utilizing quantum light to enhance imaging has been a topic of considerable current interest, much of it involving biphotons, i.e., streams of entangled-photon pairs. Biphotons have been employed for quantum versions of optical coherence tomography, ghost imaging, holography, and lithography. However, their seemingly quantum features have been mimicked with classical-sate light, questioning wherein lies the classical-quantum boundary. We have shown, for the case of Gaussian-state light, that this boundary is intimately connected to the theory of phase-sensitive partial coherence. Here we present that theory, contrasting it with the familiar case of phase-insensitive partial coherence, and use it to elucidate the classical-quantum boundary of ghost imaging. We show, both theoretically and experimentally, that classical phase-sensitive light produces ghost imaging most closely mimicking those obtained in biphotons, and we derived the spatial resolution, image contrast, and signal-to-noise ratio of a standoff-sensing ghost imager, taking into account target-induced speckle.
Investigation of heterodyne performance of quantum-well detectors. Final report
Simpson, M.L.; Hutchinson, D.P.; Calabretta, J.
1994-09-23
The purpose of this Cooperative Research and Development Agreement (CRADA) between Martin Marietta Energy Systems Inc., (Contractor) and Martin Marietta Electronic Missles (Participant) is the determination of the heterodyne characteristics of quantum-well detectors. The Participant has developed a quantum-well infrared imaging video detector with very low light level characteristics. A further improvement in low-level infrared detection could be achieved if this device can be operated in the coherent or heterodyne mode. A major program in the Physics Division of Oak Ridge National Laboratory (ORNL) presently uses individual heterodyne infrared detectors in a system under development for fusion diagnostics. An imaging infrared heterodyne detector would represent a major breakthrough in this area and would have major implications for other plasma diagnostic programs. The Participant is also studying the application of this device in the area of laser radar.
Estimating the Coherence of Noise in Quantum Control of a Solid-State Qubit
NASA Astrophysics Data System (ADS)
Feng, Guanru; Wallman, Joel J.; Buonacorsi, Brandon; Cho, Franklin H.; Park, Daniel K.; Xin, Tao; Lu, Dawei; Baugh, Jonathan; Laflamme, Raymond
2016-12-01
To exploit a given physical system for quantum information processing, it is critical to understand the different types of noise affecting quantum control. Distinguishing coherent and incoherent errors is extremely useful as they can be reduced in different ways. Coherent errors are generally easier to reduce at the hardware level, e.g., by improving calibration, whereas some sources of incoherent errors, e.g., T2* processes, can be reduced by engineering robust pulses. In this work, we illustrate how purity benchmarking and randomized benchmarking can be used together to distinguish between coherent and incoherent errors and to quantify the reduction in both of them due to using optimal control pulses and accounting for the transfer function in an electron spin resonance system. We also prove that purity benchmarking provides bounds on the optimal fidelity and diamond norm that can be achieved by correcting the coherent errors through improving calibration.
Quantum interference between a single-photon Fock state and a coherent state
NASA Astrophysics Data System (ADS)
Windhager, A.; Suda, M.; Pacher, C.; Peev, M.; Poppe, A.
2011-04-01
We derive analytical expressions for the single mode quantum field state at the individual output ports of a beam splitter when a single-photon Fock state and a coherent state are incident on the input ports. The output states turn out to be a statistical mixture between a displaced Fock state and a coherent state. Consequently we are able to find an analytical expression for the corresponding Wigner function. Because of the generality of our calculations the obtained results are valid for all passive and lossless optical four port devices. We show further how the results can be adapted to the case of the Mach-Zehnder interferometer. In addition we consider the case for which the single-photon Fock state is replaced with a general input state: a coherent input state displaces each general quantum state at the output port of a beam splitter with the displacement parameter being the amplitude of the coherent state.
Anisotropic transport in modulation doped quantum well structures
NASA Technical Reports Server (NTRS)
Radulescu, D. C.; Wicks, G. W.; Schaff, W. J.; Calawa, A. R.; Eastman, L. F.
1987-01-01
The degree of anisotropy in the anisotropic electron transport that has been observed in GaAs modulation-doped quantum wells grown by MBE on Al(0.3)Ga(0.7)As is related to the thickness and growth parameters of this substrate, which is grown just prior to the inverted interface. It is presently observed that the inverted interface has an anisotropic roughness which affects the 77 K low field electron transport parallel to the interface, and gives rise to anisotropic electron scattering in the GaAs modulation-doped quantum well.
Microscopic Theory and Simulation of Quantum-Well Intersubband Absorption
NASA Technical Reports Server (NTRS)
Li, Jianzhong; Ning, C. Z.
2004-01-01
We study the linear intersubband absorption spectra of a 15 nm InAs quantum well using the intersubband semiconductor Bloch equations with a three-subband model and a constant dephasing rate. We demonstrate the evolution of intersubband absorption spectral line shape as a function of temperature and electron density. Through a detailed examination of various contributions, such as the phase space filling effects, the Coulomb many-body effects and the non-parabolicity effect, we illuminate the underlying physics that shapes the spectra. Keywords: Intersubband transition, linear absorption, semiconductor heterostructure, InAs quantum well
Detection of electromagnetic radiation using micromechanical multiple quantum wells structures
Datskos, Panagiotis G [Knoxville, TN; Rajic, Slobodan [Knoxville, TN; Datskou, Irene [Knoxville, TN
2007-07-17
An apparatus and method for detecting electromagnetic radiation employs a deflectable micromechanical apparatus incorporating multiple quantum wells structures. When photons strike the quantum-well structure, physical stresses are created within the sensor, similar to a "bimetallic effect." The stresses cause the sensor to bend. The extent of deflection of the sensor can be measured through any of a variety of conventional means to provide a measurement of the photons striking the sensor. A large number of such sensors can be arranged in a two-dimensional array to provide imaging capability.
Superradiant modes in Fibonacci quantum wells under resonant conditions
NASA Astrophysics Data System (ADS)
Chang, C. H.; Tsao, C. W.; Hsueh, W. J.
2014-11-01
It is first presented that superradiant modes exist in Fibonacci quantum wells within the exact regions that are obtained using the gap map diagram, rather than the traditional resonant Bragg condition. The results show that three limited regions are derived from the diagram, which correspond to bandgaps with widths that differ from each other. The regions in which the superradiant modes do not occur are also defined clearly. Moreover, the proposed method can be used to determine whether superradiant modes occur in multiple quantum wells that have non-periodical arrangements, including quasiperiodic sequences and correlated disorder sequences.
All-electrical coherent control of the exciton states in a single quantum dot
NASA Astrophysics Data System (ADS)
Boyer de La Giroday, A.; Bennett, A. J.; Pooley, M. A.; Stevenson, R. M.; Sköld, N.; Patel, R. B.; Farrer, I.; Ritchie, D. A.; Shields, A. J.
2010-12-01
We demonstrate high-fidelity reversible transfer of quantum information from the polarization of photons into the spin state of an electron-hole pair in a semiconductor quantum dot. Moreover, spins are electrically manipulated on a subnanosecond time scale, allowing us to coherently control their evolution. By varying the area of the electrical pulse, we demonstrate phase-shift and spin-flip gate operations with near-unity fidelities. Our system constitutes a controllable quantum interface between flying and stationary qubits, an enabling technology for quantum logic in the solid state.
Vibration-induced coherence enhancement of the performance of a biological quantum heat engine
NASA Astrophysics Data System (ADS)
Chen, Hong-Bin; Chiu, Pin-Yi; Chen, Yueh-Nan
2016-11-01
Photosynthesis has been a long-standing research interest due to its fundamental importance. Recently, studies on photosynthesis processes also have inspired attention from a thermodynamical aspect when considering photosynthetic apparatuses as biological quantum heat engines. Quantum coherence is shown to play a crucial role in enhancing the performance of these quantum heat engines. Based on the experimentally reported structure, we propose a quantum heat engine model with a non-Markovian vibrational mode. We show that one can obtain a performance enhancement easily for a wide range of parameters in the presence of the vibrational mode. Our results provide insights into the photosynthetic processes and a design principle mimicking natural organisms.
Trading coherence and entropy by a quantum Maxwell demon
NASA Astrophysics Data System (ADS)
Lebedev, A. V.; Oehri, D.; Lesovik, G. B.; Blatter, G.
2016-11-01
The second law of thermodynamics states that the entropy of a closed system is nondecreasing. Discussing the second law in the quantum world poses different challenges and provides different opportunities, involving fundamental quantum-information-theoretic questions and interesting quantum-engineered devices. In quantum mechanics, systems with an evolution described by a so-called unital quantum channel evolve with a nondecreasing entropy. Here, we seek the opposite, a system described by a nonunital and, furthermore, energy-conserving channel that describes a system whose entropy decreases with time. We propose a setup involving a mesoscopic four-lead scatterer augmented by a microenvironment in the form of a spin that realizes this goal. Within this nonunital and energy-conserving quantum channel, the microenvironment acts with two noncommuting operations on the system in an autonomous way. We find that the process corresponds to a partial exchange or swap between the system and environment quantum states, with the system's entropy decreasing if the environment's state is more pure. This entropy-decreasing process is naturally expressed through the action of a quantum Maxwell demon and we propose a quantum-thermodynamic engine with four qubits that extracts work from a single heat reservoir when provided with a reservoir of pure qubits. The special feature of this engine, which derives from the energy conservation in the nonunital quantum channel, is its separation into two cycles, a working cycle and an entropy cycle, allowing us to run this engine with no local waste heat.
NASA Astrophysics Data System (ADS)
Hanna, S.; Eichenberg, B.; Firsov, D. A.; Vorobjev, L. E.; Ustinov, V. M.; Seilmeier, A.
2016-01-01
The coherent light-matter interaction in a 4-level cascade-type subband system of an asymmetric GaAs/AlGaAs quantum well structure is studied in pump-probe transmission experiments with picosecond (ps) time resolution. Coupling two excited subbands by an intense mid-infrared laser pulse at low sample temperatures is found to result in a substantially increased transparency of the fundamental e1-e2 transition. We find a reduction of the absorption coefficient by ~80%, which is one of the most pronounced electromagnetically induced transparency in solid state systems observed so far.
Quantum coherent tractor beam effect for atoms trapped near a nanowaveguide.
Sadgrove, Mark; Wimberger, Sandro; Nic Chormaic, Síle
2016-07-21
We propose several schemes to realize a tractor beam effect for ultracold atoms in the vicinity of a few-mode nanowaveguide. Atoms trapped near the waveguide are transported in a direction opposite to the guided mode propagation direction. We analyse three specific examples for ultracold (23)Na atoms trapped near a specific nanowaveguide (i.e. an optical nanofibre): (i) a conveyor belt-type tractor beam effect, (ii) an accelerator tractor beam effect, and (iii) a quantum coherent tractor beam effect, all of which can effectively pull atoms along the nanofibre toward the light source. This technique provides a new tool for controlling the motion of particles near nanowaveguides with potential applications in the study of particle transport and binding as well as atom interferometry.
Quantum coherent tractor beam effect for atoms trapped near a nanowaveguide
Sadgrove, Mark; Wimberger, Sandro; Nic Chormaic, Síle
2016-01-01
We propose several schemes to realize a tractor beam effect for ultracold atoms in the vicinity of a few-mode nanowaveguide. Atoms trapped near the waveguide are transported in a direction opposite to the guided mode propagation direction. We analyse three specific examples for ultracold 23Na atoms trapped near a specific nanowaveguide (i.e. an optical nanofibre): (i) a conveyor belt-type tractor beam effect, (ii) an accelerator tractor beam effect, and (iii) a quantum coherent tractor beam effect, all of which can effectively pull atoms along the nanofibre toward the light source. This technique provides a new tool for controlling the motion of particles near nanowaveguides with potential applications in the study of particle transport and binding as well as atom interferometry. PMID:27440516
Precursors and Transition to Chaos in a Quantum Well
NASA Astrophysics Data System (ADS)
Boebinger, Greg
1996-03-01
Despite great theoretical interest, there are relatively few experimental studies of simple quantum systems whose classical counterparts exhibit chaotic dynamics. Recently, using resonant tunneling spectroscopy, Fromhold, et al. footnote T. M. Fromhold, L. Eaves, F. W. Sheard, M. L. Leadbeater, T. J. Foster, and P. C. Main, Phys. Rev. Lett. 72, 2608 (1994) have demonstrated that the wide quantum well exhibits chaotic dynamics in a sufficiently intense magnetic field which is tilted away from perpendicular to the quantum well. More recently, we have discovered a distinct transition from integrable to chaotic electron dynamics in this system. footnote G. Müller, G. S. Boebinger, H. Mathur, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 75, 2875 (1995) The evolution of the chaos transition is mapped systematically by varying bias voltage, magnetic field and tilt angle. As the tilt angle is increased, the system becomes increasingly chaotic. For a perpendicular magnetic field (θ = 0 ^circ), peaks in the tunneling current correspond to the quantum well subbands. At small tilt angles, (10 ^circ < θ < 30 ^circ) the resonant tunneling spectra show the quantum well subbands, but distinct peak doubling regions have emerged. At larger tilt angles, the quantum well subbands make a transition to a peak tripling region. Poincare section calculations identify these peak doubling and tripling regions as the precursors of the chaos transition. They result from the bifurcation and trifurcation of the stable periodic orbit which corresponds to the quantum well subbands. At θ = 45 ^circ, there is a sharp transition between the region of ordered, subband-like peaks at low magnetic fields and a region of disordered peak positions at higher magnetic fields. We conclude that quantum wells provide a particularly clear manifestation of the transition from order to chaos which results from the break-up of stable periodic orbits. This work is a collaboration with G. Müller, H. Mathur
NASA Astrophysics Data System (ADS)
Allegra, Michele; Giorda, Paolo; Lloyd, Seth
2016-04-01
Assessing the role of interference in natural and artificial quantum dynamical processes is a crucial task in quantum information theory. To this aim, an appropriate formalism is provided by the decoherent histories framework. While this approach has been deeply explored from different theoretical perspectives, it still lacks of a comprehensive set of tools able to concisely quantify the amount of coherence developed by a given dynamics. In this paper, we introduce and test different measures of the (average) coherence present in dissipative (Markovian) quantum evolutions, at various time scales and for different levels of environmentally induced decoherence. In order to show the effectiveness of the introduced tools, we apply them to a paradigmatic quantum process where the role of coherence is being hotly debated: exciton transport in photosynthetic complexes. To spot out the essential features that may determine the performance of the transport, we focus on a relevant trimeric subunit of the Fenna-Matthews-Olson complex and we use a simplified (Haken-Strobl) model for the system-bath interaction. Our analysis illustrates how the high efficiency of environmentally assisted transport can be traced back to a quantum recoil avoiding effect on the exciton dynamics, that preserves and sustains the benefits of the initial fast quantum delocalization of the exciton over the network. Indeed, for intermediate levels of decoherence, the bath is seen to selectively kill the negative interference between different exciton pathways, while retaining the initial positive one. The concepts and tools here developed show how the decoherent histories approach can be used to quantify the relation between coherence and efficiency in quantum dynamical processes.
Optical properties of transition metal oxide quantum wells
Lin, Chungwei; Posadas, Agham; Choi, Miri; Demkov, Alexander A.
2015-01-21
Fabrication of a quantum well, a structure that confines the electron motion along one or more spatial directions, is a powerful method of controlling the electronic structure and corresponding optical response of a material. For example, semiconductor quantum wells are used to enhance optical properties of laser diodes. The ability to control the growth of transition metal oxide films to atomic precision opens an exciting opportunity of engineering quantum wells in these materials. The wide range of transition metal oxide band gaps offers unprecedented control of confinement while the strong correlation of d-electrons allows for various cooperative phenomena to come into play. Here, we combine density functional theory and tight-binding model Hamiltonian analysis to provide a simple physical picture of transition metal oxide quantum well states using a SrO/SrTiO{sub 3}/SrO heterostructure as an example. The optical properties of the well are investigated by computing the frequency-dependent dielectric functions. The effect of an external electric field, which is essential for electro-optical devices, is also considered.
Optical properties of transition metal oxide quantum wells
NASA Astrophysics Data System (ADS)
Lin, Chungwei; Posadas, Agham; Choi, Miri; Demkov, Alexander A.
2015-01-01
Fabrication of a quantum well, a structure that confines the electron motion along one or more spatial directions, is a powerful method of controlling the electronic structure and corresponding optical response of a material. For example, semiconductor quantum wells are used to enhance optical properties of laser diodes. The ability to control the growth of transition metal oxide films to atomic precision opens an exciting opportunity of engineering quantum wells in these materials. The wide range of transition metal oxide band gaps offers unprecedented control of confinement while the strong correlation of d-electrons allows for various cooperative phenomena to come into play. Here, we combine density functional theory and tight-binding model Hamiltonian analysis to provide a simple physical picture of transition metal oxide quantum well states using a SrO/SrTiO3/SrO heterostructure as an example. The optical properties of the well are investigated by computing the frequency-dependent dielectric functions. The effect of an external electric field, which is essential for electro-optical devices, is also considered.
Coherent control of a linear microwave cavity via single flux quantum pulses
NASA Astrophysics Data System (ADS)
Zhu, Shaojiang; Ribeill, Guilhem; Thorbeck, Ted; Leonard, Edward; Vavilov, Maxim; Plourde, Britton; McDermott, Robert
Classical Josephson digital logic based on single flux quantum (SFQ) pulses offers a path to robust, low-latency control of a large-scale quantum processor. Here we describe the coherent control of a linear superconducting cavity by direct excitation via SFQ pulses. Resonant trains of SFQ pulses are capacitively coupled to a thin-film coplanar waveguide cavity. We examine the resulting cavity states as a function of subharmonic drive and temperature. In addition, we describe first steps toward the coherent control of a superconducting qubit with SFQ pulses.
Secure quantum key distribution with a single not-so-weak coherent pulse
NASA Astrophysics Data System (ADS)
Kim, Chil-Min; Kim, Yong-Wan; Park, Young-Jai
2007-04-01
We propose a secure quantum key distribution (QKD) protocol using a single not-so-weak coherent qubit. With two preprocesses for random rotation and compensation, a key bit is encoded to a randomly polarized not-so-weak coherent qubit. We analyze the security of the QKD protocol, which counters the photon number splitting and the impersonation attacks. The estimated mean number of photon, which is less than 6.0, guarantees security. Additionally, we discuss the possibility of quantum secure direct communication.
Quantum-fluctuation-initiated coherence in multioctave Raman optical frequency combs.
Wang, Y Y; Wu, Chunbai; Couny, F; Raymer, M G; Benabid, F
2010-09-17
We show experimentally and theoretically that the spectral components of a multioctave frequency comb spontaneously created by stimulated Raman scattering in a hydrogen-filled hollow-core photonic crystal fiber exhibit strong self-coherence and mutual coherence within each 12 ns driving laser pulse. This coherence arises in spite of the field's initiation being from quantum zero-point fluctuations, which causes each spectral component to show large phase and energy fluctuations. This points to the possibility of an optical frequency comb with nonclassical correlations between all comb lines.
Quantum-Fluctuation-Initiated Coherence in Multioctave Raman Optical Frequency Combs
NASA Astrophysics Data System (ADS)
Wang, Y. Y.; Wu, Chunbai; Couny, F.; Raymer, M. G.; Benabid, F.
2010-09-01
We show experimentally and theoretically that the spectral components of a multioctave frequency comb spontaneously created by stimulated Raman scattering in a hydrogen-filled hollow-core photonic crystal fiber exhibit strong self-coherence and mutual coherence within each 12 ns driving laser pulse. This coherence arises in spite of the field’s initiation being from quantum zero-point fluctuations, which causes each spectral component to show large phase and energy fluctuations. This points to the possibility of an optical frequency comb with nonclassical correlations between all comb lines.
Dressed coherent states in finite quantum systems: A cooperative game theory approach
NASA Astrophysics Data System (ADS)
Vourdas, A.
2017-01-01
A quantum system with variables in Z(d) is considered. Coherent density matrices and coherent projectors of rank n are introduced, and their properties (e.g., the resolution of the identity) are discussed. Cooperative game theory and in particular the Shapley methodology, is used to renormalize coherent states, into a particular type of coherent density matrices (dressed coherent states). The Q-function of a Hermitian operator, is then renormalized into a physical analogue of the Shapley values. Both the Q-function and the Shapley values, are used to study the relocation of a Hamiltonian in phase space as the coupling constant varies, and its effect on the ground state of the system. The formalism is also generalized for any total set of states, for which we have no resolution of the identity. The dressing formalism leads to density matrices that resolve the identity, and makes them practically useful.
GaN/AlN Quantum Wells and Quantum Dots for Unipolar Devices at Telecommunication Wavelengths
Julien, Francois H.; Tchernycheva, Maria; Doyennette, Laetitia; Nevou, Laurent; Lupu, Anatole; Warde, Elias; Guillot, Fabien; Monroy, Eva; Bellet-Amalric, Edith
2007-04-10
We report on the latest achievements in terms of growth and optical investigation of ultrathin GaN/AlN isolated and coupled quantum wells grown by plasma-assisted molecular-beam epitaxy. We also present the observation of intraband absorption in self-organized GaN quantum dots and on the application to infrared photodetection at telecommunication wavelengths.
NASA Astrophysics Data System (ADS)
Lin, Meijin; Lin, Yanqin; Chen, Xi; Cai, Shuhui; Chen, Zhong
2012-01-01
Intermolecular multiple-quantum coherence (iMQC) is capable of improving NMR spectral resolution using a 2D shearing manipulation method. A pulse sequence termed CT-iDH, which combines intermolecular double-quantum filter (iDQF) with a modified constant-time (CT) scheme, is designed to achieve fast acquisition of high-resolution intermolecular zero-quantum coherences (iZQCs) and intermolecular double-quantum coherences (iDQCs) spectra without strong coupling artifacts. Furthermore, double-absorption lineshapes are first realized in 2D intermolecular multi-quantum coherences (iMQCs) spectra under inhomogeneous fields through a combination of iZQC and iDQC signals to double the resolution without loss of sensitivity. Theoretically the spectral linewidth can be further reduced by half compared to original iMQC high-resolution spectra. Several experiments were performed to test the feasibility of the new method and the improvements are evaluated quantitatively. The study suggests potential applications for in vivo spectroscopy.
Design and Analysis of a Quantum Well Light Emitting Triode.
NASA Astrophysics Data System (ADS)
Rajagopalan, Bharath
1992-01-01
We present, for the first time, the design and analysis of a novel, quantum well light emitting triode (QWLET), based on a bipolar junction transistor with a quantum well in the base. Modulation of the collector -base voltage controls the radiation emission from the quantum well by sweeping the space-charge region across the well. Detailed analysis is provided for an npn-Al_{.35 }Ga_{.65}As transistor with an undoped GaAs quantum well. Calculations indicate that modulation rates in excess of 1 GHz are possible. The switching-off process is limited by thermionic emission of majority carriers out of the well, whereas the turn -on is controlled by the recombination lifetime in the well. Our calculations reveal that the thermionic emission lifetime of these carriers is ~0.1 ns at an applied field of 5 times 10 ^4 V/cm, while the radiative lifetime is approximately 1-2 ns for carrier densities in excess of 10^{12} cm ^{-2} in the well. For material systems, or choice of parameters, where thermionic emission is insignificant, field induced tunneling of carriers out of the well is considered as a quenching mechanism. However, the tunneling lifetime is ~3.1 mus at a field of 1 times 10^5 V/cm, and therefore we propose a novel scheme to reduce this lifetime to ~3.3 ns through impurity assisted tunneling. Our calculated results also include a capture cross-section of 10^{-14} cm ^2 for carriers into the well, a B coefficient for radiative recombination of 2.4 times 10^{-10} cm ^3/s, and optical power generation of 0.15 muW per μm of length per mA of drive current and peaked at 855 nm. The voltage amplitude needed to modulate the radiation is on the order of 1 to 2 volts.
NASA Technical Reports Server (NTRS)
Kroemer, Herbert
1990-01-01
Researchers studied the InAs/AlSb system recently, obtaining 12nm wide quantum wells with room temperature mobilities up to 28,000 cm(exp 2)/V center dot S and low-temperature mobilities up to 325,000 cm(exp 2)/V center dot S, both at high electron sheet concentrations in the 10(exp 12)/cm(exp 2) range (corresponding to volume concentrations in the 10(exp 18)/cm(exp 2) range). These wells were not intentionally doped; the combination of high carrier concentrations and high mobilities suggest that the electrons are due to not-intentional modulation doping by an unknown donor in the AlSb barriers, presumably a stoichiometric defect, like an antisite donor. Inasmuch as not intentionally doped bulk AlSb is semi-insulating, the donor must be a deep one, being ionized only by draining into the even deeper InAs quantum well. The excellent transport properties are confirmed by other observations, like excellent quantum Hall effect data, and the successful use of the quantum wells as superconductive weak links between Nb electrodes, with unprecendentedly high critical current densities. The system is promising for future field effect transistors (FETs), but many processing problems must first be solved. Although the researchers have achieved FETs, the results so far have not been competitive with GaAs FETs.
Anisotropic transport in modulation-doped quantum-well structures
NASA Technical Reports Server (NTRS)
Radulescu, D. C.; Wicks, G. W.; Schaff, W. J.; Calawa, A.R.; Eastman, L. F.
1987-01-01
Anisotropic electron transport has been observed in GaAs modulation-doped quantum wells grown by molecular-beam epitaxy on a thick (001) Al(0.3)Ga(0.7)As buffer grown at 620 C. Thicker quantum wells (150, 200, and 300 A) show progressively less anisotropy, which vanishes for a 300-A quantum well. The degree of anisotropy is also reduced or eliminated by suspending growth of the Al(0.3)Ga(0.7)As for a period of 300 s prior to growing the GaAs quantum well. Growing the Al(0.3)Ga(0.7)As buffer at higher temperatures (680 C) also reduces the degree of anisotropy. Higher two-dimensional electron gas sheet densities result in less anisotropy.The anisotropy is eliminated by replacing the thick Al(0.3)Ga(0.7)As buffer with a periodic multilayer structure comprising 15 A of GaAs and 200 A of Al(0.3)Ga(0.7)As. The degree of anisotropy is related to the thickness and growth parameters of the Al(0.3)Ga(0.7)As layer grown just prior to the growth of the GaAs.
Multiple Quantum Wells for P T -Symmetric Phononic Crystals
NASA Astrophysics Data System (ADS)
Poshakinskiy, A. V.; Poddubny, A. N.; Fainstein, A.
2016-11-01
We demonstrate that the parity-time symmetry for sound is realized in laser-pumped multiple-quantum-well structures. Breaking of the parity-time symmetry for the phonons with wave vectors corresponding to the Bragg condition makes the structure a highly selective acoustic wave amplifier. Single-mode distributed feedback phonon lasing is predicted for structures with realistic parameters.
Comparison of coherently coupled multi-cavity and quantum dot embedded single cavity systems.
Kocaman, Serdar; Sayan, Gönül Turhan
2016-12-12
Temporal group delays originating from the optical analogue to electromagnetically induced transparency (EIT) are compared in two systems. Similar transmission characteristics are observed between a coherently coupled high-Q multi-cavity array and a single quantum dot (QD) embedded cavity in the weak coupling regime. However, theoretically generated group delay values for the multi-cavity case are around two times higher. Both configurations allow direct scalability for chip-scale optical pulse trapping and coupled-cavity quantum electrodynamics (QED).
Quantum detection of coherent-state signals in the presence of noise
NASA Technical Reports Server (NTRS)
Vilnrotter, V. A.; Lau, C. W.
2003-01-01
A general method for solving an important class of quantum detection problems will be presented and evaluated. The quantum theory for detecting pure states for communications purposes has been developed over two decades ago, however the mixed state problem representing signal plus noise states has received little attention due to its great complexity. Here we develop a practical model for solving the mixed-state problem using a discrete approximation to the coherent-state representation of signal plus noise density operators.
Coherent coupling of two quantum dots embedded in an Aharonov-Bohm interferometer.
Holleitner, A W; Decker, C R; Qin, H; Eberl, K; Blick, R H
2001-12-17
We define two laterally gated small quantum dots with less than 15 electrons in an Aharonov-Bohm geometry in which the coupling between the two dots can be changed. We measure Aharonov-Bohm oscillations for weakly coupled quantum dots. In an intermediate coupling regime we study molecular states of the double dot and extract the magnetic field dependence of the coherently coupled states.
Quantum anomalous Hall effect in magnetically doped InAs/GaSb quantum wells.
Wang, Qing-Ze; Liu, Xin; Zhang, Hai-Jun; Samarth, Nitin; Zhang, Shou-Cheng; Liu, Chao-Xing
2014-10-03
The quantum anomalous Hall effect has recently been observed experimentally in thin films of Cr-doped (Bi,Sb)(2)Te(3) at a low temperature (∼ 30 mK). In this work, we propose realizing the quantum anomalous Hall effect in more conventional diluted magnetic semiconductors with magnetically doped InAs/GaSb type-II quantum wells. Based on a four-band model, we find an enhancement of the Curie temperature of ferromagnetism due to band edge singularities in the inverted regime of InAs/GaSb quantum wells. Below the Curie temperature, the quantum anomalous Hall effect is confirmed by the direct calculation of Hall conductance. The parameter regime for the quantum anomalous Hall phase is identified based on the eight-band Kane model. The high sample quality and strong exchange coupling make magnetically doped InAs/GaSb quantum wells good candidates for realizing the quantum anomalous Hall insulator at a high temperature.
Optical control and coherence of electron or hole spins in coupled quantum dots
NASA Astrophysics Data System (ADS)
Carter, Samuel
2013-03-01
The spin of an electron or hole in an InAs quantum dot is an attractive qubit because it combines the advantages of a semiconductor platform with the power of ultrafast optical coherent control techniques. In the last few years, basic quantum operations such as initialization, rotation, and readout have become possible using single spins, but now improvements in spin coherence and demonstrations of multi-qubit systems are needed. In this work, we combine advances in the design and growth of coupled quantum dots with optical coherent control techniques to demonstrate ultrafast manipulation and coherence improvements for one or two interacting electron or hole spins in a coupled pair of InAs dots. For each of these spin systems, we use a sequence of picosecond and nanosecond pulses to initialize, manipulate, and measure the coherent spin dynamics. These dynamics include precession about a magnetic field and also entangling dynamics from the exchange interaction for coupled spins. For a single electron spin, precession dephases after only a few nanoseconds due to the hyperfine interaction with nuclear spins. For hole spins, we measure a dephasing time an order of magnitude longer due to a weaker hyperfine interaction. Coupled electron and hole spins are essential for multi-qubit systems, and they can also be used to decrease sensitivity to the environment. In these systems, we typically measure the coherent dynamics of the singlet-triplet states (ms = 0), which are much less sensitive to the nuclear environment. At present, dephasing is due to fluctuations in the electrical environment. With careful sample design, we can make these systems much less sensitive to electrical fluctuations, giving a powerful combination of long coherence times and ultrafast gates. Finally, we demonstrate that these spin qubits can be incorporated into a photonic crystal cavity and manipulated with optical pulses, a major step toward a quantum interface between photons and these spin
Electron transport in coupled double quantum wells and wires
Harff, N.E.; Simmons, J.A.; Lyo, S.K.
1997-04-01
Due to inter-quantum well tunneling, coupled double quantum wells (DQWs) contain an extra degree of electronic freedom in the growth direction, giving rise to new transport phenomena not found in single electron layers. This report describes work done on coupled DQWs subject to inplane magnetic fields B{sub {parallel}}, and is based on the lead author`s doctoral thesis, successfully defended at Oregon State University on March 4, 1997. First, the conductance of closely coupled DQWs in B{sub {parallel}} is studied. B{sub {parallel}}-induced distortions in the dispersion, the density of states, and the Fermi surface are described both theoretically and experimentally, with particular attention paid to the dispersion anticrossing and resulting partial energy gap. Measurements of giant distortions in the effective mass are found to agree with theoretical calculations. Second, the Landau level spectra of coupled DQWs in tilted magnetic fields is studied. The magnetoresistance oscillations show complex beating as Landau levels from the two Fermi surface components cross the Fermi level. A third set of oscillations resulting from magnetic breakdown is observed. A semiclassical calculation of the Landau level spectra is then performed, and shown to agree exceptionally well with the data. Finally, quantum wires and quantum point contacts formed in DQW structures are investigated. Anticrossings of the one-dimensional DQW dispersion curves are predicted to have interesting transport effects in these devices. Difficulties in sample fabrication have to date prevented experimental verification. However, recently developed techniques to overcome these difficulties are described.
Theoretical investigation of photonic quantum wells and defects
NASA Astrophysics Data System (ADS)
Jiang, Yuankai
In this dissertation, band gaps of photonic crystal slabs are calculated and single and multiple photonic quantum well systems are theoretically investigated. A comprehensive study of defects in the photonic crystal is also presented in the dissertation. The major milestones and current developments in the photonic crystal research are briefly outlined in the introduction. Four theoretical approaches most commonly applied in the photonic crystal studies are reviewed. They are the plane wave expansion method, finite difference time domain method, transfer matrix method and modal expansion with R-matrix propagation algorithm. A comparison of these theoretical methods is discussed and the R-matrix formalism is implemented in the present work. The modal expansion with R-matrix propagation algorithm is applied to calculate the band gap for two-dimensional photonic crystal slabs and the results are compared with experimental measurements and with other numerical calculations. Excellent agreement with experiments is found and the R-matrix formalism proves to be more advantageous than other approaches. These advantages include its stability, efficiency and the fact that it can deal with finite photonic crystal slabs. The effect of the finite photonic slab on the band gap is also discussed. It is demonstrated that the band gap for a photonic slab structure can be controlled by the dielectric contrast, filling factor, filling geometry, lattice structure and polarization of the electric field. A photonic quantum well structure is proposed and investigated by the R-matrix algorithm. The band gap of photonic materials with periodic spatial modulation of the refractive index greater than unity can actually be regarded as a potential barrier for photons. Similar to the semiconductor quantum well systems, a photonic quantum well can be constructed by sandwiching a uniform medium between two photonic barriers due to the photonic band gap mismatch. The transmission and reflection
Coherent control of quantum fluctuations using cavity electromagnetically induced transparency.
Souza, J A; Figueroa, E; Chibani, H; Villas-Boas, C J; Rempe, G
2013-09-13
We study the all-optical control of the quantum fluctuations of a light beam via a combination of single-atom cavity quantum electrodynamics (CQED) and electromagnetically induced transparency (EIT). Specifically, the EIT control field is used to tune the CQED transition frequencies in and out of resonance with the probe light. In this way, photon blockade and antiblockade effects are employed to produce sub-Poissonian and super-Poissonian light fields, respectively. The achievable quantum control paves the way towards the realization of a prototype of a novel quantum transistor which amplifies or attenuates the relative intensity noise of a light beam. Its feasibility is demonstrated by calculations using realistic parameters from recent experiments.
Magnetotransport in p-type Ge quantum well narrow wire arrays
NASA Astrophysics Data System (ADS)
Newton, P. J.; Llandro, J.; Mansell, R.; Holmes, S. N.; Morrison, C.; Foronda, J.; Myronov, M.; Leadley, D. R.; Barnes, C. H. W.
2015-04-01
We report magnetotransport measurements of a SiGe heterostructure containing a 20 nm p-Ge quantum well with a mobility of 800 000 cm2 V-1 s-1. By dry etching arrays of wires with widths between 1.0 μm and 3.0 μm, we were able to measure the lateral depletion thickness, built-in potential, and the phase coherence length of the quantum well. Fourier analysis does not show any Rashba related spin-splitting despite clearly defined Shubnikov-de Haas oscillations being observed up to a filling factor of ν = 22. Exchange-enhanced spin-splitting is observed for filling factors below ν = 9. An analysis of boundary scattering effects indicates lateral depletion of the hole gas by 0.5 ± 0.1 μm from the etched germanium surface. The built-in potential is found to be 0.25 ± 0.04 V, presenting an energy barrier for lateral transport greater than the hole confinement energy. A large phase coherence length of 3.5 ± 0.5 μm is obtained in these wires at 1.7 K.
Magnetotransport in p-type Ge quantum well narrow wire arrays
Newton, P. J. Llandro, J.; Mansell, R.; Barnes, C. H. W.; Holmes, S. N.; Morrison, C.; Foronda, J.; Myronov, M.; Leadley, D. R.
2015-04-27
We report magnetotransport measurements of a SiGe heterostructure containing a 20 nm p-Ge quantum well with a mobility of 800 000 cm{sup 2} V{sup −1} s{sup −1}. By dry etching arrays of wires with widths between 1.0 μm and 3.0 μm, we were able to measure the lateral depletion thickness, built-in potential, and the phase coherence length of the quantum well. Fourier analysis does not show any Rashba related spin-splitting despite clearly defined Shubnikov-de Haas oscillations being observed up to a filling factor of ν = 22. Exchange-enhanced spin-splitting is observed for filling factors below ν = 9. An analysis of boundary scattering effects indicates lateral depletion of the hole gas by 0.5 ± 0.1 μm from the etched germanium surface. The built-in potential is found to be 0.25 ± 0.04 V, presenting an energy barrier for lateral transport greater than the hole confinement energy. A large phase coherence length of 3.5 ± 0.5 μm is obtained in these wires at 1.7 K.
Joo, Jaewoo; Ginossar, Eran
2016-01-01
We propose a deterministic scheme for teleporting an unknown qubit state through continuous-variable entangled states in superconducting circuits. The qubit is a superconducting two-level system and the bipartite quantum channel is a microwave photonic entangled coherent state between two cavities. A Bell-type measurement performed on the hybrid state of solid and photonic states transfers a discrete-variable unknown electronic state to a continuous-variable photonic cat state in a cavity mode. In order to facilitate the implementation of such complex protocols we propose a design for reducing the self-Kerr nonlinearity in the cavity. The teleporation scheme enables quantum information processing operations with circuit-QED based on entangled coherent states. These include state verification and single-qubit operations with entangled coherent states. These are shown to be experimentally feasible with the state of the art superconducting circuits. PMID:27245775
NASA Astrophysics Data System (ADS)
Joo, Jaewoo; Ginossar, Eran
2016-06-01
We propose a deterministic scheme for teleporting an unknown qubit state through continuous-variable entangled states in superconducting circuits. The qubit is a superconducting two-level system and the bipartite quantum channel is a microwave photonic entangled coherent state between two cavities. A Bell-type measurement performed on the hybrid state of solid and photonic states transfers a discrete-variable unknown electronic state to a continuous-variable photonic cat state in a cavity mode. In order to facilitate the implementation of such complex protocols we propose a design for reducing the self-Kerr nonlinearity in the cavity. The teleporation scheme enables quantum information processing operations with circuit-QED based on entangled coherent states. These include state verification and single-qubit operations with entangled coherent states. These are shown to be experimentally feasible with the state of the art superconducting circuits.
Joo, Jaewoo; Ginossar, Eran
2016-06-01
We propose a deterministic scheme for teleporting an unknown qubit state through continuous-variable entangled states in superconducting circuits. The qubit is a superconducting two-level system and the bipartite quantum channel is a microwave photonic entangled coherent state between two cavities. A Bell-type measurement performed on the hybrid state of solid and photonic states transfers a discrete-variable unknown electronic state to a continuous-variable photonic cat state in a cavity mode. In order to facilitate the implementation of such complex protocols we propose a design for reducing the self-Kerr nonlinearity in the cavity. The teleporation scheme enables quantum information processing operations with circuit-QED based on entangled coherent states. These include state verification and single-qubit operations with entangled coherent states. These are shown to be experimentally feasible with the state of the art superconducting circuits.
Frustrated quantum phase diffusion and increased coherence of solitons due to nonlocality
Batz, Sascha; Peschel, Ulf
2011-03-15
We investigate the quantum properties of solitons with nonlocal self-interaction. We find significant changes when compared to the local interaction. Quantum phase diffusion of nonlocal solitons is always reduced with respect to the local interaction and vanishes in the strongly nonlocal limit. Thus, coherence is increased in the nonlocal case. Furthermore, we compare the intrinsic quantum wave packet spreading to the recently discussed classical Gordon-Haus effect for nonlocal solitons [V. Folli and C. Conti, Phys. Rev. Lett. 104, 193901 (2010)].
Quantum cascade light emitting diodes based on type-2 quantum wells
NASA Technical Reports Server (NTRS)
Lin, C. H.; Yang, R. Q.; Zhang, D.; Murry, S. J.; Pei, S. S.; Allerman, A. A.; Kurtz, S. R.
1997-01-01
The authors have demonstrated room-temperature CW operation of type-2 quantum cascade (QC) light emitting diodes at 4.2 (micro)m using InAs/InGaSb/InAlSb type-2 quantum wells. The type-2 QC configuration utilizes sequential multiple photon emissions in a staircase of coupled type-2 quantum wells. The device was grown by molecular beam epitaxy on a p-type GaSb substrate and was compared of 20 periods of active regions separated by digitally graded quantum well injection regions. The maximum average output power is about 250 (micro)W at 80 K, and 140 (micro)W at 300 K at a repetition rate of 1 kHz with a duty cycle of 50%.
Quantum cascade light emitting diodes based on type-II quantum wells
Lin, C.H.; Yang, R.Q.; Zhang, D.; Murry, S.J.; Pei, S.S.; Allerman, A.A.; Kurtz, S.R.
1997-01-21
The authors have demonstrated room-temperature CW operation of type-II quantum cascade (QC) light emitting diodes at 4.2 {micro}m using InAs/InGaSb/InAlSb type-II quantum wells. The type-II QC configuration utilizes sequential multiple photon emissions in a staircase of coupled type-II quantum wells. The device was grown by molecular beam epitaxy on a p-type GaSb substrate and was compared of 20 periods of active regions separated by digitally graded quantum well injection regions. The maximum average output power is about 250 {micro}W at 80 K, and 140 {micro}W at 300 K at a repetition rate of 1 kHz with a duty cycle of 50%.
Ferroelectric tunnel junctions with multi-quantum well structures
Ma, Zhijun; Zhang, Tianjin; Liang, Kun; Qi, Yajun; Wang, Duofa; Wang, Jinzhao; Jiang, Juan
2014-06-02
Ferroelectric tunnel junctions (FTJs) with multi-quantum well structures are proposed and the tunneling electroresistance (TER) effect is investigated theoretically. Compared with conventional FTJs with monolayer ferroelectric barriers, FTJs with single-well structures provide TER ratio improvements of one order of magnitude, while FTJs with optimized multi-well structures can enhance this improvement by another order of magnitude. It is believed that the increased resonant tunneling strength combined with appropriate asymmetry in these FTJs contributes to the improvement. These studies may help to fabricate FTJs with large TER ratio experimentally and put them into practice.
NASA Astrophysics Data System (ADS)
Chen, Siming; Li, Wei; Zhang, Ziyang; Childs, David; Zhou, Kejia; Orchard, Jonathan; Kennedy, Ken; Hugues, Maxime; Clarke, Edmund; Ross, Ian; Wada, Osamu; Hogg, Richard
2015-08-01
A high-performance superluminescent light-emitting diode (SLD) based upon a hybrid quantum well (QW)/quantum dot (QD) active element is reported and is assessed with regard to the resolution obtainable in an optical coherence tomography system. We report on the appearance of strong emission from higher order optical transition from the QW in a hybrid QW/QD structure. This additional emission broadening method contributes significantly to obtaining a 3-dB linewidth of 290 nm centered at 1200 nm, with 2.4 mW at room temperature.
Intersubband Transitions in InAs/AlSb Quantum Wells
NASA Technical Reports Server (NTRS)
Li, J.; Koloklov, K.; Ning, C. Z.; Larraber, D. C.; Khodaparast, G. A.; Kono, J.; Ueda, K.; Nakajima, Y.; Sasa, S.; Inoue, M.
2003-01-01
We have studied intersubband transitions in InAs/AlSb quantum wells experimentally and theoretically. Experimentally, we performed polarization-resolved infrared absorption spectroscopy to measure intersubband absorption peak frequencies and linewidths as functions of temperature (from 4 K to room temperature) and quantum well width (from a few nm to 10 nm). To understand experimental results, we performed a self-consistent 8-band k-p band-structure calculation including spatial charge separation. Based on the calculated band structure, we developed a set of density matrix equations to compute TE and TM optical transitions self-consistently, including both interband and intersubband channels. This density matrix formalism is also ideal for the inclusion of various many-body effects, which are known to be important for intersubband transitions. Detailed comparison between experimental data and theoretical simulations is presented.
EPR and Ferromagnetism in Diluted Magnetic Semiconductor Quantum Wells
NASA Astrophysics Data System (ADS)
König, Jürgen; MacDonald, Allan H.
2003-08-01
Motivated by recent measurements of electron paramagnetic resonance spectra in modulation-doped CdMnTe quantum wells [
Anomalous capacitance of quantum well double-barrier diodes
NASA Technical Reports Server (NTRS)
Boric, Olga; Tolmunen, Timo J.; Kollberg, Erik; Frerking, Margaret A.
1992-01-01
The S-parameters of several different quantum well double barrier diodes have been measured. A technique has been developed for measuring whisker contacted diodes with an HP 8510B automatic network analyzer. Special coaxial mounts using K-connectors were designed to enable measurements up to 20 GHz. The voltage-dependent conductance and capacitance were derived from the measured reflection coefficient of each device. The C/V characteristics were observed to exhibit an anomalous increase at voltages corresponding to the negative differential resistance region (NDR). These are the first reported S-parameter measurements in the negative differential resistance region of quantum well double barrier diodes. A theory is presented that explains, in part, the observed results.
Gehring, Tobias; Händchen, Vitus; Duhme, Jörg; Furrer, Fabian; Franz, Torsten; Pacher, Christoph; Werner, Reinhard F; Schnabel, Roman
2015-10-30
Secret communication over public channels is one of the central pillars of a modern information society. Using quantum key distribution this is achieved without relying on the hardness of mathematical problems, which might be compromised by improved algorithms or by future quantum computers. State-of-the-art quantum key distribution requires composable security against coherent attacks for a finite number of distributed quantum states as well as robustness against implementation side channels. Here we present an implementation of continuous-variable quantum key distribution satisfying these requirements. Our implementation is based on the distribution of continuous-variable Einstein-Podolsky-Rosen entangled light. It is one-sided device independent, which means the security of the generated key is independent of any memoryfree attacks on the remote detector. Since continuous-variable encoding is compatible with conventional optical communication technology, our work is a step towards practical implementations of quantum key distribution with state-of-the-art security based solely on telecom components.
NASA Astrophysics Data System (ADS)
Gehring, Tobias; Händchen, Vitus; Duhme, Jörg; Furrer, Fabian; Franz, Torsten; Pacher, Christoph; Werner, Reinhard F.; Schnabel, Roman
2015-10-01
Secret communication over public channels is one of the central pillars of a modern information society. Using quantum key distribution this is achieved without relying on the hardness of mathematical problems, which might be compromised by improved algorithms or by future quantum computers. State-of-the-art quantum key distribution requires composable security against coherent attacks for a finite number of distributed quantum states as well as robustness against implementation side channels. Here we present an implementation of continuous-variable quantum key distribution satisfying these requirements. Our implementation is based on the distribution of continuous-variable Einstein-Podolsky-Rosen entangled light. It is one-sided device independent, which means the security of the generated key is independent of any memoryfree attacks on the remote detector. Since continuous-variable encoding is compatible with conventional optical communication technology, our work is a step towards practical implementations of quantum key distribution with state-of-the-art security based solely on telecom components.
Gehring, Tobias; Händchen, Vitus; Duhme, Jörg; Furrer, Fabian; Franz, Torsten; Pacher, Christoph; Werner, Reinhard F.; Schnabel, Roman
2015-01-01
Secret communication over public channels is one of the central pillars of a modern information society. Using quantum key distribution this is achieved without relying on the hardness of mathematical problems, which might be compromised by improved algorithms or by future quantum computers. State-of-the-art quantum key distribution requires composable security against coherent attacks for a finite number of distributed quantum states as well as robustness against implementation side channels. Here we present an implementation of continuous-variable quantum key distribution satisfying these requirements. Our implementation is based on the distribution of continuous-variable Einstein–Podolsky–Rosen entangled light. It is one-sided device independent, which means the security of the generated key is independent of any memoryfree attacks on the remote detector. Since continuous-variable encoding is compatible with conventional optical communication technology, our work is a step towards practical implementations of quantum key distribution with state-of-the-art security based solely on telecom components. PMID:26514280
Multimode dynamics in quantum cascade lasers: From coherent instability to mode locking
NASA Astrophysics Data System (ADS)
Wang, Christine Yi-Ting
Quantum Cascade Lasers (QCLs) are unipolar semiconductor lasers based on intersubband transitions in quantum wells. Since their invention in 1994, these lasers have undergone tremendous improvement, and have become the most prominent coherent light source in the mid-infrared and terahertz spectral ranges. However, the understanding of multimode regimes in QCLs is still in its infancy, and there has not been much effort toward generating ultrafast pulses from QCLs. The recent development of low loss, high power QCLs enables the study of those previously under-investigated aspects. This thesis can be divided into two main parts. In the first part, we study the multimode regimes in QCLs. We find that QCLs, because of their extremely fast gain recovery time, differ from diode lasers in multimode operation. While a saturable absorber can often lead to self mode-locking in lasers with long gain recovery compared to the cavity round-trip time, in QCLs it lowers the threshold of a coherent multimode instability, which is driven by the same fundamental mechanism of Rabi oscillations as the elusive Risken-Nummedal-Graham-Haken (RNGH) instability predicted 40 years ago. The main experimental signature of RNGH instability is a splitting corresponding to twice the Rabi frequency in optical spectrum. In QCLs this coherent instability is enhanced due to the large Rabi frequency compared to the relaxation rates. We have also shown that spatial hole burning, which is not so readily observable in diode lasers, also plays an important role in QCLs. Both experimental data and simulations based on Maxwell-Bloch equations are presented. In the second part of this thesis, we demonstrate active mode-locking in QCLs. The stable mode-locked pulse train was generated by actively modulating the pumping current of a small section on a QCL. Stable mode locking was confirmed by second-order interferometric autocorrelation measurements, and a FWHM of 3 ps and about 0.5 pJ per pulse were deduced
A realization of the dynamical group for the square-well potential and its coherent states
NASA Astrophysics Data System (ADS)
Lemus, R.; Frank, A.
2003-05-01
A realization of the ladder operators for the infinitely deep well potential is presented. Both the asymmetric and symmetric wells are discussed. It is shown that these operators satisfy the commutation relations for the SU(1, 1) group. Closed analytical expressions for the matrix elements of some relevant functions are obtained. Perelomov coherent states for this system are explicitly constructed. The behaviour of the dispersion relation and the time evolution for such states are examined.
Terahertz quantum well photodetectors with reflection-grating couplers
Zhang, R.; Fu, Z. L.; Gu, L. L.; Guo, X. G.; Cao, J. C.
2014-12-08
The design, fabrication, and characterization of terahertz (THz) quantum well photodetectors with one-dimensional reflection-grating coupler are presented. It is found that the reflection gratings could effectively couple the THz waves normally incident to the device. Compared with the 45-degree facet sample, the peak responsivity of this grating-coupled detector is enhanced by over 20%. The effects of the gratings on the photocurrent spectra are also analyzed.
Linear and Nonlinear Spectroscopy of Quantum Well Intersubband Transitions
NASA Astrophysics Data System (ADS)
Yoo, Sung-Joo Ben
An efficient nonlinear optical device requires a material with large nonlinearity, phasematching capability and transparency in the interacting frequencies. This dissertation examines the optical characteristics of intersubband transitions in quantum wells that are of special interest to nonlinear device applications. Intersubband transitions in quantum wells have large oscillator strengths and narrow linewidths, a condition that suggests large second order susceptibilities ( chi^{(2)}] in a non-centrosymmetric structure. I consider two cases: structures whose inversion symmetry is broken through external electric fields and those with compositional asymmetry. Experimental values of chi^{(2)} for second harmonic generation of tunable CO_2 laser radiation are 14 nm/V for the electric field -biased structure and 58 nm/V for the compositionally asymmetric structure. These are extremely large compared to the chi^{(2)} of other nonlinear materials, including that of bulk GaAs, which is 0.18 nm/V at similar wavelengths. Second, the chi^{(2)} can be patterned for quasi-phasematching. I demonstrate that such patterning can be achieved by alternating the polarity of the applied bias field or by proton bombarding the sample. Third, it is possible to design a quantum well structure that has reasonable transparency without much sacrifice in the optical nonlinearity. One simple method is to detune from resonance and note that the absorption decays faster than the optical nonlinearity with detuning. These three characteristics, combined with relatively mature processing techniques for semiconductor devices make possible fabrication of a high quality waveguide device where the large quantum well nonlinearity is patterned for efficient quasi-phasematching interaction. In such a device, the theoretical conversion efficiency is as high as 30% with an input power of several hundred milliwatts.
Advanced semiconductor quantum well devices for infrared applications
NASA Astrophysics Data System (ADS)
Kuznetsov, Vladimir V.
High performance mid-wavelength infrared (MWIR) light emitting diodes (LEDs) are needed for chemical sensing, analysis and medical imaging. Efficient long wavelength infrared (LWIR) photodetectors are highly desirable for remote sensing and space exploration. The goal of this work is to investigate new mid-infrared LEDs and to optimize existing LWIR quantum well infrared photodetectors (QWIPs). Type-II "W" InAs/InGaSb/AlGaAsSb quantum wells were incorporated as optically active layers in MWIR LEDs. Influence of MBE crystal growth conditions on the density of Shockley-Read-Hall centers in the "W" quantum wells was studied and the optimal growth conditions were identified. A qualitative physical model was developed to describe relative importance of the radiative and non-radiative processes for various temperature ranges. MWIR LED structures lattice-matched to InAs and GaSb substrates were grown. Devices on InAs substrates were found to be at least twice as efficient as devices grown on GaSb. LEDs on InAs had 4.5 mum emission wavelength and 26.5 muW/A external efficiency. Possibility to operate GaAs/AIGaAs QWIP under normal-to-surface light incidence was studied. Metal nano-particle surface coating was developed and processes responsible for, light coupling into the QWIP were investigated. QWIP structure itself was optimized to eliminate Si-diffusion-assisted dark current enhancement by employing a new doping profile in the quantum wells. Devices with the new doping profile had an order of magnitude lower dark current and 20% higher photoresponse than commercially available QWIPs.
Electron scattering and mobility in a quantum well heterolayer
NASA Astrophysics Data System (ADS)
Arora, Vijay K.; Naeem, Athar
1984-11-01
The theory of electron-lattice scattering is analyzed for a quantum-well heterolayer under the conditions that the de Broglie wavelength of an electron is comparable to or larger than the width of the layer, and donor impurities are removed in an adjacent nonconducting layer. The mobility due to isotropic scattering by acoustic phonons, point defects, and alloy scattering is found to increase whereas that due to polar-optic phon scattering is found to decrease with increasing thickness.
Millimeter-Wave Quantum-Well Frequency Multipliers
NASA Technical Reports Server (NTRS)
Batelaan, Paul D.; Frerking, Margaret A.
1990-01-01
Double-barrier quantum-well GaAs diode achieved 0.61 percent efficiency as frequency tripler when operated at input frequency of 63.7 GHz, nearly one-tenth cutoff frequency of diode. Efficiency increased with drive level up to maximum input power available, 40 mW. Diodes studied for use in local-oscillator chains in microwave radiometers. Potential applications include spectrometers and radar.
Temperature independent quantum well FET with delta channel doping
NASA Technical Reports Server (NTRS)
Young, P. G.; Mena, R. A.; Alterovitz, S. A.; Schacham, S. E.; Haugland, E. J.
1992-01-01
A temperature independent device is presented which uses a quantum well structure and delta doping within the channel. The device requires a high delta doping concentration within the channel to achieve a constant Hall mobility and carrier concentration across the temperature range 300-1.4 K. Transistors were RF tested using on-wafer probing and a constant G sub max and F sub max were measured over the temperature range 300-70 K.
Coherent Control and Manipulation of Three Spin States in a Triple Quantum Dot
NASA Astrophysics Data System (ADS)
Sachrajda, Andrew
2013-03-01
The triple quantum dot energy level spectrum is far more complex than its double quantum dot counterpart. As a result it is a challenge to cleanly manipulate only the two required qubit states without invoking more complex multi- state coherent evolution. In this talk I will describe experiments and modeling of lateral triple quantum dot devices where by suitable device gate (i.e. energy level spectrum) tuning and pulse characteristics we were able to characterize and manipulate various three spin qubit species. In particular I will describe measurements where the Landau-Zener -Stückelberg approach previously demonstrated in double dots is extended to three- interacting spin states permitting us to demonstrate phenomena such as pairwise exchange control. I will also demonstrate how by tuning the experimental parameters one can controllably switch to coherent oscillations originating from alternative potentially useful qubit states and how to distinguish them. This work was funded by NRC, NSERC and CIFAR.
Yin, H-L; Cao, W-F; Fu, Y; Tang, Y-L; Liu, Y; Chen, T-Y; Chen, Z-B
2014-09-15
Measurement-device-independent quantum key distribution (MDI-QKD) with decoy-state method is believed to be securely applied to defeat various hacking attacks in practical quantum key distribution systems. Recently, the coherent-state superpositions (CSS) have emerged as an alternative to single-photon qubits for quantum information processing and metrology. Here, in this Letter, CSS are exploited as the source in MDI-QKD. We present an analytical method that gives two tight formulas to estimate the lower bound of yield and the upper bound of bit error rate. We exploit the standard statistical analysis and Chernoff bound to perform the parameter estimation. Chernoff bound can provide good bounds in the long-distance MDI-QKD. Our results show that with CSS, both the security transmission distance and secure key rate are significantly improved compared with those of the weak coherent states in the finite-data case.
Quantum well structures for plasma instability-based terahertz radiation sources
NASA Astrophysics Data System (ADS)
Butler, Justin John
This thesis is a theoretical study of the electron transport and response properties of epitaxially grown, low-dimensional semiconductor quantum well heterostructures, under steady-state, current driven (nonequilibrium) conditions. These structures operate in the Terahertz (THz) frequency and submillimeter wavelength range, and are the leading candidates for compact, coherent sources of THz radiation. This work is divided into two parts: Part I consists of an analytical study of the individual quantum well units, and the tunneling transmission characteristics, for which reasonably accurate algebraic expressions are obtained. An underlying philosophy of this work is the desire to describe each of the key components involved, independently, through these simple analytical expressions. In Part II the numerical study of the transport and radiation response of the quantum well structures specially designed to generate THz radiation based on the plasma instability concept is presented. Several models are proposed which describe the overall electron transport and which determine the underlying nonequilibrium steady state. In particular, the key features of the experimental current-voltage (IV) curves for such structures are explained, and the corresponding response properties are determined. The modeling and simulation of these potential optoelectronic devices is a crucial tool for elucidating the precise mechanisms and interplay of the many microscopic processes which give rise to the observed behavior. Key features of the radiation response arise from the intersubband plasma instability which occurs due to the resonant interaction of an emission and an absorption mode, and these features are compared with the experimental observations.
A real-time spectrum acquisition system design based on quantum dots-quantum well detector
NASA Astrophysics Data System (ADS)
Zhang, S. H.; Guo, F. M.
2016-01-01
In this paper, we studied the structure characteristics of quantum dots-quantum well photodetector with response wavelength range from 400 nm to 1000 nm. It has the characteristics of high sensitivity, low dark current and the high conductance gain. According to the properties of the quantum dots-quantum well photodetectors, we designed a new type of capacitive transimpedence amplifier (CTIA) readout circuit structure with the advantages of adjustable gain, wide bandwidth and high driving ability. We have implemented the chip packaging between CTIA-CDS structure readout circuit and quantum dots detector and tested the readout response characteristics. According to the timing signals requirements of our readout circuit, we designed a real-time spectral data acquisition system based on FPGA and ARM. Parallel processing mode of programmable devices makes the system has high sensitivity and high transmission rate. In addition, we realized blind pixel compensation and smoothing filter algorithm processing to the real time spectrum data by using C++. Through the fluorescence spectrum measurement of carbon quantum dots and the signal acquisition system and computer software system to realize the collection of the spectrum signal processing and analysis, we verified the excellent characteristics of detector. It meets the design requirements of quantum dot spectrum acquisition system with the characteristics of short integration time, real-time and portability.
Quantum phase transition of electron-hole liquid in coupled quantum wells
NASA Astrophysics Data System (ADS)
Babichenko, V. S.; Polishchuk, I. Ya.
2016-10-01
Many-component electron-hole plasma is considered in the coupled quantum wells. The electrons are assumed to be localized in one quantum well (QW) while the holes are localized in the other QW. It is found that the homogeneous charge distribution within the QWs is unstable if the carrier density is sufficiently small. The instability results in the breakdown of the homogeneous charge distribution into two coexisting phases—a low-density phase and a high-density phase, which is electron-hole liquid. In turn, the homogeneous state of the electron-hole liquid is stable if the distance between the quantum wells ℓ is sufficiently small. However, as the distance ℓ increases and reaches a certain critical value ℓcr, the plasmon spectrum of the electron-hole liquid becomes unstable. Hereupon, a quantum phase transition occurs, resulting in the appearance of charge-density waves of finite amplitude in both quantum wells. Strong mass renormalization and the strong Z -factor renormalization are found for the electron-hole liquid as the quantum phase transition occurs.
Charge transport and localization in atomically coherent quantum dot solids
NASA Astrophysics Data System (ADS)
Whitham, Kevin; Yang, Jun; Savitzky, Benjamin H.; Kourkoutis, Lena F.; Wise, Frank; Hanrath, Tobias
2016-05-01
Epitaxial attachment of quantum dots into ordered superlattices enables the synthesis of quasi-two-dimensional materials that theoretically exhibit features such as Dirac cones and topological states, and have major potential for unprecedented optoelectronic devices. Initial studies found that disorder in these structures causes localization of electrons within a few lattice constants, and highlight the critical need for precise structural characterization and systematic assessment of the effects of disorder on transport. Here we fabricated superlattices with the quantum dots registered to within a single atomic bond length (limited by the polydispersity of the quantum dot building blocks), but missing a fraction (20%) of the epitaxial connections. Calculations of the electronic structure including the measured disorder account for the electron localization inferred from transport measurements. The calculations also show that improvement of the epitaxial connections will lead to completely delocalized electrons and may enable the observation of the remarkable properties predicted for these materials.
Interaction of a quantum well with squeezed light: Quantum-statistical properties
Sete, Eyob A.; Eleuch, H.
2010-10-15
We investigate the quantum statistical properties of the light emitted by a quantum well interacting with squeezed light from a degenerate subthreshold optical parametric oscillator. We obtain analytical solutions for the pertinent quantum Langevin equations in the strong-coupling and low-excitation regimes. Using these solutions we calculate the intensity spectrum, autocorrelation function, and quadrature squeezing for the fluorescent light. We show that the fluorescent light exhibits bunching and quadrature squeezing. We also show that the squeezed light leads to narrowing of the width of the spectrum of the fluorescent light.
Apparent quantum number paradox in Ag quantum wells on Si(111)
NASA Astrophysics Data System (ADS)
Brinkley, M. K.; Speer, N. J.; Liu, Y.; Miller, T.; Chiang, T.-C.
2011-12-01
Angle-resolved photoemission studies of the quantum well electronic structure in atomically uniform Ag films grown on Si(111)-(7×7) reveal an anomalous bifurcation of one of the subbands as it disperses toward the Fermi level. This bifurcation leads to an apparent quantum number paradox, since subbands must be associated with consecutive integer quantum numbers. Evident only in films thicker than 15 monolayers, the bifurcation migrates upon annealing from subband to subband toward the zone center and ultimately vanishes. Various tests indicate that this perplexing behavior arises from transverse resonances in the film electronic structure caused by the reconstruction at the interface.
NASA Astrophysics Data System (ADS)
Kelleher, B.; Bonatto, C.; Huyet, G.; Hegarty, S. P.
2011-02-01
Excitability is a generic prediction for an optically injected semiconductor laser. However, the details of the phenomenon differ depending on the type of device in question. For quantum-well lasers very complicated multipulse trajectories can be found, while for quantum-dot lasers the situation is much simpler. Experimental observations show the marked differences in the pulse shapes while theoretical considerations reveal the underlying mechanism responsible for the contrast, identifying the increased stability of quantum-dot lasers to perturbations as the root.
Kelleher, B; Bonatto, C; Huyet, G; Hegarty, S P
2011-02-01
Excitability is a generic prediction for an optically injected semiconductor laser. However, the details of the phenomenon differ depending on the type of device in question. For quantum-well lasers very complicated multipulse trajectories can be found, while for quantum-dot lasers the situation is much simpler. Experimental observations show the marked differences in the pulse shapes while theoretical considerations reveal the underlying mechanism responsible for the contrast, identifying the increased stability of quantum-dot lasers to perturbations as the root.
Quantum Well and Quantum Dot Modeling for Advanced Infrared Detectors and Focal Plane Arrays
NASA Technical Reports Server (NTRS)
Ting, David; Gunapala, S. D.; Bandara, S. V.; Hill, C. J.
2006-01-01
This viewgraph presentation reviews the modeling of Quantum Well Infrared Detectors (QWIP) and Quantum Dot Infrared Detectors (QDIP) in the development of Focal Plane Arrays (FPA). The QWIP Detector being developed is a dual band detector. It is capable of running on two bands Long-Wave Infrared (LWIR) and Medium Wavelength Infrared (MWIR). The same large-format dual-band FPA technology can be applied to Quantum Dot Infrared Photodetector (QDIP) with no modification, once QDIP exceeds QWIP in single device performance. Details of the devices are reviewed.
Hybrid quantum well/quantum dot structures for broad spectral bandwidth devices
NASA Astrophysics Data System (ADS)
Chen, Siming; Zhou, Kejia; Zhang, Ziyang; Childs, David T. D.; Orchard, Jonathan R.; Hogg, Richard A.; Kennedy, Kenneth; Hughes, Max.
2012-02-01
In this paper we report a hybrid quantum well (QW) and quantum dot (QD) structure to achieve a broad spontaneous emission and gain spectra. A single quantum well is introduced into a multi-layer stack of quantum dots, spectrally positioned to cancel the losses due to the second excited state of the dots. Attributed to the combined effect of QW and QDs, we show room temperature spontaneous emission with a 3dB bandwidth of ~250 nm and modal gain spanning over ~300 nm. We describe how this is achieved by careful design of the structure, balancing thermal emission from the QW and transport/capture processes in the QDs. We will also compare results from a QD-only epitaxial structure to describe how broadband gain/emission can be achieved in this new type of structure.
Coherent scattering in two dimensions: Graphene and quantum corrals
NASA Astrophysics Data System (ADS)
Barr, Matthew Christopher
Two dimensional electronic materials provide a vibrant area for applying basic quantum mechanics and scattering theory. In quantum corrals, multiple scattering leads to resonances closely approximating eigenstates of an equivalently shaped billiard. We extend the analogy using methods from acoustics to demonstrate that the billiard conception of quantum corrals is a useful one even in wavelength regimes close to corral size. Resonance widths can be described by a simple relationship proportional to the perimeter to area ratio of the enclosure and the average reflection of a classical path. In graphene, we study the unique behavior strain induces on the effective Dirac Hamiltonian by creating an effective pseudomagnetic field. These fields induce an energy splitting of degenerate eigenstates of certain graphene quantum dots which is distinct from that of an applied scalar potential and can in principle be observed in the conductance through the dot. Additionally, for strain bubbles smaller than the effective Dirac wavelength, the scattering is shown to be distinct from other impurity types. This leads to characteristic features in the conductance, as a function of the bubble position, through regions containing a strong strain bubble.
Metallic behaviour in SOI quantum wells with strong intervalley scattering
Renard, V. T.; Duchemin, I.; Niida, Y.; Fujiwara, A.; Hirayama, Y.; Takashina, K.
2013-01-01
The fundamental properties of valleys are recently attracting growing attention due to electrons in new and topical materials possessing this degree-of-freedom and recent proposals for valleytronics devices. In silicon MOSFETs, the interest has a longer history since the valley degree of freedom had been identified as a key parameter in the observation of the controversial “metallic behaviour” in two dimensions. However, while it has been recently demonstrated that lifting valley degeneracy can destroy the metallic behaviour, little is known about the role of intervalley scattering. Here, we show that the metallic behaviour can be observed in the presence of strong intervalley scattering in silicon on insulator (SOI) quantum wells. Analysis of the conductivity in terms of quantum corrections reveals that interactions are much stronger in SOI than in conventional MOSFETs, leading to the metallic behaviour despite the strong intervalley scattering. PMID:23774638
Non-linear optics of coupled quantum dots and atomic systems with coherent control fields
NASA Astrophysics Data System (ADS)
Mumba, Mambwe
Presented herein is an investigation of quantum systems with coherent optical control fields. Three such systems are examined. The first consists of two dipole-dipole coupled quantum dots or dimers which behave as an effective three or four-level system whose susceptibility and hence transmissivity for an optical beam at some frequency may be switched on or off in response to a coherent control field. The second quantum system consists of a model cluster of three coupled dots that is shown to display light intermittency or blinking when irradiated by a coherent field. Results indicate that the observed variation in rate, intensity and duration of blinking times occasioned by the rare but observable rapid blinking at higher rate and intensity (superradiance) can be traced back to the groupings of states in different manifolds that the coupled system is capable of being found in at any given time. It is shown, however, that the experimentally observed blinking can not be entirely accounted for by dipole-dipole coupling alone. The third system investigated consists of Rubidium atoms in a cell placed in a ring cavity. A coherent control field drives the system. A mathematical model of the system is developed which consists of propagating a gaussian beam around the system and examining the output spectrum when a steady state value of the electromagnetic field is attained in the Rubidium cell. Some interesting features occurring in the output spectrum of the field at some cavity detuning are reproduced and match those experimentally observed.
State-conditional coherent charge qubit oscillations in a Si/SiGe quadruple quantum dot
Ward, Daniel R.; Kim, Dohun; Savage, Donald E.; ...
2016-10-18
Universal quantum computation requires high-fidelity single-qubit rotations and controlled two-qubit gates. Along with high-fidelity single-qubit gates, strong efforts have been made in developing robust two-qubit logic gates in electrically gated quantum dot systems to realise a compact and nanofabrication-compatible architecture. Here we perform measurements of state-conditional coherent oscillations of a charge qubit. Using a quadruple quantum dot formed in a Si/SiGe heterostructure, we show the first demonstration of coherent two-axis control of a double quantum dot charge qubit in undoped Si/SiGe, performing Larmor and Ramsey oscillation measurements. We extract the strength of the capacitive coupling between a pair of doublemore » quantum dots by measuring the detuning energy shift (≈75 μeV) of one double dot depending on the excess charge configuration of the other double dot. Finally, we further demonstrate that the strong capacitive coupling allows fast, state-conditional Landau–Zener–Stückelberg oscillations with a conditional π phase flip time of about 80 ps, showing a promising pathway towards multi-qubit entanglement and control in semiconductor quantum dots.« less
State-conditional coherent charge qubit oscillations in a Si/SiGe quadruple quantum dot
Ward, Daniel R.; Kim, Dohun; Savage, Donald E.; Lagally, Max G.; Foote, Ryan H.; Friesen, Mark; Coppersmith, Susan N.; Eriksson, Mark A.
2016-10-18
Universal quantum computation requires high-fidelity single-qubit rotations and controlled two-qubit gates. Along with high-fidelity single-qubit gates, strong efforts have been made in developing robust two-qubit logic gates in electrically gated quantum dot systems to realise a compact and nanofabrication-compatible architecture. Here we perform measurements of state-conditional coherent oscillations of a charge qubit. Using a quadruple quantum dot formed in a Si/SiGe heterostructure, we show the first demonstration of coherent two-axis control of a double quantum dot charge qubit in undoped Si/SiGe, performing Larmor and Ramsey oscillation measurements. We extract the strength of the capacitive coupling between a pair of double quantum dots by measuring the detuning energy shift (≈75 μeV) of one double dot depending on the excess charge configuration of the other double dot. Finally, we further demonstrate that the strong capacitive coupling allows fast, state-conditional Landau–Zener–Stückelberg oscillations with a conditional π phase flip time of about 80 ps, showing a promising pathway towards multi-qubit entanglement and control in semiconductor quantum dots.
State-conditional coherent charge qubit oscillations in a Si/SiGe quadruple quantum dot
NASA Astrophysics Data System (ADS)
Ward, Daniel R.; Kim, Dohun; Savage, Donald E.; Lagally, Max G.; Foote, Ryan H.; Friesen, Mark; Coppersmith, Susan N.; Eriksson, Mark A.
2016-10-01
Universal quantum computation requires high-fidelity single-qubit rotations and controlled two-qubit gates. Along with high-fidelity single-qubit gates, strong efforts have been made in developing robust two-qubit logic gates in electrically gated quantum dot systems to realise a compact and nanofabrication-compatible architecture. Here we perform measurements of state-conditional coherent oscillations of a charge qubit. Using a quadruple quantum dot formed in a Si/SiGe heterostructure, we show the first demonstration of coherent two-axis control of a double quantum dot charge qubit in undoped Si/SiGe, performing Larmor and Ramsey oscillation measurements. We extract the strength of the capacitive coupling between a pair of double quantum dots by measuring the detuning energy shift (≈75 μeV) of one double dot depending on the excess charge configuration of the other double dot. We further demonstrate that the strong capacitive coupling allows fast, state-conditional Landau-Zener-Stückelberg oscillations with a conditional π phase flip time of about 80 ps, showing a promising pathway towards multi-qubit entanglement and control in semiconductor quantum dots.
No-Go Theorem for the Characterization of Work Fluctuations in Coherent Quantum Systems.
Perarnau-Llobet, Martí; Bäumer, Elisa; Hovhannisyan, Karen V; Huber, Marcus; Acin, Antonio
2017-02-17
An open question of fundamental importance in thermodynamics is how to describe the fluctuations of work for quantum coherent processes. In the standard approach, based on a projective energy measurement both at the beginning and at the end of the process, the first measurement destroys any initial coherence in the energy basis. Here we seek extensions of this approach which can possibly account for initially coherent states. We consider all measurement schemes to estimate work and require that (i) the difference of average energy corresponds to average work for closed quantum systems and that (ii) the work statistics agree with the standard two-measurement scheme for states with no coherence in the energy basis. We first show that such a scheme cannot exist. Next, we consider the possibility of performing collective measurements on several copies of the state and prove that it is still impossible to simultaneously satisfy requirements (i) and (ii). Nevertheless, improvements do appear, and in particular, we develop a measurement scheme that acts simultaneously on two copies of the state and allows us to describe a whole class of coherent transformations.
No-Go Theorem for the Characterization of Work Fluctuations in Coherent Quantum Systems
NASA Astrophysics Data System (ADS)
Perarnau-Llobet, Martí; Bäumer, Elisa; Hovhannisyan, Karen V.; Huber, Marcus; Acin, Antonio
2017-02-01
An open question of fundamental importance in thermodynamics is how to describe the fluctuations of work for quantum coherent processes. In the standard approach, based on a projective energy measurement both at the beginning and at the end of the process, the first measurement destroys any initial coherence in the energy basis. Here we seek extensions of this approach which can possibly account for initially coherent states. We consider all measurement schemes to estimate work and require that (i) the difference of average energy corresponds to average work for closed quantum systems and that (ii) the work statistics agree with the standard two-measurement scheme for states with no coherence in the energy basis. We first show that such a scheme cannot exist. Next, we consider the possibility of performing collective measurements on several copies of the state and prove that it is still impossible to simultaneously satisfy requirements (i) and (ii). Nevertheless, improvements do appear, and in particular, we develop a measurement scheme that acts simultaneously on two copies of the state and allows us to describe a whole class of coherent transformations.
Electron bilayers in an undoped Si/SiGe double-quantum-well heterostructure
NASA Astrophysics Data System (ADS)
Lu, Tzu-Ming; Laroche, Dominique; Huang, Shih-Hsien; Nielsen, Erik; Chuang, Yen; Li, Jiun-Yun; Liu, Cheewee
We report the design, fabrication, and the magneto-transport study of an undoped Si/SiGe double quantum well heterostructure. We show that employing asymmetric quantum wells for our single-side-gated devices allows us to observe a cross-over from single-layer-like to bi-layer-llike behavior in the mobility-density dependence. We also observe an integer quantum Hall state at filling factor ν = 2, which is expected to arise from inter-layer effects for Si electrons. This state could be due to either inter-layer coherence, or the symmetric-antisymmetric tunneling gap. This work has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy (DOE). 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. DOE's National Nuclear Security Administration under contract DE-AC04-94AL85000.
Characterization and Analysis of Multi-Quantum Well Solar Cells
NASA Astrophysics Data System (ADS)
Bradshaw, Geoffrey Keith
GaAs layers form quantum well capable of absorbing lower energy wavelengths than GaAs which leads to an increase in current. Absorption due to quantum wells is proportional to the number of quantum wells in the intrinsic region. Therefore, in order to grow the maximum number of the absorbing quantum wells within the background doping limited intrinsic region, it is necessary to reduce the width of the non-absorbing GaAsP barriers to as thin as possible. The research presented within shows this concept by exploring the fabrication and electrical characterization of these quantum well devices when balanced with ultra-thin GaAsP layers with very high phosphorus content (˜75-80%). By reducing the width of the barriers to approximately 30 A, tunneling of carriers dominates carrier transport across the structure as opposed to the traditional quantum well approach with very thick, low phosphorus GaAsP barriers that rely on thermionic emission of carriers to escape the InGaAs quantum wells. This research shows the strong effect and sensitivity to not only the thickness the GaAsP barriers, but also to the polarity of the device and the dependence of electric field. As well widths are decreased, quantum confinement of carriers within the InGaAs quantum wells increases. This leads to a blue-shift in the wavelengths of light absorbed and limits the current gain potential of the quantum well structure. To combat this blue-shift, the staggered MQW is introduced. The staggering technique can be use to not only improve wavelength absorption extension, but also lead to an enhancement in the absorption coefficient. These structures were also included into a GaInP/GaAs(MQW) tandem device to see the effects of the structure on the GaInP top cell.
Coherent frequency combs produced by self frequency modulation in quantum cascade lasers
Khurgin, J. B.; Dikmelik, Y.; Hugi, A.; Faist, J.
2014-02-24
One salient characteristic of Quantum Cascade Laser (QCL) is its very short τ ∼ 1 ps gain recovery time that so far thwarted the attempts to achieve self-mode locking of the device into a train of single pulses. We show theoretically that four wave mixing, combined with the short gain recovery time causes QCL to operate in the self-frequency-modulated regime characterized by a constant power in time domain and stable coherent comb in the frequency domain. Coherent frequency comb may enable many potential applications of QCL's in sensing and measurement.
Recent Developments in Quantum-Well Infrared Photodetectors
NASA Technical Reports Server (NTRS)
Gunapala, S. D.; Bandara, K. M. S. V.
1995-01-01
Intrinsic infrared (IR) detectors in the long wavelength range (8-20 Am) are based on an optically excited interband transition, which promotes an electron across the band gap (E(sub g)) from the valence band to the conduction band as shown. These photoelectrons can be collected efficiently, thereby producing a photocurrent in the external circuit. Since the incoming photon has to promote an electron from the valence band to the conduction band, the energy of the photon (h(sub upsilon)) must be higher than the E(sub g) of the photosensitive material. Therefore, the spectral response of the detectors can be controlled by controlling the E(sub g) of the photosensitive material. Examples for such materials are Hg(1-x), Cd(x), Te, and Pb(1-x), Sn(x), Te, in which the energy gap can be controlled by varying x. This means detection of very-long-wavelength IR radiation up to 20 microns requires small band gaps down to 62 meV. It is well known that these low band gap materials, characterized by weak bonding and low melting points, are more difficult to grow and process than large-band gap semiconductors such as GaAs. These difficulties motivate the exploration of utilizing the intersub-band transitions in multiquantum well (MQW) structures made of more refractory large-band gap semiconductors. The idea of using MQW structures to detect IR radiation can be explained by using the basic principles of quantum mechanics. The quantum well is equivalent to the well-known particle in a box problem in quantum mechanics, which can be solved by the time independent Schroudiner equation.
Increased coherence time in narrowed bath states in quantum dots
NASA Astrophysics Data System (ADS)
Gravert, Lars B.; Lorenz, Peter; Nase, Carsten; Stolze, Joachim; Uhrig, Götz S.
2016-09-01
We study the influence of narrowed distributions of the nuclear Overhauser field on the decoherence of a central electron spin in quantum dots. We describe the spin dynamics in quantum dots by the central spin model. We use analytic solutions for uniform couplings and the time dependent density-matrix renormalization group (tDMRG) for nonuniform couplings. With these tools we calculate the dynamics of the central spin for large baths of nuclear spins with or without external magnetic field applied to the central spin. The focus of our study is the influence of initial mixtures with narrowed distributions of the Overhauser field and of applied magnetic fields on the decoherence of the central spin.
Superconducting proximity effect in inverted InAs/GaSb quantum well structures with Ta electrodes
Yu, Wenlong; Jiang, Yuxuan; Huan, Chao; Chen, Xunchi; Jiang, Zhigang; Hawkins, Samuel D.; Klem, John F.; Pan, Wei
2014-11-10
We present our recent electronic transport results in top-gated InAs/GaSb quantum well hybrid structures with superconducting Ta electrodes. We show that the transport across the InAs−Ta junction depends largely on the interfacial transparency, exhibiting distinct zero-bias behavior. For a relatively resistive interface, a broad conductance peak is observed at zero bias. When a transparent InAs−Ta interface is achieved, a zero-bias conductance dip appears with two coherent-peak-like features forming at bias voltages corresponding to the superconducting gap of Ta. The conductance spectra of the transparent InAs−Ta junction at different gate voltages can be fit well using the standard Blonder-Tinkham-Klapwijk theory.
Quantum coherence, decoherence and entanglement in light harvesting complexes
NASA Astrophysics Data System (ADS)
Plenio, Martin; Caruso, Filippo; Chin, Alex; Datta, Animesh; Huelga, Susana
2009-03-01
Transport phenomena in networks allow for information and energy to be exchanged between individual constituents of communication systems, networks or light-harvesting complexes. Environmental noise is generally expected to hinder transport. Here we show that transport of excitations across dissipative quantum networks can be enhanced by dephasing noise. We identify two key processes that underly this phenomenon and provide instructive examples of quantum networks for each. We argue that Nature may be routinely exploiting this effect by showing that exciton transport in light harvesting complexes and other networks benefits from noise and is remarkably robust against static disorder. These results point towards the possibility for designing optimized structures for transport, for example in artificial nano-structures, assisted by noise. Furthermore, we demonstrate that quantum entanglement may be present for short times in light-harvesting complexes. We describe how the presence of such entanglement may be verified without the need for full state tomography and with minimal model assumptions. This work is based on M.B. Plenio & S.F. Huelga, New J. Phys. 10, 113019 (2008) and F. Caruso, A. Chin, A. Datta, S.F. Huelga & M.B. Plenio, in preparation
Single-channel 40 Gbit/s digital coherent QAM quantum noise stream cipher transmission over 480 km.
Yoshida, Masato; Hirooka, Toshihiko; Kasai, Keisuke; Nakazawa, Masataka
2016-01-11
We demonstrate the first 40 Gbit/s single-channel polarization-multiplexed, 5 Gsymbol/s, 16 QAM quantum noise stream cipher (QNSC) transmission over 480 km by incorporating ASE quantum noise from EDFAs as well as the quantum shot noise of the coherent state with multiple photons for the random masking of data. By using a multi-bit encoded scheme and digital coherent transmission techniques, secure optical communication with a record data capacity and transmission distance has been successfully realized. In this system, the signal level received by Eve is hidden by both the amplitude and the phase noise. The highest number of masked signals, 7.5 x 10(4), was achieved by using a QAM scheme with FEC, which makes it possible to reduce the output power from the transmitter while maintaining an error free condition for Bob. We have newly measured the noise distribution around I and Q encrypted data and shown experimentally with a data size of as large as 2(25) that the noise has a Gaussian distribution with no correlations. This distribution is suitable for the random masking of data.
Probing quantum coherence in a biological system by means of DNA amplification.
Bieberich, E
2000-07-01
As a result of rapid decoherence, quantum effects in biological systems are usually confined to single electron or hydrogen delocalizations. In principle, molecular interactions at high temperatures can be guided by quantum coherence if embedded in a dynamics preventing decoherence. This was experimentally investigated by analyzing the thermodynamics, kinetics, and quantum mechanics of the primer/template duplex formation during DNA amplification by polymerase chain reaction. The structures of the two oligonucleotide primers used for amplification of a cDNA template were derived either from a repetitive motif or a fractal distribution of nucleotide residues. Contrary to the computer-based calculation of the primer melting temperatures (T(m)) that predicted a higher T(m) for the non-fractal primer due to nearest-neighbor effects, it was found that the T(m) of the non-fractal primer was actually 2 degrees C lower than that of its fractal counterpart. A thermodynamic analysis of the amplification reaction indicated that the primer annealing process followed Bose-Einstein instead of Boltzmann statistics, with an additional binding potential of mu=500 J/mol or 10(-21) J/molecule due to a superposition of binding states within the primer/template duplex. The temporal evolution of the Bose-Einstein state was determined by enzyme kinetic analysis of the association of the primer/template duplex to Taq polymerase. Assuming that collision with the enzyme interrupted the superposition, it was found that the Bose-Einstein state lasted for t(dec)=0.7x10(-12) s, corresponding to the energy dispersion (DeltaE) of quantum coherent states (mu=DeltaE>/=h/t(dec)). A quantum mechanical analysis revealed that the coherent state was stabilized by almost vanishing separation energies between distinct binding states during a temperature-driven shifting of the two DNA strands in the primer/template duplex. The additional binding potential is suggested to arise from a short-lived electron
Excimer laser induced diffusion in magnetic semiconductor quantum wells
NASA Astrophysics Data System (ADS)
Howari, H.; Sands, D.; Nicholls, J. E.; Hogg, J. H. C.; Stirner, T.; Hagston, W. E.
2000-08-01
Studies of pulsed laser annealing (PLA) of CdTe/CdMnTe quantum well structures are made in order to examine depth dependent effects in laser irradiated semiconductors. Since diffusion coefficients are strongly dependent on the temperature, depth resolution is achieved because the diffusion of Mn from the barriers into the quantum wells is depth dependent. Multiple quantum well (MQW) structures of CdTe/CdMnTe were annealed with single pulses from an XeCl laser at 308 nm. At a threshold of 90 mJ cm-2 two new emission bands are observed that are attributed to the diffusion of Mn from barrier layers to QWs. The diffusion associated with these bands, measured as the integrated product of the diffusion constant and time, is found to be 300 and 30 Å2. Calculations of the temperature, reached within the surface following PLA, using an analytical solution of the heat diffusion equation coupled with known high temperature diffusion coefficients predict the diffusion to decrease by one order of magnitude within one period at the top of the MQW stack. It is suggested that at the threshold surface melting occurs and that these emission bands arise from the QWs immediately beneath the melt front. The diffusion of Mn ions into the QWs is confirmed by magneto-optical data. A further emission band occurs at this same threshold with a Mn concentration above that of the concentration in the barrier layers of the MQW stack. This emission is attributed tentatively to the segregation of the Mn ion within the molten region following recrystallization.
Cavity-photon-switched coherent transient transport in a double quantum waveguide
Abdullah, Nzar Rauf Gudmundsson, Vidar; Tang, Chi-Shung; Manolescu, Andrei
2014-12-21
We study a cavity-photon-switched coherent electron transport in a symmetric double quantum waveguide. The waveguide system is weakly connected to two electron reservoirs, but strongly coupled to a single quantized photon cavity mode. A coupling window is placed between the waveguides to allow electron interference or inter-waveguide transport. The transient electron transport in the system is investigated using a quantum master equation. We present a cavity-photon tunable semiconductor quantum waveguide implementation of an inverter quantum gate, in which the output of the waveguide system may be selected via the selection of an appropriate photon number or “photon frequency” of the cavity. In addition, the importance of the photon polarization in the cavity, that is, either parallel or perpendicular to the direction of electron propagation in the waveguide system is demonstrated.
Thermopower enhancement in quantum wells with the Rashba effect
Wu, Lihua; Yang, Jiong; Wang, Shanyu; Wei, Ping; Yang, Jihui E-mail: wqzhang@mail.sic.ac.cn; Zhang, Wenqing E-mail: wqzhang@mail.sic.ac.cn; Chen, Lidong
2014-11-17
We theoretically demonstrate that the thermopower in two-dimensional quantum wells (QWs) can be significantly enhanced by its Rashba spin-splitting effect, governed by the one-dimensional density of states in the low Fermi energy region. The thermopower enhancement is due to the lower Fermi level for a given carrier concentration in Rashba QWs, as compared with that in normal two-dimensional systems without the spin-splitting effect. The degenerate approximation directly shows that larger strength of Rashba effect leads to higher thermopower and consequently better thermoelectric performance in QWs.
Reliability assessment of multiple quantum well avalanche photodiodes
NASA Technical Reports Server (NTRS)
Yun, Ilgu; Menkara, Hicham M.; Wang, Yang; Oguzman, Isamil H.; Kolnik, Jan; Brennan, Kevin F.; May, Gray S.; Wagner, Brent K.; Summers, Christopher J.
1995-01-01
The reliability of doped-barrier AlGaAs/GsAs multi-quantum well avalanche photodiodes fabricated by molecular beam epitaxy is investigated via accelerated life tests. Dark current and breakdown voltage were the parameters monitored. The activation energy of the degradation mechanism and median device lifetime were determined. Device failure probability as a function of time was computed using the lognormal model. Analysis using the electron beam induced current method revealed the degradation to be caused by ionic impurities or contamination in the passivation layer.
Properties of the magnetopolaron in a triangular quantum well
NASA Astrophysics Data System (ADS)
Yali, Li; Shuping, Shan
2015-08-01
We study the properties of the magnetopolaron in a triangular quantum well within LLP variational method. At different electron-phonon coupling strength, we derive the relations between the ground state energy, the ground state binding energy with the electron areal density and the cyclotron frequency of magnetic field, respectively. Our numerical results show that the ground state energy is an increasing function of the electron areal density and the cyclotron frequency of the magnetic field. However, the ground state binding energy is a decreasing function of those.
Ultrafast excitonic room temperature nonlinearity in neutron irradiated quantum wells
Ten, S.; Williams, J.G.; Guerreiro, P.T.; Khitrova, G.; Peyghambarian, N.
1997-01-01
Sharp room temperature exciton features and complete recovery of the excitonic absorption with 21 ps time constant are demonstrated in neutron irradiated (Ga,Al)As/GaAs multiple quantum wells. Carrier lifetime reduction is consistent with the EL2 midgap defect which is efficiently generated by fast neutrons. Influence of gamma rays accompanying neutron irradiation is discussed. Neutron irradiation provides a straightforward way to control carrier lifetime in semiconductor heterostructures with minor deterioration of their excitonic properties. {copyright} {ital 1997 American Institute of Physics.}
Strong photoluminescence emission from resonant Fibonacci quantum wells.
Chang, C H; Chen, C H; Hsueh, W J
2013-06-17
Strong photoluminescence (PL) emission from a resonant Fibonacci quantum well (FQW) is demonstrated. The maximum PL intensity in the FQW is significantly stronger than that in a periodic QW under the Bragg or anti-Bragg conditions. Moreover, the peaks of the squared electric field in the FQW are located very near each of the QWs. The optimal PL spectrum in the FQW has an asymmetrical form rather than the symmetrical one in the periodic case. The maximum PL intensity and the corresponding thickness filling factor in the FQW become greater with increasing generation order.
Magnetic field induced minigap in double quantum wells
Simmons, J.A.; Lyo, S.K.; Klem, J.F.; Harff, N.E. |
1994-07-01
We report discovery of a partial energy gap, or minigap, in strongly coupled double quantum wells (QWs), due to an anticrossing of the two QW dispersion curves. The anticrossing and minigap are induced by an in-plane magnetic field B{sub {parallel}}, and give rise to large distortions in the Fermi surface and density of states, including a Van Hove singularity. Sweeping B{sub {parallel}} moves the minigap through the Fermi level, with the upper and lower gap edges producing a sharp maximum and minimum in the low-temperature in-plane conductance, in agreement with theoretical calculations. The gap energy may be directly determined from the data.
Observing coherence effects in an overdamped quantum system
Lien, Y. -H.; Barontini, G.; Scheucher, M.; Mergenthaler, M.; Goldwin, J.; Hinds, E. A.
2016-01-01
It is usually considered that the spectrum of an optical cavity coupled to an atomic medium does not exhibit a normal-mode splitting unless the system satisfies the strong coupling condition, meaning the Rabi frequency of the coherent coupling exceeds the decay rates of atom and cavity excitations. Here we show that this need not be the case, but depends on the way in which the coupled system is probed. Measurements of the reflection of a probe laser from the input mirror of an overdamped cavity reveal an avoided crossing in the spectrum that is not observed when driving the atoms directly and measuring the Purcell-enhanced cavity emission. We understand these observations by noting a formal correspondence with electromagnetically induced transparency of a three-level atom in free space, where our cavity acts as the absorbing medium and the coupled atoms play the role of the control field. PMID:28000674
Observing coherence effects in an overdamped quantum system
NASA Astrophysics Data System (ADS)
Lien, Y.-H.; Barontini, G.; Scheucher, M.; Mergenthaler, M.; Goldwin, J.; Hinds, E. A.
2016-12-01
It is usually considered that the spectrum of an optical cavity coupled to an atomic medium does not exhibit a normal-mode splitting unless the system satisfies the strong coupling condition, meaning the Rabi frequency of the coherent coupling exceeds the decay rates of atom and cavity excitations. Here we show that this need not be the case, but depends on the way in which the coupled system is probed. Measurements of the reflection of a probe laser from the input mirror of an overdamped cavity reveal an avoided crossing in the spectrum that is not observed when driving the atoms directly and measuring the Purcell-enhanced cavity emission. We understand these observations by noting a formal correspondence with electromagnetically induced transparency of a three-level atom in free space, where our cavity acts as the absorbing medium and the coupled atoms play the role of the control field.
Mid- and Long-IR Broadband Quantum Well Photodetector
NASA Technical Reports Server (NTRS)
Soibel, Alexander; Ting, David Z.; Khoshakhlagh, Arezou; Gunapala, Sarath D.
2012-01-01
A single-stack broadband quantum well infrared photodetector (QWIP) has been developed that consists of stacked layers of GaAs/AlGaAs quantum wells with absorption peaks centered at various wavelengths spanning across the 9- to-11- m spectral regions. The correct design of broadband QWIPs was a critical step in this task because the earlier implementation of broadband QWIPs suffered from a tuning of spectral response curve with an applied bias. Here, a new QWIP design has been developed to overcome the spectral tuning with voltage that results from non-uniformity and bias variation of the electrical field across the detector stacks with different absorption wavelengths. In this design, a special effort has been made to avoid non-uniformity and bias tuning by changing the doping levels in detector stacks to compensate for variation of dark current generation rate across the stacks with different absorption wavelengths. Single-pixel photodetectors were grown, fabricated, and tested using this new design. The measured dark current is comparable with the dark measured current for single-color QWIP detectors with similar cutoff wavelength, thus indicating high material quality as well as absence of performance degradation resulting from broadband design. The measured spectra clearly demonstrate that the developed detectors cover the desired special range of 8 to 12 m. Moreover, the shape of the spectral curves does not change with applied biases, thus overcoming the problem plaguing previous designs of broadband QWIPs.
Highly efficient metallic optical incouplers for quantum well infrared photodetectors
Liu, Long; Chen, Yu; Huang, Zhong; Du, Wei; Zhan, Peng; Wang, Zhenlin
2016-01-01
Herein, we propose a highly efficient metallic optical incoupler for a quantum well infrared photodetector (QWIP) operating in the spectrum range of 14~16 μm, which consists of an array of metal micropatches and a periodically corrugated metallic back plate sandwiching a semiconductor active layer. By exploiting the excitations of microcavity modes and hybrid spoof surface plasmons (SSPs) modes, this optical incoupler can convert infrared radiation efficiently into the quantum wells (QWs) layer of semiconductor region with large electrical field component (Ez) normal to the plane of QWs. Our further numerical simulations for optimization indicate that by tuning microcavity mode to overlap with hybrid SSPs mode in spectrum, a coupled mode is formed, which leads to 33-fold enhanced light absorption for QWs centered at wavelength of 14.5 μm compared with isotropic absorption of QWs without any metallic microstructures, as well as a large value of coupling efficiency (η) of |Ez|2 ~ 6. This coupled mode shows a slight dispersion over ~40° and weak polarization dependence, which is quite beneficial to the high performance infrared photodetectors. PMID:27456691
Highly efficient metallic optical incouplers for quantum well infrared photodetectors
NASA Astrophysics Data System (ADS)
Liu, Long; Chen, Yu; Huang, Zhong; Du, Wei; Zhan, Peng; Wang, Zhenlin
2016-07-01
Herein, we propose a highly efficient metallic optical incoupler for a quantum well infrared photodetector (QWIP) operating in the spectrum range of 14~16 μm, which consists of an array of metal micropatches and a periodically corrugated metallic back plate sandwiching a semiconductor active layer. By exploiting the excitations of microcavity modes and hybrid spoof surface plasmons (SSPs) modes, this optical incoupler can convert infrared radiation efficiently into the quantum wells (QWs) layer of semiconductor region with large electrical field component (Ez) normal to the plane of QWs. Our further numerical simulations for optimization indicate that by tuning microcavity mode to overlap with hybrid SSPs mode in spectrum, a coupled mode is formed, which leads to 33-fold enhanced light absorption for QWs centered at wavelength of 14.5 μm compared with isotropic absorption of QWs without any metallic microstructures, as well as a large value of coupling efficiency (η) of |Ez|2 ~ 6. This coupled mode shows a slight dispersion over ~40° and weak polarization dependence, which is quite beneficial to the high performance infrared photodetectors.
Quantum Well Thermoelectrics for Converting Waste Heat to Electricity
Saeid Ghamaty
2007-04-01
Fabrication development of high efficiency quantum well (QW) thermoelectric continues with the P-type and N-type Si/Si{sub 80}Ge{sub 20} films with encouraging results. These films are fabricated on Si substrates and are being developed for low as well as high temperature operation. Both isothermal and gradient life testing are underway. One couple has achieved over 4000 hours at T{sub H} of 300 C and T{sub C} of 50 C with little or no degradation. Emphasis is now shifting towards couple and module design and fabrication, especially low resistance joining between N and P legs. These modules can be used in future energy conversion systems as well as for air conditioning.
Quantum dual signature scheme based on coherent states with entanglement swapping
NASA Astrophysics Data System (ADS)
Liu, Jia-Li; Shi, Rong-Hua; Shi, Jin-Jing; Lv, Ge-Li; Guo, Ying
2016-08-01
A novel quantum dual signature scheme, which combines two signed messages expected to be sent to two diverse receivers Bob and Charlie, is designed by applying entanglement swapping with coherent states. The signatory Alice signs two different messages with unitary operations (corresponding to the secret keys) and applies entanglement swapping to generate a quantum dual signature. The dual signature is firstly sent to the verifier Bob who extracts and verifies the signature of one message and transmits the rest of the dual signature to the verifier Charlie who verifies the signature of the other message. The transmission of the dual signature is realized with quantum teleportation of coherent states. The analysis shows that the security of secret keys and the security criteria of the signature protocol can be greatly guaranteed. An extensional multi-party quantum dual signature scheme which considers the case with more than three participants is also proposed in this paper and this scheme can remain secure. The proposed schemes are completely suited for the quantum communication network including multiple participants and can be applied to the e-commerce system which requires a secure payment among the customer, business and bank. Project supported by the National Natural Science Foundation of China (Grant Nos. 61272495, 61379153, and 61401519) and the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20130162110012).
Quantum entanglement generation in trapped ions using coherent and dissipative methods
NASA Astrophysics Data System (ADS)
Lin, Yiheng
Entangled states are a key resource in fundamental quantum physics, quantum cryptography, and quantum computation. In this thesis, we focus on the demonstrations of two novel methods to generate entanglement. First, we implement dissipative production of a maximally entangled steady state on two trapped ions. Dissipative and coherent processes are combined and implemented in a continuous time-independent fashion, analogous to optical pumping of atomic states, continuously driving the system towards the steady entangled state. With this method, we obtain a Bell state fidelity up to 0.89(2). Second, we propose and demonstrate a novel coherent process to confine quantum evolution in a subspace between an initial separable state and the target entangled state. We demonstrate this scheme on two and three ions obtaining a Bell state fidelity up to 0.992(2). Both of these methods are robust against certain types of experimental noise and decoherence. Additionally, we demonstrate sympathetic cooling of ion chains to near the ground state of motion with an electromagnetically-induced-transparency (EIT) method. This results in roughly an order of magnitude faster cooling time while using significantly lower laser power compared to the conventional resolved sideband cooling method. These techniques may be helpful for scaled-up quantum computing.
Non-Gaussianity of quantum states: An experimental test on single-photon-added coherent states
Barbieri, Marco; Ferreyrol, Franck; Blandino, Remi; Grangier, Philippe; Tualle-Brouri, Rosa; Spagnolo, Nicolo; Genoni, Marco G.; Paris, Matteo G. A.
2010-12-15
Non-Gaussian states and processes are useful resources in quantum information with continuous variables. An experimentally accessible criterion has been proposed to measure the degree of non-Gaussianity of quantum states based on the conditional entropy of the state with a Gaussian reference. Here we adopt such a criterion to characterize an important class of nonclassical states: single-photon-added coherent states. Our studies demonstrate the reliability and sensitivity of this measure and use it to quantify how detrimental is the role of experimental imperfections in our implementation.
Observation of "partial coherence" in an Aharonov-Bohm interferometer with a quantum dot.
Aikawa, Hisashi; Kobayashi, Kensuke; Sano, Akira; Katsumoto, Shingo; Iye, Yasuhiro
2004-04-30
We report experiments on the interference through spin states of electrons in a quantum dot (QD) embedded in an Aharonov-Bohm (AB) interferometer. We have picked up a spin-pair state, for which the environmental conditions are ideally similar. The AB amplitude is traced in a range of gate voltage that covers the pair. The behavior of the asymmetry in the amplitude around the two Coulomb peaks agrees with the theoretical prediction that the spin-flip process in a QD is related to the quantum dephasing of electrons. These results constitute evidence of "partial coherence" due to an entanglement of spins in the QD and in the interferometer.
Secure coherent-state quantum key distribution protocols with efficient reconciliation
Assche, G. van; Cerf, N.J.
2005-05-15
We study the equivalence of a realistic quantum key distribution protocol using coherent states and homodyne detection with a formal entanglement purification protocol. Maximally entangled qubit pairs that one can extract in the formal protocol correspond to secret key bits in the realistic protocol. More specifically, we define a qubit encoding scheme that allows the formal protocol to produce more than one entangled qubit pair per entangled oscillator pair or, equivalently for the realistic protocol, more than one secret key bit per coherent state. The entanglement parameters are estimated using quantum tomography. We analyze the properties of the encoding scheme and investigate the resulting secret key rate in the important case of the attenuation channel.
Theory of Transport Phenomena in Coherent Quantum Hall Bilayers
NASA Astrophysics Data System (ADS)
MacDonald, Allan H.; Chen, Hua; Sodemann, Inti
2015-03-01
We will describe a theory that allows to understand the anomalous transport properties of the excitonic condensate state occurring in quantum quantum Hall bilayers in terms of a picture in which the condensate phase is nearly uniform across the sample, and the strength of condensate coupling to interlayer tunneling processes is substantially reduced compared to the predictions of disorder-free microscopic mean-field theory. These ingredients provide a natural explanation for recently established I-V characteristics which feature a critical current above which the tunneling resistance abruptly increases and a non-local interaction between interlayer tunneling at the inner and outer edges of Corbino rings. We propose a microscopic picture in which disorder is the main agent responsible for the reduction of the effective interlayer tunneling strength. IS is supported by the Pappalardo Fellowship in Physics. HC and AHM are supported by DOE Division of Materials Sciences and Engineering Grant DE-FG03- 02ER45958 and Welch Foundation Grant TBF1473.
NASA Astrophysics Data System (ADS)
Zhou, Jian; Guo, Ying
2017-02-01
A continuous-variable measurement-device-independent (CV-MDI) multipartite quantum communication protocol is designed to realize multipartite communication based on the GHZ state analysis using Gaussian coherent states. It can remove detector side attack as the multi-mode measurement is blindly done in a suitable Black Box. The entanglement-based CV-MDI multipartite communication scheme and the equivalent prepare-and-measurement scheme are proposed to analyze the security and guide experiment, respectively. The general eavesdropping and coherent attack are considered for the security analysis. Subsequently, all the attacks are ascribed to coherent attack against imperfect links. The asymptotic key rate of the asymmetric configuration is also derived with the numeric simulations illustrating the performance of the proposed protocol.
Lee, Su-Yong; Kim, Ho-Joon; Ji, Se-Wan; Nha, Hyunchul
2011-07-15
We investigate how the entanglement properties of a two-mode state can be improved by performing a coherent superposition operation ta+ra{sup {dagger}} of photon subtraction and addition, proposed by Lee and Nha [Phys. Rev. A 82, 053812 (2010)], on each mode. We show that the degree of entanglement, the Einstein-Podolsky-Rosen-type correlation, and the performance of quantum teleportation can be all enhanced for the output state when the coherent operation is applied to a two-mode squeezed state. The effects of the coherent operation are more prominent than those of the mere photon subtraction a and the addition a{sup {dagger}} particularly in the small-squeezing regime, whereas the optimal operation becomes the photon subtraction (case of r=0) in the large-squeezing regime.
Andrea Rozzi, Carlo; Maria Falke, Sarah; Spallanzani, Nicola; Rubio, Angel; Molinari, Elisa; Brida, Daniele; Maiuri, Margherita; Cerullo, Giulio; Schramm, Heiko; Christoffers, Jens; Lienau, Christoph
2013-01-01
The efficient conversion of light into electricity or chemical fuels is a fundamental challenge. In artificial photosynthetic and photovoltaic devices, this conversion is generally thought to happen on ultrafast, femto-to-picosecond timescales and to involve an incoherent electron transfer process. In some biological systems, however, there is growing evidence that the coherent motion of electronic wavepackets is an essential primary step, raising questions about the role of quantum coherence in artificial devices. Here we investigate the primary charge-transfer process in a supramolecular triad, a prototypical artificial reaction centre. Combining high time-resolution femtosecond spectroscopy and time-dependent density functional theory, we provide compelling evidence that the driving mechanism of the photoinduced current generation cycle is a correlated wavelike motion of electrons and nuclei on a timescale of few tens of femtoseconds. We highlight the fundamental role of the interface between chromophore and charge acceptor in triggering the coherent wavelike electron-hole splitting. PMID:23511467
Quantum coherence oscillations in InSb and InAs
NASA Astrophysics Data System (ADS)
Peters, J. A.; Chen, Hong; Heremans, J. J.; Goel, N.; Chung, S. J.; Santos, M. B.; van Roy, W.; Borghs, G.
2006-03-01
Quantum oscillation phenomena in parallel arrays of loops have been investigated in InSb/AlInSb and InAs/AlGaSb heterostructures, notable for their strong spin-orbit interaction. The arrays consist of parallel lines of hexagonal lattice cells, forming linear concatenations of loops. From the h/2e periodicity, the dominance of Altshuler-Aronov-Spivak (AAS) oscillations is deduced. Measurement of the temperature dependence of the oscillations enables the extraction of spin and phase coherence lengths in InSb and InAs. The spin coherence lengths show a weak drop with increasing temperature, akin to the mobility mean free path behavior, and consistent with a dominant Elliott-Yafet related spin relaxation mechanism in both heterostructures. The phase coherence lengths follow a power law without observed saturation at the lowest temperatures. NSF DMR-0094055 (JJH), DMR-0080054, DMR-0209371 (MBS).
Electron-interface phonon interaction in multiple quantum well structures
NASA Astrophysics Data System (ADS)
Sun, J. P.; Teng, H. B.; Haddad, G. I.; Stroscio, M. A.
1998-08-01
Intersubband relaxation rates due to electron interactions with the interface phonons are evaluated for multiple quantum well structures designed for step quantum well lasers operating at mid-infrared to submillimetre wavelengths. The interface phonon modes and electron-phonon interaction Hamiltonians for the structures are derived using the transfer matrix method, based on the macroscopic dielectric continuum model, whereas the electron wavefunctions are obtained by solving the Schrödinger equation. Fermi's golden rule is employed to calculate the electron relaxation rates between the subbands in these structures. The relaxation rates for two different structures are examined and compared with those calculated using the bulk phonon modes and the Fröhlich interaction Hamiltonian. The sum rule for the relationship between the form factors of the various localized phonon modes and the bulk phonon modes is verified. The results obtained in this work illustrate that the transfer matrix method provides a convenient way for deriving the properties of the interface phonon modes in different structures of current interest and that, for preferential electron relaxation in intersubband laser structures, the effects of the interface phonon modes are significant and should be considered for optimal design of these laser structures.
Optical properties of transition metal oxide quantum wells
NASA Astrophysics Data System (ADS)
Demkov, Alexander; Choi, Miri; Butcher, Matthew; Rodriguez, Cesar; He, Qian; Posadas, Agham; Borisevich, Albina; Zollner, Stefan; Lin, Chungwei; Ortmann, Elliott
2015-03-01
We report on the investigation of SrTiO3/LaAlO3 quantum wells (QWs) grown by molecular beam epitaxy (MBE) on LaAlO3 substrate. Structures with different QW thicknesses ranging from two to ten unit cells were grown and characterized using x-ray photoemission spectroscopy, reflection high-energy electron diffraction (RHEED), scanning transmission electron microscopy (STEM). Optical properties (complex dielectric function) were measured by spectroscopic ellipsometry (SE) in the range of 1.0 eV to 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 (uc). Density functional theory and tight-binding are used to model the optical response of these heterostructures. Our results demonstrate that the energy level of the first sub-band can be controlled by the QW thickness in a complex oxide material. We acknowledge support from Air Force Office of Scientific Research (FA9550-12-10494).
Optimized photonic crystal design for quantum well infrared photodetectors
NASA Astrophysics Data System (ADS)
Reininger, P.; Kalchmair, S.; Gansch, R.; Andrews, A. M.; Detz, H.; Zederbauer, T.; Ahn, S. I.; Schrenk, W.; Strasser, G.
2012-06-01
The performance of quantum well infrared photodetectors (QWIP) can be significantly enhanced combining it with a photonic crystal slab (PCS) resonator. In such a system the chosen PCS mode is designed to coincide with the absorption maximum of the photodetector by adjusting the lattice parameters. However there is a multitude of parameter sets that exhibit the same resonance frequency of the chosen PCS mode. We have investigated how the choice of the PC design can be exploited for a further enhancement of QWIPs. Several sets of lattice parameters that exhibit the chosen PCS mode at the same resonance frequency have been obtained and the finite difference time domain method was used to simulate the absorption spectra of the different PCS. A photonic crystal slab quantum well infrared photodetector with three different photonic crystal lattice designs that exhibit the same resonance frequency of the chosen PCS mode were designed, fabricated and measured. This work shows that the quality factor of a PCS-QWIP and therefore the absorption enhancement can be increased by an optimized PCS design. The improvement is a combined effect of a changed lattice constant, PC normalized radius and normalized slab thickness. An enhancement of the measured photocurrent of more than a factor of two was measured.
Ultra-broad band, low power, highly efficient coherent wavelength conversion in quantum dot SOA.
Contestabile, G; Yoshida, Y; Maruta, A; Kitayama, K
2012-12-03
We report broadband, all-optical wavelength conversion over 100 nm span, in full S- and C-band, with positive conversion efficiency with low optical input power exploiting dual pump Four-Wave-Mixing in a Quantum Dot Semiconductor Optical Amplifier (QD-SOA). We also demonstrate by Error Vector Magnitude analysis the full transparency of the conversion scheme for coherent modulation formats (QPSK, 8-PSK, 16-QAM, OFDM-16QAM) in the whole C-band.
Das, Kunal K.
2011-09-15
We propose a way to simulate mesoscopic transport processes with counterpropagating wave packets of ultracold atoms in quasi-one-dimensional (1D) waveguides and show quantitative agreement with analytical results. The method allows the study of a broad range of transport processes at the level of individual modes, not possible in electronic systems. Typically suppressed effects of quantum coherence become manifest, along with the effects of tunable interactions, which can be used to develop a simpler type of sensitive atom interferometer.
Origins of low energy-transfer efficiency between patterned GaN quantum well and CdSe quantum dots
Xu, Xingsheng
2015-03-02
For hybrid light emitting devices (LEDs) consisting of GaN quantum wells and colloidal quantum dots, it is necessary to explore the physical mechanisms causing decreases in the quantum efficiencies and the energy transfer efficiency between a GaN quantum well and CdSe quantum dots. This study investigated the electro-luminescence for a hybrid LED consisting of colloidal quantum dots and a GaN quantum well patterned with photonic crystals. It was found that both the quantum efficiency of colloidal quantum dots on a GaN quantum well and the energy transfer efficiency between the patterned GaN quantum well and the colloidal quantum dots decreased with increases in the driving voltage or the driving time. Under high driving voltages, the decreases in the quantum efficiency of the colloidal quantum dots and the energy transfer efficiency can be attributed to Auger recombination, while those decreases under long driving time are due to photo-bleaching and Auger recombination.
NASA Astrophysics Data System (ADS)
Chen, Disheng; Lander, Gary R.; Solomon, Glenn S.; Flagg, Edward B.
2017-01-01
Resonant photoluminescence excitation (RPLE) spectra of a neutral InGaAs quantum dot show unconventional line shapes that depend on the detection polarization. We characterize this phenomenon by performing polarization-dependent RPLE measurements and simulating the measured spectra with a three-level quantum model. The spectra are explained by interference between fields coherently scattered from the two fine structure split exciton states, and the measurements enable extraction of the steady-state coherence between the two exciton states.
2. QUANTUM HALL EFFECT: Hidden SU(4) symmetry in bilayer quantum well at integer filling factors
NASA Astrophysics Data System (ADS)
Fal'ko, V. I.; Iordanskii, S. V.; Kashuba, A. B.
2001-10-01
Phase diagram of a bilayer quantum well at integer filling factors is established using the hidden symmetry method. Three phases: ferromagnetic, canted antiferromagnetic (CAP) and spin-singlet, have been found. We confirm early results of Das Sarma et al. Each phase violates the SU(4) hidden symmetry and is stabilized by the anisotropy interactions.
Efficiency of GaInAs/GaAs quantum-well lasers upon inhomogeneous excitation of quantum wells
Ushakov, D V; Afonenko, A A; Aleshkin, V Ya
2013-11-30
A model for calculating the power characteristics of laser structures taking into account inhomogeneous excitation of quantum wells (QWs), recombination in the barrier regions, and nonlinear gain effects is developed. It is shown that, with increasing number of QWs, the output power of the Ga{sub 0.8}In{sub 0.2}As/GaAs/InGaP structures at first considerably increases and then slightly decreases. In a wide range of injection currents, the optimal number of QWs is 5 ± 1. The inhomogeneity of QW excitation increases with increasing injection current and decreases the laser power compared to homogeneous excitation. (lasers)
Time-optimal excitation of maximum quantum coherence: Physical limits and pulse sequences
NASA Astrophysics Data System (ADS)
Köcher, S. S.; Heydenreich, T.; Zhang, Y.; Reddy, G. N. M.; Caldarelli, S.; Yuan, H.; Glaser, S. J.
2016-04-01
Here we study the optimum efficiency of the excitation of maximum quantum (MaxQ) coherence using analytical and numerical methods based on optimal control theory. The theoretical limit of the achievable MaxQ amplitude and the minimum time to achieve this limit are explored for a set of model systems consisting of up to five coupled spins. In addition to arbitrary pulse shapes, two simple pulse sequence families of practical interest are considered in the optimizations. Compared to conventional approaches, substantial gains were found both in terms of the achieved MaxQ amplitude and in pulse sequence durations. For a model system, theoretically predicted gains of a factor of three compared to the conventional pulse sequence were experimentally demonstrated. Motivated by the numerical results, also two novel analytical transfer schemes were found: Compared to conventional approaches based on non-selective pulses and delays, double-quantum coherence in two-spin systems can be created twice as fast using isotropic mixing and hard spin-selective pulses. Also it is proved that in a chain of three weakly coupled spins with the same coupling constants, triple-quantum coherence can be created in a time-optimal fashion using so-called geodesic pulses.
Time-optimal excitation of maximum quantum coherence: Physical limits and pulse sequences.
Köcher, S S; Heydenreich, T; Zhang, Y; Reddy, G N M; Caldarelli, S; Yuan, H; Glaser, S J
2016-04-28
Here we study the optimum efficiency of the excitation of maximum quantum (MaxQ) coherence using analytical and numerical methods based on optimal control theory. The theoretical limit of the achievable MaxQ amplitude and the minimum time to achieve this limit are explored for a set of model systems consisting of up to five coupled spins. In addition to arbitrary pulse shapes, two simple pulse sequence families of practical interest are considered in the optimizations. Compared to conventional approaches, substantial gains were found both in terms of the achieved MaxQ amplitude and in pulse sequence durations. For a model system, theoretically predicted gains of a factor of three compared to the conventional pulse sequence were experimentally demonstrated. Motivated by the numerical results, also two novel analytical transfer schemes were found: Compared to conventional approaches based on non-selective pulses and delays, double-quantum coherence in two-spin systems can be created twice as fast using isotropic mixing and hard spin-selective pulses. Also it is proved that in a chain of three weakly coupled spins with the same coupling constants, triple-quantum coherence can be created in a time-optimal fashion using so-called geodesic pulses.
Quantum dynamics in strong fields with Fermion Coupled Coherent States
NASA Astrophysics Data System (ADS)
Kirrander, Adam; Shalashilin, Dmitrii V.
2012-06-01
We present a new version of the Coupled Coherent State method, specifically adapted for solving the time-dependent Schr"odinger equation for multi-electron dynamics in atoms and molecules. This new theory takes explicit account of the exchange symmetry of fermion particles, and uses fermion molecular dynamics to propagate trajectories. As a demonstration, calculations in the He atom are performed using the full Hamiltonian and accurate experimental parameters. Single and double ionization yields by 160 fs and 780 nm laser pulses are calculated as a function of field intensity in the range 10^14 - 10^16 W/cm^2 and good agreement with experiments by Walker et al. is obtained. Since this method is trajectory based, mechanistic analysis of the dynamics is straightforward. We also calculate semiclassical momentum distributions for double ionization following 25 fs and 795 nm pulses at 1.5 10^15 W/cm^2, in order to compare to the detailed experiments by Rudenko et al. For this more challenging task, full convergence is not achieved, but however major effects such as the finger-like structures in the momentum distribution are reproduced.
Enhancement of electron mobility in asymmetric coupled quantum well structures
Das, S.; Nayak, R. K.; Sahu, T. Panda, A. K.
2014-02-21
We study the low temperature multisubband electron mobility in a structurally asymmetric GaAs/Al{sub x}Ga{sub 1-x}As delta doped double quantum well. We calculate the subband energy levels and wave functions through selfconsistent solution of the coupled Schrodinger equation and Poisson's equation. We consider ionized impurity scattering, interface roughness scattering, and alloy disorder scattering to calculate the electron mobility. The screening of the scattering potentials is obtained by using static dielectric response function formalism within the random phase approximation. We analyze, for the first time, the effect of asymmetric structure parameters on the enhancement of multisubband electron mobility through intersubband interactions. We show that the asymmetric variation of well width, doping concentration, and spacer width considerably influences the interplay of scattering mechanisms on mobility. Our results of asymmetry induced enhancement of electron mobility can be utilized for low temperature device applications.
A Secure Key Distribution System of Quantum Cryptography Based on the Coherent State
NASA Technical Reports Server (NTRS)
Guo, Guang-Can; Zhang, Xiao-Yu
1996-01-01
The cryptographic communication has a lot of important applications, particularly in the magnificent prospects of private communication. As one knows, the security of cryptographic channel depends crucially on the secrecy of the key. The Vernam cipher is the only cipher system which has guaranteed security. In that system the key must be as long as the message and most be used only once. Quantum cryptography is a method whereby key secrecy can be guaranteed by a physical law. So it is impossible, even in principle, to eavesdrop on such channels. Quantum cryptography has been developed in recent years. Up to now, many schemes of quantum cryptography have been proposed. Now one of the main problems in this field is how to increase transmission distance. In order to use quantum nature of light, up to now proposed schemes all use very dim light pulses. The average photon number is about 0.1. Because of the loss of the optical fiber, it is difficult for the quantum cryptography based on one photon level or on dim light to realize quantum key-distribution over long distance. A quantum key distribution based on coherent state is introduced in this paper. Here we discuss the feasibility and security of this scheme.
Coherent Optical Control of Quantum Dots: Spin Qubits and Flying Qubits
NASA Astrophysics Data System (ADS)
Burgers, Alex
2015-03-01
Coherent control of solid-state qubits lies at the heart of most quantum information architectures. In quantum dots (QDs), optical fields are an attractive medium for qubit manipulation and readout. The entanglement between a QD spin qubit and an emitted photonic qubit allows for the transport of quantum information between distant quantum memories via decoherence resistant photon channels. I will present recent experimental work showing the entanglement between a single electron spin confined to an InAs QD and its spontaneously emitted photon. This entanglement is significant for the further development of quantum information technologies using QDs and forms the foundation of on-chip technologies using photonic crystal pathways. In addition, I will discuss on-going work on teleportation of information from a single photon generated in a spontaneous parametric down conversion (SPDC) process to a QD spin through intermediate interference between the SPDC photon and the dot's emitted photon. The ability to integrate two quantum information platforms is not only exciting in its own right, but this technique could allow for an entanglement swapping bridge between other matter-qubit (ions, NV centers, etc.) based quantum memories. This work is funded by NSF, ARO, AFOSR, ONR and DARPA.
NASA Astrophysics Data System (ADS)
Huang, Zhiming; Situ, Haozhen
2017-02-01
In this article, the dynamics of quantum correlation and coherence for two atoms interacting with a bath of fluctuating massless scalar field in the Minkowski vacuum is investigated. We firstly derive the master equation that describes the system evolution with initial Bell-diagonal state. Then we discuss the system evolution for three cases of different initial states: non-zero correlation separable state, maximally entangled state and zero correlation state. For non-zero correlation initial separable state, quantum correlation and coherence can be protected from vacuum fluctuations during long time evolution when the separation between the two atoms is relatively small. For maximally entangled initial state, quantum correlation and coherence overall decrease with evolution time. However, for the zero correlation initial state, quantum correlation and coherence are firstly generated and then drop with evolution time; when separation is sufficiently small, they can survive from vacuum fluctuations. For three cases, quantum correlation and coherence first undergo decline and then fluctuate to relatively stable values with the increasing distance between the two atoms. Specially, for the case of zero correlation initial state, quantum correlation and coherence occur periodically revival at fixed zero points and revival amplitude declines gradually with increasing separation of two atoms.
Phase control of light propagation via Fano interference in asymmetric double quantum wells
Yang, Wen-Xing; Lu, Jia-Wei; Zhou, Zhi-Kang; Yang, Long; Lee, Ray-Kuang
2014-05-28
We investigate the light propagation and dynamical control of a weak pulsed probe field in asymmetric double quantum wells via Fano interference, which is caused by tunneling from the excited subbands to the same continuum. Our results show that the system can produce anomalous and normal dispersion regions with negligible absorption by choosing appropriate coupling strength of the tunneling and the Fano interference. Interesting enough, the dispersion can be switched between normal and anomalous by adjusting the relative phase between the pulsed probe and coherent control fields owing to the existence of the perfectly Fano interference. Thus, the relative phase can be regarded as a switch to manipulate light propagation with subluminal or superluminal. The temporal and spatial dynamics of the pulsed probe field with hyperbolic secant envelope are analyzed.
NASA Astrophysics Data System (ADS)
Bao, Jianfeng; Cui, Xiaohong; Huang, Yuqing; Zhong, Jianhui; Chen, Zhong
2015-08-01
High-resolution 1H magnetic resonance spectroscopy (MRS) is generally inaccessible in red bone marrow (RBM) tissues using conventional MRS techniques. This is because signal from these tissues suffers from severe inhomogeneity in the main static B0 field originated from the intrinsic honeycomb structures in trabecular bone. One way to reduce effects of B0 field inhomogeneity is by using the intermolecular double quantum coherence (iDQC) technique, which has been shown in other systems to obtain signals insensitive to B0 field inhomogeneity. In the present study, we employed an iDQC approach to enhance the spectral resolution of RBM. The feasibility and performance of this method for achieving high resolution MRS was verified by experiments on phantoms and pig vertebral bone samples. Unsaturated fatty acid peaks which overlap in the conventional MRS were well resolved and identified in the iDQC spectrum. Quantitative comparison of fractions of three types of fatty acids was performed between iDQC spectra on the in situ RMB and conventional MRS on the extracted fat from the same RBM. Observations of unsaturated fatty acids with iDQC MRS may provide valuable information and may hold potential in diagnosis of diseases such as obesity, diabetes, and leukemia.
Vignesh, G.; Nithiananthi, P.
2015-06-24
Diamagnetic susceptibility of a randomly distributed donor in a GaAs/Al{sub 0.3}Ga{sub 0.7}As Double Quantum Well has been calculated in its ground state as a function of barrier and well width. It is shown that the modification in the barrier and well dimension significantly influences the dimensional character of the donor through modulating the subband distribution and in turn the localization of the donor. The effect of barrier and well thickness on the interparticle distance has also been observed. Interestingly it opens up the possibility of tuning the susceptibility and monitoring the tunnel coupling among the wells.
Self-coherent phase reference sharing for continuous-variable quantum key distribution
NASA Astrophysics Data System (ADS)
Marie, Adrien; Alléaume, Romain
2017-01-01
We develop a comprehensive framework to model and optimize the performance of continuous-variable quantum key distribution (CV-QKD) with a local local oscillator (LLO), when phase reference sharing and QKD are jointly implemented. We first analyze the limitations of the only existing approach, called LLO-sequential, and show that it requires high modulation dynamics and can only tolerate small phase noise. Our main contribution is to introduce two designs to perform LLO CV-QKD, respectively called LLO-delayline and LLO-displacement, and to study their performance. Both designs rely on a self-coherent approach, in which phase reference information and quantum information are coherently obtained from a single optical wavefront. We show that these designs can lift some limitations of the existing LLO-sequential approach. The LLO-delayline design can in particular tolerate much stronger phase noise and thus appears to be an appealing alternative to LLO-sequential in terms of network integrability. We also investigate, with the LLO-displacement design, how phase reference information and quantum information can be multiplexed within a single optical pulse. By studying the trade-off between phase reference recovery and phase noise induced by displacement, we, however, demonstrate that this design can only tolerate low phase noise. On the other hand, the LLO-displacement design has the advantage of minimal hardware requirements and provides a simple approach to multiplex classical and quantum communications, opening a practical path towards the development of ubiquitous coherent classical-quantum communications systems compatible with next-generation network requirements.
Quantum heat engine power can be increased by noise-induced coherence.
Scully, Marlan O; Chapin, Kimberly R; Dorfman, Konstantin E; Kim, Moochan Barnabas; Svidzinsky, Anatoly
2011-09-13
Laser and photocell quantum heat engines (QHEs) are powered by thermal light and governed by the laws of quantum thermodynamics. To appreciate the deep connection between quantum mechanics and thermodynamics we need only recall that in 1901 Planck introduced the quantum of action to calculate the entropy of thermal light, and in 1905 Einstein's studies of the entropy of thermal light led him to introduce the photon. Then in 1917, he discovered stimulated emission by using detailed balance arguments. Half a century later, Scovil and Schulz-DuBois applied detailed balance ideas to show that maser photons were produced with Carnot quantum efficiency (see Fig. 1A). Furthermore, Shockley and Quiesser invoked detailed balance to obtain the efficiency of a photocell illuminated by "hot" thermal light (see Fig. 2A). To understand this detailed balance limit, we note that in the QHE, the incident light excites electrons, which can then deliver useful work to a load. However, the efficiency is limited by radiative recombination in which the excited electrons are returned to the ground state. But it has been proven that radiatively induced quantum coherence can break detailed balance and yield lasing without inversion. Here we show that noise-induced coherence enables us to break detailed balance and get more power out of a laser or photocell QHE. Surprisingly, this coherence can be induced by the same noisy (thermal) emission and absorption processes that drive the QHE (see Fig. 3A). Furthermore, this noise-induced coherence can be robust against environmental decoherence.Fig. 1.(A) Schematic of a laser pumped by hot photons at temperature T(h) (energy source, blue) and by cold photons at temperature T(c) (entropy sink, red). The laser emits photons (green) such that at threshold the laser photon energy and pump photon energy is related by Carnot efficiency (4). (B) Schematic of atoms inside the cavity. Lower level b is coupled to the excited states a and β. The laser power
NASA Astrophysics Data System (ADS)
Derkach, Ivan D.; Peuntinger, Christian; Ruppert, László; Heim, Bettina; Gunthner, Kevin; Usenko, Vladyslav C.; Elser, Dominique; Marquardt, Christoph; Filip, Radim; Leuchs, Gerd
2016-10-01
Continuous-variable quantum key distribution is a practical application of quantum information theory that is aimed at generation of secret cryptographic key between two remote trusted parties and that uses multi-photon quantum states as carriers of key bits. Remote parties share the secret key via a quantum channel, that presumably is under control of of an eavesdropper, and which properties must be taken into account in the security analysis. Well-studied fiber-optical quantum channels commonly possess stable transmittance and low noise levels, while free-space channels represent a simpler, less demanding and more flexible alternative, but suffer from atmospheric effects such as turbulence that in particular causes a non-uniform transmittance distribution referred to as fading. Nonetheless free-space channels, providing an unobstructed line-of-sight, are more apt for short, mid-range and potentially long-range (using satellites) communication and will play an important role in the future development and implementation of QKD networks. It was previously theoretically shown that coherent-state CV QKD should be in principle possible to implement over a free-space fading channel, but strong transmittance fluctuations result in the significant modulation-dependent channel excess noise. In this regime the post-selection of highly transmitting sub-channels may be needed, which can even restore the security of the protocol in the strongly turbulent channels. We now report the first proof-of-principle experimental test of coherent state CV QKD protocol using different levels Gaussian modulation over a mid-range (1.6-kilometer long) free-space atmospheric quantum channel. The transmittance of the link was characterized using intensity measurements for the reference but channel estimation using the modulated coherent states was also studied. We consider security against Gaussian collective attacks, that were shown to be optimal against CV QKD protocols . We assumed a
Scattering theory of nonlinear thermoelectricity in quantum coherent conductors.
Meair, Jonathan; Jacquod, Philippe
2013-02-27
We construct a scattering theory of weakly nonlinear thermoelectric transport through sub-micron scale conductors. The theory incorporates the leading nonlinear contributions in temperature and voltage biases to the charge and heat currents. Because of the finite capacitances of sub-micron scale conducting circuits, fundamental conservation laws such as gauge invariance and current conservation require special care to be preserved. We do this by extending the approach of Christen and Büttiker (1996 Europhys. Lett. 35 523) to coupled charge and heat transport. In this way we write relations connecting nonlinear transport coefficients in a manner similar to Mott's relation between the linear thermopower and the linear conductance. We derive sum rules that nonlinear transport coefficients must satisfy to preserve gauge invariance and current conservation. We illustrate our theory by calculating the efficiency of heat engines and the coefficient of performance of thermoelectric refrigerators based on quantum point contacts and resonant tunneling barriers. We identify, in particular, rectification effects that increase device performance.
II-VI semiconductor quantum dot quantum wells: a tight-binding study
NASA Astrophysics Data System (ADS)
Pérez-Conde, J.; Bhattacharjee, A. K.
2006-05-01
We have studied the electronic structure, exciton states and optical spectra of spherical semiconductor quantum dot quantum wells (QDQW's) by means of a symmetry-adapted tight-binding (TB) method. We have investigated two classes of QDQW's: CdS/HgS/CdS, based on a CdS core which acts as a barrier, with a thin HgS well layer intercalated between the core and a clad layer of CdS. The second class of QDQW's is based on ZnS cores covered with CdS layers which act in this case as a well. The calculated values of the absorption onset show a good agreement with the experimental data. Large photoluminescence Stokes shifts are also predicted.
NASA Astrophysics Data System (ADS)
Giorgioni, Anna; Paleari, Stefano; Cecchi, Stefano; Vitiello, Elisa; Grilli, Emanuele; Isella, Giovanni; Jantsch, Wolfgang; Fanciulli, Marco; Pezzoli, Fabio
2016-12-01
Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the desirable but contrasting requirements of spin robustness against relaxation mechanisms and sizeable coupling between spin and orbital motion of the carriers. Here, we focus on Ge, which is a prominent candidate for shuttling spin quantum bits into the mainstream Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome these fundamental limitations by investigating a two-dimensional electron gas in quantum wells of pure Ge grown on Si. These epitaxial systems demonstrate exceptionally long spin lifetimes. In particular, by fine-tuning quantum confinement we demonstrate that the electron Landé g factor can be engineered in our CMOS-compatible architecture over a range previously inaccessible for Si spintronics.
Giorgioni, Anna; Paleari, Stefano; Cecchi, Stefano; Vitiello, Elisa; Grilli, Emanuele; Isella, Giovanni; Jantsch, Wolfgang; Fanciulli, Marco; Pezzoli, Fabio
2016-01-01
Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the desirable but contrasting requirements of spin robustness against relaxation mechanisms and sizeable coupling between spin and orbital motion of the carriers. Here, we focus on Ge, which is a prominent candidate for shuttling spin quantum bits into the mainstream Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome these fundamental limitations by investigating a two-dimensional electron gas in quantum wells of pure Ge grown on Si. These epitaxial systems demonstrate exceptionally long spin lifetimes. In particular, by fine-tuning quantum confinement we demonstrate that the electron Landé g factor can be engineered in our CMOS-compatible architecture over a range previously inaccessible for Si spintronics. PMID:28000670