Neural networks with quantum architecture and quantum learning

A method is proposed for solving the two key problems facing quantum neural networks: introduction of nonlinearity in the neuron operation and efficient use of quantum superposition in the learning algorithm. The former is indirectly solved by using suitable Boolean functions. The latter is based on the use of a suitable nonlinear quantum circuit. The resulting learning procedure does not apply any optimization method. The optimal neural network is obtained by applying an exhaustive search among all the possible solutions. The exhaustive search is carried out by the proposed quantum circuit composed of both linear and nonlinear components. Copyright © 2009 John Wiley & Sons, Ltd.     

Semiconductor quantum devices

The operation of devices with small enough dimensions for electrons to exhibit wavelike behavior is explained. Two types of device are examined: vertical quantum devices, which include the resonant tunneling structure and the single-electron transistor, and lateral, which include the quantum interference transistor and the spin precession device. The advantages and drawbacks of the two types are identified     

Classical and quantum mechanical transport simulations in open quantum dots

Simulations of open dot arrays are performed where the electron motion is considered in a harmonic potential and in a static magnetic field. Results concern electron trajectories and Poincaré sections from the classical calculation. The analysis of the Poincaré sections reveals a strong influence of the length of the constriction on the chaotic regions in phase space. Density probabilities from the quantum mechanical calculation show a significant correspondence to classically calculated trajectories.     

Non-equilibrium quantum transport theory: current and gain in quantum cascade lasers

We have developed a self-consistent non-equilibrium Green’s function theory (NEGF) for charge transport and optical gain in THz quantum cascade lasers (QCL) and present quantitative results for the I-V characteristics, optical gain, as well as the temperature dependence of the current density for a concrete GaAs/Al_.15Ga_.85As QCL structure. Phonon scattering, impurity, Hartree electron-electron and interface roughness scattering within the self-consistent Born approximation are taken into account. We show that the characteristic QCL device properties can be successfully modeled by taking into account a single period of the structure, provided the system is consistently treated as open quantum system. In order to support this finding, we have developed two different numerically efficient contact models and compare single-period results with a quasi-periodic NEGF calculation. Both approaches show good agreement with experiment as well as with one another.     

Inelastic cotunneling through an interacting quantum dot with a quantum Langevin equation approach

We present a theoretical analysis of several aspects of nonequilibrium cotunneling through a strong Coulomb-blockaded quantum dot (QD) subject to a finite magnetic field in the weak coupling limit. We carry this out by developing a generic quantum Heisenberg-Langevin equation approach leading to a set of Bloch dynamic equations which describe the nonequilibrium cotunneling in a convenient and compact way. This scheme not only provides analytical expressions for the relaxation and decoherence of the localized spin induced by cotunneling, but it also facilitates evaluations of the nonequilibrium magnetization, the charge current, and the spin current at arbitrary bias-voltage, magnetic field, and temperature, and their frequency-independent fluctuations as well.     

Effect of quantum dots on InAs/GaAs p-i-p quantum dots infrared photodetectors

ABSTRACT

In this paper, the InAs/GaAs p-i-p quantum dots infrared photodetectors (QDIPs) were successfully demonstrated by Apsys software. It consists of Al_0.3Ga_0.7As/GaAs structure to reduce dark current and InAs quantum dots (QDs) embedded in In_0.15Ga_0.85As as an active layer. The effect of structure parameters of InAs QDs on the dark current, photocurrent of the device and SNR (signal to noise) is discussed respectively, including different QDs density, the number of QD layer, GaAs thickness between QDs layers and Al_0.3Ga_0.7As, and GaAs thickness between two the QD layers.     

Transport in open quantum systems: comparing classical and quantum phase space dynamics

Transport in open quantum systems is of great interest. We show that the discrete states of an open quantum system may be classified into three distinct groups, dependent upon the manner in which they influence transport when connected to an external environment. A first class of states is current-carrying states, which are naturally strongly connected to the environment. A second class of states is discrete, but stable and isolated, and thought to be the pointer states of decoherence theory. Finally, we identify backscattered states, which do not propagate through the system.     

Optical properties of quantum dots versus quantum antidots: Effects of hydrostatic pressure and temperature

The effects of hydrostatic pressure and temperature on the linear, nonlinear and total absorption coefficients (ACs) of a hydrogenic impurity in the center of spherical quantum dot (QD) and quantum antidot (QAD) have been investigated. The comparative approach is used for presenting the results of both models. Our numerical results indicate that for QD nano-systems, by increasing the pressure, the resonance peak positions (RPPs) of ACs shift towards higher energies, while for QAD nano-systems, RPPs of ACs approximately remain unchanged. Furthermore, the larger pressure leads to the smaller height of resonance peak in both models. Also, our results show that the temperature increasing imposes the opposite effect on RPPs than the pressure increasing to the both models.     

Hydrodynamic modeling of silicon quantum wires

A hydrodynamic model for silicon quantum wires is formulated by taking the moments of the multisubband Boltzmann equation, coupled to the Schr?dinger-Poisson system. Explicit closure relations for the fluxes and production terms (i.e. the moments on the collisional operator) are obtained by means of the Maximum Entropy Principle of Extended Thermodynamics, including scattering of electrons with acoustic and non-polar optical phonons. By using this model, thermoelectric effects are investigated.     

Ultrafast Wigner transport in quantum wires

Two quantum-kinetic models, governing the transport of an initial highly non-equilibrium carrier distribution generated locally in a nanowire, are explored. Dissipation processes due to phonons govern the carrier relaxation, which at early stages of the evolution is characterized by the lack of energy conservation in the collisions. The models are analyzed and approached numerically by a backward Monte Carlo method. The basic difference between them is in the way of treatment of the finite collision duration time. The latter introduces quantum effects of broadening and retardation, ultrafast spatial transfer and modification of the classical trajectories, which are demonstrated in the presented simulation results.     

量子点太阳电池研究进展

分析了量子点太阳电池的物理机理。详细阐述了提高量子点太阳电池光电转换效率的两个效应:第一个效应是来自具有充足能量的单光子激发产生多激子;第二个效应是在带隙里形成中间带,可以有多个带隙起作用,来产生电子空穴对。综述了近年来量子点太阳电池材料和器件的研究进展,并对几个有待于进一步研究的问题进行展望。     

量子密码技术及其应用

量子密码是信息理论的一个重要方面,它利用量子力学的一些独特性质,突破了传统密码学的限制,能够绝对安全地传送信息.本文介绍了量子密码的基本原理,探讨了量子密码的一些技术,最后简单介绍了量子密码的应用以及未来的发展前景.     

The influence of the zigzag and armchair leads on quantum transport through the graphene based quantum rings

The conductivity of a graphene ring with two semi-infinite, armchair and zigzag leads has been investigated. We have performed numerical calculations based on the nearest neighbor tight-binding Hamiltonian and Dirac point approximation. A Non-Equilibrium Green’s Function (NEGF) approach has been employed to calculate the electric current under an applied bias voltage, in this two terminal mesoscopic system. We have studied the effect of the external magnetic field on transport characteristics of this graphene-based quantum ring. Coherent transport features of the system have been studied. It was shown that there is significant distinction between the I–V characteristics (and also the Aharonov-Bohm effect) of the graphene rings depending on the edge structure of the leads.     

Unity gain and nonunity gain quantum teleportation

We investigate continuous variable quantum teleportation. We discuss the methods presently used to characterize teleportation in this regime, and propose an extension of the measures proposed by Grangier and Grosshans , and Ralph and Lam . This new measure, the gain normalized conditional variance product M, turns out to be highly significant for continuous variable entanglement swapping procedures, which we examine using a necessary and sufficient criterion for entanglement. We elaborate on our recent experimental continuous variable quantum teleportation results , demonstrating success over a wide range of teleportation gains. We analyze our results using fidelity; signal transfer, and the conditional variance product; and a measure derived in this paper, the gain normalized conditional variance product.     

Probing quantum coherent states in bilayer graphene

An active area of post-CMOS device research is to study the possibility of realizing and exploiting exotic quantum states in nanostructures. In this paper we consider one such system, two layers of graphene separated by an oxide insulator. This system has been predicted to have an excitonic condensate that survives above room temperature. We describe a computational technique—path integral quantum Monte Carlo (PIMC)—that directly simulates many-body quantum phenomena, including excitonic condensation. Starting from a simplified quasiparticle model, the many-body PIMC simulations show excitonic pairing and a confirm a superfluid phase that persists above room temperature. We then present an atomistic PIMC model that captures more details of graphene than our quasiparticle model, and discuss how to extract parameters for a non-equilibrium Green’s function calculation.     

High-efficiency photon-number detection for quantum information processing

The visible light photon counter (VLPC) features high quantum efficiency (QE) and low pulse height dispersion. These properties make it ideal for efficient photon-number state detection. The ability to perform efficient photon-number state detection is important in many quantum information processing applications, including recent proposals for performing quantum computation with linear optical elements. In this paper, we investigate the unique capabilities of the VLPC. The efficiency of the detector and cryogenic system is measured at 543 nm wavelengths to be 85%. A picosecond pulsed laser is then used to excite the detector with pulses having average photon numbers ranging from 3-5. The output of the VLPC is used to discriminate photon numbers in a pulse. The error probability for number state discrimination is an increasing function of the number of photons, due to buildup of multiplication noise. This puts an ultimate limit on the ability of the VLPC to do number state detection. For many applications, it is sufficient to discriminate between 1 and more than one detected photon. The VLPC can do this with 99% accuracy.     

Towards realistic time-resolved simulations of quantum devices

We report on our recent efforts to perform realistic simulations of large quantum devices in the time domain. In contrast to d.c. transport where the calculations are explicitly performed at the Fermi level, the presence of time-dependent terms in the Hamiltonian makes the system inelastic so that it is necessary to explicitly enforce the Pauli principle in the simulations. We illustrate our approach with calculations for a flying qubit interferometer, a nanoelectronic device that is currently under experimental investigation. Our calculations illustrate the fact that many degrees of freedom (16,700 tight-binding sites in the scattering region) and long simulation times (9500 times the inverse bandwidth of the tight-binding model) can be easily achieved on a local computer.