Brun-Type Formalism for Decoherence in Two-Dimensional Quantum Walks

We study decoherence in the quantum walk on the xy-plane.We generalize the method of decoherent coin quantum walk,introduced by [T.A.Brun,et al.,Phys.Rev.A 67(2003) 032304],which could be applicable to all sorts of decoherence in two-dimensional quantum walks,irrespective of the unitary transformation governing the walk.As an application we study decoherence in the presence of broken line noise in which the quantum walk is governed by the two-dimensional Hadamard operator.     

Directional quantum random walk induced by coherence

Quantum walk (QW), which is considered as the quantum counterpart of the classical random walk (CRW), is actually the quantum extension of CRW from the single-coin interpretation. The sequential unitary evolution engenders correlation between different steps in QW and leads to a non-binomial position distribution. In this paper, we propose an alternative quantum extension of CRW from the ensemble interpretation, named quantum random walk (QRW), where the walker has many unrelated coins, modeled as two-level systems, initially prepared in the same state. We calculate the walker's position distribution in QRW for different initial coin states with the coin operator chosen as Hadamard matrix. In one-dimensional case, the walker's position is the asymmetric binomial distribution. We further demonstrate that in QRW, coherence leads the walker to perform directional movement. For an initially decoherenced coin state, the walker's position distribution is exactly the same as that of CRW. Moreover, we study QRW in 2D lattice, where the coherence plays a more diversified role in the walker's position distribution.     

Limitations of discrete-time quantum walk on a one-dimensional infinite chain

How well can we manipulate the state of a particle via a discrete-time quantum walk? We show that the discrete-time quantum walk on a one-dimensional infinite chain with coin operators that are independent of the position can only realize product operators of the form $e^{i\xi }A?{\mathbb{1}}_p$, which cannot change the position state of the walker. We present a scheme to construct all possible realizations of all the product operators of the form $e^{i\xi }A?{\mathbb{1}}_p$. When the coin operators are dependent on the position, we show that the translation operators on the position can not be realized via a DTQW with coin operators that are either the identity operator $\mathbb{1}$ or the Pauli operator ${\sigma }_x$.     

(001)和(111)取向的张应变量子阱光学特性的比较

以(001)和(111)价带Lutinger-Kohn哈密顿量为基础,讨论了这两种取向的量子阱的能带结构、态密度(DOS)、跃迁矩阵元和增益特性。由于材料结构的各向异性,(111)取向的量子阱在平行生长面内有较小的重空穴有效质量,无应变和压应变激光器可以很好地利用这一点。而(001)取向的量子阱虽有较小的平面内轻空穴有效质量,但并不比(111)量子阱的小多少;加上(001)量子阱的价带耦合效应强,使其DOS比(111)量子阱的大,在相同载流子注入下,张应变(111)量子阱的增益系数比相应(001)量子阱的要高。所以张应变(111)量子阱激光器仍然比相应的(001)量子阱激光器性能要好。可以认为匹配和压应变的(111)激光器优异的性能不仅来自小的面内有效质量,也来自于弱的价带耦合效应。     

光纤光栅的温度增敏实验

用热膨胀系数较大的聚合物材料对光纤光栅进行封装处理,极大地提高了光纤光栅的温度灵敏度。我们将光纤光栅封装于两种不同的聚合物材料中,其温度灵敏度分别提高6倍和23倍之多。这是迄今有文献报道的最大的光纤光栅温度灵敏度。     

Donor impurity states in zinc-blende GaN/AlGaN quantum well: Quantum confinement and laser-dressed effects

Within the framework of effective-mass approximation, the effects of a laser field on the ground-state donor binding energy in zinc-blende (ZB) GaN/AlGaN quantum well (QW) have been investigated variationally. Numerical results show that the donor binding energy is highly dependent on QW structure parameters and Al composition in ZB GaN/AlGaN QW. The laser field effects are more noticeable on the donor binding energy of an impurity localized inside the QW with small well width and low Al composition. However, for the impurity located in the vicinity of the well edge of the QW, the donor binding energy is insensible to the variation of Al composition, well width and laser field intensity in ZB GaN/AlGaN QW. In particular, the competition effects between laser field and quantum confinement on impurity states have also been investigated in this paper.     

Donor impurity states in direct-gap SiGe quantum well: Applied electric field and quantum size effects

Within the framework of the effective-mass approximation and variational procedure, competition effects between applied electric field and quantum size on donor impurity states in the direct-gap Ge/SiGe quantum well (QW) have been investigated theoretically. Numerical results show that the applied electric field (quantum size) dominates electron and impurity states in direct-gap Ge/SiGe QW with large (small) well width. Moreover, the competition effects also induce that the donor binding energies show obviously different behaviors with respect to electric field in the QW with different well widths. In particular, when the impurity is located at left boundary of the QW, the donor binding energy is insensitive to the variation of well width when well width is large for any electric field case.     

Negative differential conductivity in quantum well with complex potential profile for electron–phonon scattering

The effect of phonon scattering on electrical conductivity (EC) of 2D electron gas in quantum well (QW) systems with a complicated potential profile is described. Dependence of QW electrical conductivity on QW parameters (such as QW width, Fermi level positions etc.) when phonon scattering is employed has been calculated. NDC in EC when it varies with width of the QW has been found.     

Controllable simulation of topological phases and edge states with quantum walk

We simulate various topological phenomena in condense matter, such as formation of different topological phases, boundary and edge states, through two types of quantum walk with step-dependent coins. Particularly, we show that one-dimensional quantum walk with step-dependent coin simulates all types of topological phases in BDI family, as well as all types of boundary and edge states. In addition, we show that step-dependent coins provide the number of steps as a controlling factor over the simulations. In fact, with tuning number of steps, we can determine the occurrences of boundary, edge states and topological phases, their types and where they should be located. These two features make quantum walks versatile and highly controllable simulators of topological phases, boundary, edge states, and topological phase transitions. We also report on emergences of cell-like structures for simulated topological phenomena. Each cell contains all types of boundary (edge) states and topological phases of BDI family.     

Classical random walk with memory versus quantum walk on a one-dimensional infinite chain

Quantum walk is a very useful tool for building quantum algorithms due to the faster spreading of probability distributions as compared to a classical random walk. Comparing the spreading of the probability distributions of a quantum walk with that of a mnemonic classical random walk on a one-dimensional infinite chain, we find that the classical random walk could have a faster spreading than that of the quantum walk conditioned on a finite number of walking steps. Quantum walk surpasses classical random walk with memory in spreading speed when the number of steps is large enough. However, in such a situation, quantum walk would seriously suffer from decoherence. Therefore, classical walk with memory may have some advantages in practical applications.     

Measurements of far-infrared intersubband absorption linewidths in GaAs/AlGaAs quantum wells as a function of temperature and charge density

Intersubband transitions in doped quantum wells are of fundamental scientific interest, as well as technological interest for potential devices. One of the important characteristics of the transitions is their linewidth. We present the results of linear absorption spectroscopy on a coupled double asymmetric QW structure. Using a backgated sample, we can independently vary the charge density and DC field at the well. The absorption lines appear to be homogeneously broadened. The lines appear to become lifetime broadened for temperatures above 70 K.     

Magneto-optical study of nonmagnetic quantum dots coupled to a magnetic semiconductor quantum well

We have investigated a series of double-layer structures consisting of a layer of self-assembled non-magnetic CdSe quantum dots (QDs) separated by a thin ZnSe barrier from a ZnCdMnSe diluted magnetic semiconductor (DMSs) quantum well (QW). In the series, the thickness of the ZnSe barrier ranged between 12 and 40 nm. We observe two clearly defined photoluminescence (PL) peaks in all samples, corresponding to the CdSe QDs and the ZnCdMnSe QW, respectively. The PL intensity of the QW peak is observed to decrease systematically relative to the QD peak as the thickness of the ZnSe barrier decreases, indicating a corresponding increase in carrier tunneling from the QW to the QDs. Furthermore, polarization-selective PL measurements reveal that the degree of polarization of the PL emitted by the CdSe QDs increases with decreasing thickness of the ZnSe barriers. The observed behavior is discussed in terms of anti-parallel spin interaction between carriers localized in the non-magnetic QDs and in the magnetic QWs.     

Effects of hydrostatic pressure and temperature on the nonlinear optical properties of semiparabolic plus semi-inverse squared quantum wells

In this study, the effects of hydrostatic pressure and temperature on nonlinear optical rectification(OR), second-harmonic generation(SHG), third-harmonic generation(THG) and the linear,nonlinear, and total optical absorption coefficients(OACs) of a semiparabolic plus semi-inverse squared quantum well(QW) are theoretically investigated. The results show that hydrostatic pressure and temperature have significant effects on the optical properties of semiparabolic plus semi-inverse squared QWs, and that the energy levels and magnitudes of the resonant peaks of OR, SHG, THG, and the total OACs vary according to the shape of the limiting potential, the hydrostatic pressure, and the temperature. It is easily seen that the peak positions of the resonant peaks of OR, SHG, THG, and the total OACs in the semiparabolic plus semi-inverse squared QW show a red shift with increasing hydrostatic pressure, but a blue shift with increasing temperature. Therefore, the magnitude and position of the resonant peaks of OR, SHG, THG,and the total OACs can be adjusted by changing the hydrostatic pressure and the temperature,which promise a new degree of freedom in the tunability of various electro-optical devices.     

One-dimensional quantum walks with a position-dependent coin

We investigate the evolution of a discrete-time one-dimensional quantum walk driven by a position-dependent coin. The rotation angle, which depends upon the position of a quantum particle, parameterizes the coin operator. For different values of the rotation angle, we observe that such a coin leads to a variety of probability distributions, e.g. localized, periodic, classicallike, semi-classical-like, and quantum-like. Further, we study the Shannon entropy associated with position and the coin space of a quantum particle, and compare them with the case of the position-independent coin. Our results show that the entropy is smaller for most values of the rotation angle as compared to the case of the position-independent coin. We also study the effect of entanglement on the behavior of probability distribution and Shannon entropy by considering a quantum walk with two identical position-dependent entangled coins. We observe that in general, a wave function becomes more localized as compared to the case of the positionindependent coin and hence the corresponding Shannon entropy is lower. Our results show that a position-dependent coin can be used as a controlling tool of quantum walks.     

Resonant elastic scattering of light from single quantum wells in semiconductor multilayers

A theory is developed for steady-state elastic scattering of light via quasi-2D excitons from a quantum well (QW) whose interfaces are randomly rough. The study is mainly focused on the angle dependences of radiation giving direct information about static disorder responsible for the elastic scattering. A nonlocal excitonic susceptibility is expressed in terms of random profile functions of QW interfaces. Treated is elastic scattering of light from a disordered QW in the following actual dielectric environments: (i) a uniform background, (ii) a Fabry–Perot film with rough boundaries, and (iii) a semiconductor microcavity. The cross-sections are derived analytically for scattering of linearly polarized light to the lowest (Born's) approximation with arbitrary roughness statistics. The spectral and angle dependencies of scattering intensity are analyzed numerically in the absolute-value scale with Gaussian correlation of interface roughness. The probability 10⁻² was found for the exciton-mediated scattering of a photon from a QW interface roughness whose root-mean-square height is on the level of 2×10⁻¹ nm. This probability is shown to exceed by two orders of magnitude that is typical of resonant scattering from either a single semiconductor surface or rough boundaries of a semiconductor Fabry–Perot film containing the QW. The scattering spectrum of a QW placed in a microcavity is predicted to have a doublet structure whose components are associated with the cavity exciton–polaritons.     

Stopping time of a one-dimensional bounded quantum walk

The stopping time of a one-dimensional bounded classical random walk(RW) is defined as the number of steps taken by a random walker to arrive at a fixed boundary for the first time.A quantum walk(QW) is a non-trivial generalization of RW,and has attracted a great deal of interest from researchers working in quantum physics and quantum information.In this paper,we develop a method to calculate the stopping time for a one-dimensional QW.Using our method,we further compare the properties of stopping time for QW and RW.We find that the mean value of the stopping time is the same for both of these problems.However,for short times,the probability for a walker performing a QW to arrive at the boundary is larger than that for a RW.This means that,although the mean stopping time of a quantum and classical walker are the same,the quantum walker has a greater probability of arriving at the boundary earlier than the classical walker.     

Electron transport properties through double-barrier structures sandwiching a wide band-gap layer

We investigate the time-dependent transport properties of the quantum well (QW) with a wide band-gap material layer, where Al atoms doped in the middle GaAs QW. We find that the raised potential well bottom can affect the position of current hysteresis, current oscillation frequency and final steady-state mean current value. Moreover in this special structure, we find a negative differential conductance and only a current hysteresis region, the plateau structure of I–V curve found in the AlGaAs/GaAs/AlGaAs QW disappears.