Spinor-electron wave guided modes in coupled quantum wells structures by solving the Dirac equation

A quantum analysis based on the Dirac equation of the propagation of spinor-electron waves in coupled quantum wells, or equivalently coupled electron waveguides, is presented. The complete optical wave equations for Spin-Up (SU) and Spin-Down (SD) spinor-electron waves in these electron guides couplers are derived from the Dirac equation. The relativistic amplitudes and dispersion equations of the spinor-electron wave-guided modes in a planar quantum coupler formed by two coupled quantum wells, or equivalently by two coupled slab electron waveguides, are exactly derived. The main outcomes related to the spinor modal structure, such as the breaking of the non-relativistic degenerate spin states, the appearance of phase shifts associated with the spin polarization and so on, are shown.     

The temperature effect of the triangular bound potential quantum dot qubit

We study the eigenenergies and eigenfunctions of the ground and the first-excited states of an electron, which is strongly coupled to LO-phonon in a quantum dot with triangular bound potential by using the Pekar variational method. This system may be used as a two-level qubit. Numerical calculations are performed on the electron probability density varying with respect to the time, the temperature, the electron–LO-phonon coupling strength, the confinement length of the quantum dot and the polar angle. The relationship between the oscillating period and the polar angle is derived.     

Electron entanglement detected by quantum spin Hall systems

We propose a promising electron entanglement detector consisting of two quantum spin Hall systems weakly coupled to a superconductor. The detection of electron spins along various polarization directions, which is a prerequisite for testing Bell's inequality on solid state spins, can be achieved in an all-electrical-controlled manner utilizing the helical edge states. It is found that the violation of Bell's inequality exists in a large range of the tunneling parameters, which can be realized in mercury telluride quantum wells.     

Coulomb bound potential quantum rod qubit

We study the eigenenergies and the eigenfunctions of the ground and the first excited states of an electron, which is strongly coupled to LO-phonon in a quantum rod with a hydrogen-like impurity at the center by using the variational method of Pekar type. This quantum rod system may be used as a two-level quantum qubit. When the electron is in the superposition state of the ground and the first-excited states, the probability density of the electron oscillates in the quantum rod. It is found that the probability density and the oscillation period are individually increased and decreased due to the presence of the Coulomb interaction between the electron and the hydrogen-like impurity. The oscillation period is an increasing function of the ellipsoid aspect ratio and the effective confinement lengths of the quantum rod, whereas it is a decreasing one of the electron–phonon coupling strength.     

Tunneling-induced coherent electron population transfer in an asymmetric quantum well

We propose an asymmetric double quantum wells structure with a common continuum and investigate the effect of resonant tunneling on the control of coherent electron population transfer between the two quantum wells. By numerically solving the motion equations of element moments, the almost complete electron population transfer from the initial subband to the target subband could be realized due to the constructive interference via flexibly adjusting the structure parameters.     

Optical electron transfer between quantum dots

The continuous spectrum in the problem of resonance optical transitions between bound states of quantum wells is taken into account by the method of equivalence transformation of the initial Hamiltonian. The effective Hamiltonian of resonance interaction, describing the decay of levels to a continuous spectrum, is obtained. The formulas obtained are applied to the problem of resonance electron transfer between quantum dots. The conditions for effective resonance electron transfer are determined.     

Optical absorption and magnetotransport in quantum wires in a perpendicular magnetic field

A periodic array of δ function potentials are used to simulate the potential barriers between quantum wires in the presence or absence of lattice site dislocation. The exact eigenenergies and eigenfunctions are found by employing a numerical diagonalization procedure. Based on these results, a self-consistent field theory is derived for the mid-infrared absorption coefficient of the system. The crossover from a cyclotron mode to two tunneling coupled modes and finally to edge and 1D lattice magnetoplasmon modes with increasing modulation strength is investigated. The magnetic field enhanced and suppressed electron tunneling, associated with the evolution to cyclotron modes at strong magnetic fields passing through the formation of tunneling coupled modes, is observed. The edge mode excitation energy oscillates as a function of the electron density. These oscillations correspond to a soft or hard potential wall for which the electron states are extended or localized, respectively. The displacement of the 1D lattice magnetoplasmon modes under strong modulation is found to be periodic and corresponds to the evolution from a complex unit cell which is composed of one narrow and one wide quantum wire to a simple unit cell containing only one quantum wire. The magnetoresistivities and the associated conductivities are also calculated for the lateral surface superlattice. At strong potential modulation there is a giant peak in the Hall conductivity and many peaks in its resistivity in the quantum regime. With strong modulation, the suppression of the transverse conductivity along with oscillations in its resistivity are obtained.     

Thickness effects on the Coulomb drag rate in double quantum layer systems

In this paper, we have investigated the effect of quantum layer thickness on Coulomb drag phenomenon in a double quantum well (DQW) system, in which the electrons momentum can transfer from one layer to another. We have applied the full random phase approximation (RPA) in dynamical dielectric matrix of this coupled two-dimensional electron gas (2DEG) system in order to obtain an improved result for temperature-dependent rate of momentum transfer. We have calculated the drag rate transresistivity for various well thicknesses at low and intermediate temperatures in Fermi-scale and for different electron gas densities. It has been obtained that the Coulomb drag rate increases with increasing the well width when the separation between the wells remains unchanged.     

Parametric instabilities in quantum plasmas with electron exchange-correlation effects

Parametric instabilities induced by the nonlinear interaction between high frequency quantum Langmuir waves and low frequency quantum ion-acoustic waves in quantum plasmas with the electron exchange-correlation effects are presented.By using the quantum hydrodynamic equations with the electron exchange-correlation correction,we obtain an effective quantum Zaharov model,which is then used to derive the modified dispersion relations and the growth rates of the decay and four-wave instabilities.The influences of the electron exchange-correlation effects and the quantum effects on the existence of quantum Langmuir waves and the parametric instabilities are discussed in detail.It is shown that the electron exchange-correlation effects and quantum effects are strongly coupled.The quantum Langmuir wave can propagate in quantum plasmas only when the electron exchange-correlation effects and the quantum effects satisfy a certain condition.The electron exchange-correlation effects tend to enhance the parametric instabilities,while quantum effects suppress the instabilities.     

Specific features of the hybridization of Frenkel and Wannier–Mott excitons in a microcavity

Polariton states have been investigated in a microcavity, where the energy of the Frenkel exciton in an organic quantum well and the energy of the semiconductor Wannier–Mott exciton in an inorganic quantum well are close to the microcavity optical mode. It has been shown that the interaction of each of these excitons with the microcavity optical mode leads to their interaction with each other and to the formation of mutually coupled hybrid excitations. The influence of the location of the quantum wells in a microcavity on the spectra of hybrid states with different polarizations has been analyzed.     

Spectrum of indirect magnetoexcitons in coupled quantum wells

The indirect Mott exciton (spatially-separated electron and hole) in coupled quantum wells in crossed electric and magnetic fields is discussed. The exciton spectrum is calculated for the case where the distance between the quantum wells of the electron and hole is larger than the exciton Bohr radius. The magnetoexciton creation probability is calculated and its dependence on the electric field is found. The absorption of electromagnetic radiation between the indirect magnetoexciton levels in coupled quantum wells is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 2220–2223 (December 1997)     

The decoherence of the triangular and Coulomb bound potential quantum dot qubit

We study the eigenenergies and eigenfunctions of the ground and first-excited states of an electron which is strongly coupled to an LO-phonon in a quantum dot with a triangular bound potential and Coulomb bound potential by using the Pekar variational method. This system may be used as a two-level qubit. Phonon spontaneous emission causes the decoherence of the qubit. Numerical calculations are performed on the decoherence rate as a function of the polar angle, the Coulomb binding parameter, the coupling strength, the confinement length of the quantum dot and the dispersion coefficient.     

Signature of a Fano resonance in a plasmonic metamolecule's local density of optical states

We present measurements on plasmonic metamolecules under local excitation using cathodoluminescence which show a spatial redistribution of the local density of optical states at the same frequency where a sharp spectral Fano feature in extinction has been observed. Our analytical model shows that both near- and far-field effects arise due to interference of the same two eigenmodes of the system. We present quantitative insights both in a bare state, and in a dressed state picture that describe Fano interference either as near-field amplitude transfer between coupled bare states, or as interference of uncoupled eigenmodes in the far field. We identify the same eigenmode causing a dip in extinction to strongly enhance the radiative local density of optical states, making it a promising candidate for spontaneous emission control.     

Intersubband relaxation of hot carriers in quantum well systems

We use an ensemble Monte Carlo simulation of coupled electrons, holes and nonequilibrium polar optical phonons in multiple quantum well systems to model the intersubband relaxation of hot carriers measured in ultra-fast optical experiments. We have investigated the effect of various models of confined photon modes on the energy relaxation and intersubband transition rate in single quantum well and coupled well systems. In particular, the symmetry of the atomic displacement with respect to the quantum well has a marked effect on the relative intersubband versus intrasubband scattering rates, depending on whether one considers electrostatic boundary conditions(slab modes) or mechanical boundary conditions(guided modes). In single quantum wells systems, the overall intersubband relaxation time is not found to be strongly dependent on the confined mode model used due to competing effects of hot phonons and the relative intrasubband scattering rates. For coupled well systems, the relaxation rate is much more dependent on the exact nature of the phonon amplitude. Large effects are found associated with localized AlAs interface modes which dominate the intersubband relaxation time.     

Detection of the electron spin resonance of two-dimensional electrons at large wave vectors

We have investigated the electron spin resonance at nonzero wave vector in GaAs single quantum wells by combining the virtues of high frequency surface acoustic wave generation to produce excitations with large wave numbers with a sensitive optical scheme to detect resonant absorption. The observed large deviations from the single particle Zeeman energy are attributed to the exchange interaction. The enhancement of the electronic g* factor is, however, substantially smaller compared with theoretical predictions for spin waves when adopting a bare Coulomb interaction potential.     

Interaction of Wannier–Stark levels in a wide well superlattice

We have investigated the energy spectrum of a superlattice with wide quantum wells under the bias of an electric field perpendicular to the superlattice layers. By using photocurrent spectroscopy, transitions of Wannier–Stark levels for the various electron and hole states are observed, and at low fields, further structures corresponding to miniband edge transitions are found. Various anticrossings could be observed at higher and lower electric fields. The anticrossings at high electric fields are due to energy alignment of different electronic sublevels in adjacent wells. The anticrossing structures at low fields could be interpreted as resonances between intrawell and interwell excitonic Wannier–Stark states with equal sublevel states, where the anticrossing is caused by differences in exciton binding energy. Fitting of transitions and anticrossings was done by using a semi-empirical model and we have extracted relevant fitting parameters like the quantum-confined Stark coefficient, binding energies for the excitonic Wannier–Stark levels and the resonant coupling strength for states involved in the various anticrossing transitions. Finally, insight into the excitonic influences on the coupling of the WS states could be obtained by comparing the fitted parameters for the various transitions.     

Quantum crystallization of two-dimensional dipole systems

An analysis is made of the region of existence of crystalline order in a system of spatially separated electrons (e) and holes (h) in two coupled quantum wells for various concentrations n, temperatures T, and distances D between the layers. A study is also made of crystallization in a system of electrons in semiconductor structures near a metal electrode for various distances d between the semiconductor and the metal. Calculations of the crystalline phase were made using variational calculations of the ground-state energy of the system allowing for pairing of quasiparticles with nonzero momentum. For a system of two coupled quantum wells, regions in (T,n,D) space are determined in which electron (or hole) charge-density waves exist in each layer and regions where these charge-density waves are in phase, in other words, indirect excitons (or pairs with spatially separated electrons and holes) interacting as electric dipoles, become crystallized. In the electron system in semiconductor structures near a metal electrode, regions of existence of an electron crystal are also obtained in (T,n,D) space, where over large distances the electrons interact as electric dipoles because of image forces. Fiz. Tverd. Tela (St. Petersburg) 40, 1350–1355 (July 1998)     

Controlling polariton coupling in intersubband microcavities

We report the external control of the intersubband polariton coupling by manipulating the carrier density in quantum wells resonantly coupled to a GaAs/AlGaAs microcavity. The electrons in the wells were tuned by means of a depletion gate bias or by utilizing charge transfer between the energetically aligned ground subbands of asymmetric tunnel-coupled quantum wells. We propose the use of tunnel-assisted control of the polariton ground state in an asymmetrically coupled quantum well for implementing ultrafast modulation of intersubband polaritons.     

多个量子节点确定性纠缠的建立

量子纠缠是一种重要的量子资源,在多个空间分离的量子存储器间建立确定性的量子纠缠,然后在用户控制的时刻将所存储的量子纠缠转移到量子信道中进行信息的分发和传送,这对于实现量子信息网络是至关重要的.本文介绍了用光学参量放大器制备与铷原子D1吸收线对应的非经典光场,而且在三个空间分离的原子系综中确定性量子纠缠的产生、存储和转移.利用电磁感应透明光和原子相互作用的原理,将制备的多组分光场纠缠态模式映射到三个远距离的原子系综以建立原子自旋波之间的纠缠.然后,存储在原子系综中的纠缠态通过三个量子通道,纠缠态的量子噪声被转移到三束空间分离的正交纠缠光场.三束释放的光场间纠缠的存在验证了该系统具有保持多组分纠缠的能力.这个方案实现了三个量子节点间的纠缠,并且可以直接扩展到具有更多节点的量子网络,为未来实现大型量子网络通信奠定了基础.     

Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.