Superconducting resonators and charge qubits: Spectroscopy and quantum operations. Part I

For the past few years, considerable progress has been made in the fabrication of quantum devices using mesoscopic semiconductor elements. These devices can represent the elemental base for a wide class of high-tech devices from supersensitive detectors of an electromagnetic field to reliable sources of single photons and quantum computers. The article reviews the main experimental results obtained to date with the participation of superconducting quantum devices.     

Superconducting cavities and charge qubits: Spectroscopy and quantum operations. Part II

In the second part of our review, we focus on such urgent problems of modern physics as the use of superconductors in quantum informatics and coherent control of the state of mesoscopic systems.     

二维和三维量子点量子比特的性质

在电子-体纵光学声子强耦合情况下, 我们采用Pekar类型的变分方法获得了二维和三维抛物势限制的量子点中的电子的基态和第一激发态的本征能量和本征波函数. 在量子点中这样的二能级体系可以作为一个量子比特. 当电子处于基态和第一激发态的叠加态时, 计算了电子态密度的时间演化。得出了电子的几率密度和振荡周期与电子-体纵光学声子耦合强度,量子点的限制长度的关系.     

量子化光场作用下超导电荷比特的Pancharatnam相

考虑一个在经典电压偏置和量子化微波场作用下的电荷库伯对箱,研究了该系统的Pancharatnam相和几何相的时间演化行为,研究发现Pancharatnam相随时间是减小的,几何相随时间是增加的,并且当Pancharatnam相减小的速率增大时,几何相增加的速率减小;反之当Panchaxatnam相减少的速率减小时,几何相增加的速率增加.     

The influences of an anisotropic parabolic potential on the quantum dot qubit* Zhao Cui-Lan(赵翠兰), Cai Chun-Yu(蔡春雨), Xiao Jing-Lin(肖景林)?

To study the influence of an anisotropic parabolic potential(APP)on the properties of a quantum dot(QD)qubit,we obtain the eigenenergies and eigenfunctions of the ground and first excited state of an electron,which is strongly coupled to the bulk longitudinal optical(LO)phonons,in a QD under the influence of an APP by the celebrated Lee–Low–Pines(LLP)unitary transformation and the Pekar type variational(PTV)methods.Then,this kind of two-level quantum system can be excogitated to constitute a single qubit.When the electron locates at the superposition state of its related eigenfunctions,we get the time evolution of the electron’s probability density.Finally,the influence of an APP on the QD qubit is investigated.The numerical calculations indicate that the probability density will oscillate periodically and it is a decreasing function of the effective confinement lengths of theAPPindifferentdirections.Whereasitsoscillatoryperiodisanincreasingoneandwilldiminishwithenhancing the electron–phonon(EP)coupling strength.     

三结磁通量子比特中处于亚稳态粒子的热逃逸和量子隧穿

运用热力学统计中Kramers理论和Wentzel-Kramers-Brillouin(WKB)近似理论,分析了三结磁通量子比特中处于亚稳态粒子的热逃逸和量子隧穿逃逸。在0.01K到4.2K温度下,计算出了粒子的逃逸率,得到了跳变磁通随磁场和温度变化的分布规律,并同两结超导干涉仪的分布规律进行了定量比较。结果显示,三结磁通量子比特自身噪声比两结超导干涉仪大。     

库仑场对抛物线性限制势二能级系统量子点量子比特概率密度的影响

在抛物量子点中电子与体纵光学声子强耦合且在库仑场束缚条件下, 应用Pekar变分方法, 得出了电子的基态和第一激发态的本征能量及基态和第一激发态的本征波函数。 以量子点中这样的二能级体系作为一个量子比特。当电子处于基态和第一激发态的叠加态时, 计算出电子在时空中作周期性振荡的概率分布。并且得出了概率分布与库仑场、耦合强度、受限强度的变化关系。     

基于第三方控制的量子双向传态

提出一个基于第三方控制的量子双向传态方案：通信双方Alice、Bob和控制方Charlie事先共享一个六粒子团簇态,通信开始后,Alice、Bob分别对自己拥有的部分粒子作Bell基联合测量（BSM）,若控制方同意双方通信,则对自己拥有的粒子对作测量,将粒子所处的态公布;通信双方根据控制方公布的测量结果,对各自的某些粒子作适当的幺正变换,即可在这些粒子上重建对方要传的态。由于加入了第三方的控制,双向传态的安全性得到了提高。     

Capture of charge carriers and output power of a quantum well laser

The effect of noninstantaneous carrier capture by a nanoscale active region on the power characteristics of a semiconductor laser is studied. A laser structure based on a single quantum well is considered. It is shown that delayed carrier capture by the quantum well results in a decrease in the internal differential quantum efficiency and sublinearity of the light-current characteristic of the laser. The main parameter of the developed theoretical model is the velocity of carrier capture from the bulk (waveguide) region to the two-dimensional region (quantum well). The effect of the capture velocity on the dependence of the following laser characteristics on the pump current density is studied: the output optical power, internal quantum efficiency of stimulated emission, current of stimulated recombination in the quantum well, current of spontaneous recombination in the optical confinement layer, and carrier concentration in the optical confinement layer. A decrease in the carrier capture velocity results in a larger sublinearity of the light-current characteristic, which results from an increase in the injection current fraction expended to parasitic spontaneous recombination in the optical confinement layer and, hence, a decrease in the injection current fraction expended to stimulated recombination in the quantum well. A comparison of calculated and experimental light-current characteristics for a structure considered as an example shows that good agreement between them (up to a very high injection current density of 45 kA/cm²) is attained at a capture velocity of 2 × 10⁶ cm/s. The results of this study can be used to optimize quantum well lasers for generating high optical powers.     

Quantum operations on charge qubits with the electrostatic control in semiconductor cavities

We consider one- and two-qubit operations on charge qubits that represent double quantum dots with one electron in each of them. The dots are formed inside a high-Q semiconductor cavity (disk, toroidal, or spherical) in the antinodes of one of its optical eigenmodes; the frequencies of the transitions between the ground (logic) and excited (auxiliary) states of the discrete electron spectrum in quantum dots are close to the frequency of this mode. The precise tuning of the transition frequency is performed by applying an electric potential on the qubit control gate. Within the model of qubits coherently interacting with a cavity quantum field, a few methods for controlling their states are developed. In particular, we propose different variants for implementing two-qubit CNOT and CZ gates and generating qubit entangled states. By the example of a micro-disk cavity, we calculate the operating characteristics that ensure a high rate of quantum gate implementation.     

Biphase amplifier for precision controlled rectification and polarlogic operations

A biphase amplifier is a positive-negative controlled unity gain amplifier. Its circuit is simple but very useful in the fields of control, instrumentation, and communications. The use of a biphase amplifier for some novel applications, e.g. polar logic operations and precision controlled rectifications, is described. The circuits described were experimentally tested and found to be quite satisfactory     

Alternating space charge and ambiguity of quantum states in double-barrier structures

Summation of a series of perturbation theory was used to obtain a solution to the time-dependent self-consistent Schrödinger and Poisson equations describing the resonance interaction of electrons that tunnel through asymmetric double-barrier structures with a high-frequency electric field. In the case where electrons are uniformly distributed in energy within the width of the quasi-level, the solution is obtained analytically; if the beam is monoenergetic, solution of the equation is reduced to finding the roots of a fifth-degree algebraic polynomial. It is shown that, in a number of cases, the influence of alternating space charge gives rise to an effect that is quite new for the systems under consideration: several different wave functions may correspond to the same amplitude of the high-frequency voltage applied to the structure; consequently, the values obtained for the high-frequency conductivity and the coefficients of transmission and reflection can differ by several times. As a result, instability of the current flow and hysteresis of the current-voltage characteristics can be observed in these structures. Furthermore, the dependence of the coefficients of transmission and reflection of the electrons and high-frequency conductivity on the voltage amplitude are combinations of the N-and S-shaped characteristics for uniform distribution of electrons and are looplike in the case of a monoenergetic beam.     

TM and TE Diffraction by a Perfectly Conducting Half-Plane in Presence of a Perfectly Conducting Strip: Solution by Exponentially Converging Nystrom and Galerkin Methods

We study TM and TE diffraction by a perfectly conducting half-plane in presence of a perfectly conducting flat strip of finite width and infinite length; the edges of the strip are parallel to the edge of the half-plane. The relevant integral equations with unknowns the induced current densities on the strip are solved by a Nystrom and a Galerkin method that fully account for the singular nature of the kernels and the singularities of the solution at the edges. The proposed algorithms are highly accurate, and appear to converge exponentially as shown by detailed numerical examples and case studies. The Nystrom method is the most efficient of the two methods because of its simple formulation, fast computer implementation, and much less effort in computing the matrix elements. When the strip is coplanar to the half-plane, an additional formulation method in terms of equivalent surface magnetic currents is possible in the framework of the field equivalence principles. This alternative method is applied in order to validate the algorithms and test their accuracy.     

Modelling the centroid and charge density in double-gate MOSFETs including quantum effects

An analytical model for double-gate metal–oxide–semiconductor field-effect transistors considering quantum effects is presented in this article. The variational method is used to solve the Schrodinger–Poisson's equation. The simple and accurate mathematical expressions for centroid and inversion charge density are obtained. It is seen that the minimum energy of the sub-bands and the inversion charge density decreases as the silicon thickness of the device increases. The analytical results obtained from the proposed model are compared and agreed well with a device simulation tool named SCHRED.     

Synergistic enhancement of charge generation and transfer in organic solar cells via a separate PbS quantum dot layer

The major impediment to the high photovoltaic performance of organic solar cells (OSCs) involves deficient photon harvesting and ineffective charge transfer from the photoactive layer to the electrodes. To improve these constraints, in this study, a new OSC device architecture is demonstrated by incorporating PbS colloidal quantum dots (QDs) between the organic photoactive layer and the top electrode. PbS QDs were spin-coated on top of an organic blend via a layer-by-layer deposition process, which formed a separate PbS QD layer with high density and uniformity. The PbS QD layer reinforced the optical property of the OSC by harvesting photons that were not absorbed by the underlying organic photoactive layer. In addition, the OSC employing the QD layer showed the enhanced charge transfer and suppressed recombination loss through the hybrid organic-inorganic interfacial contacts. Thus, a significant increase in the efficiency was achieved compared with the OSC with no PbS QD layer (10.12 vs 8.84%). Accompanied with the improved optoelectronic properties, a superior stability of the proposed architecture advances the practical viability of OSCs in various applications.     

Quantum interference and space charge effects in double barrier structures incorporating wide quantum wells

The current-voltage characteristics of resonant tunneling devices with well widths between 12 and 180 nm are studied. The voltage interval between the resonant peaks in the current is measured as a function of well width. For the wide wells the amplitude of the peaks in the differential conductance is modulated by an “over the barrier” interference effect involving the collector barrier. Space charge buildup and intrinsic bistability effects for a particular resonant state are found to depend critically on its energy difference from the top of the collector barrier and from lower lying standing wave states of the quantum well.     

Optical absorption and scattering at one-particle states of charge carriers in semiconductor quantum dots

A theory is developed for the interaction of an electromagnetic field with one-particle quantum-confined states of charge carriers in semiconductor quantum dots. It is shown that the oscillator strengths and dipole moments for the transitions involving one-particle states in quantum dots are rather large, exceeding the corresponding typical parameters of bulk semiconductor materials. In the context of dipole approximation it is established that the large optical absorption cross sections and attenuation coefficients in the quasi-zero-dimensional systems make it possible to use the systems as new efficient absorbing materials.     

Electronic characteristics of the singly ionized pair of phosphorus donors in silicon and operations with charge qubits

The electron energy spectrum of the singly ionized pair of phosphorus donors in silicon, P ₂ ⁺ , is calculated numerically, taking into account the sixfold degeneracy of the conduction band. The envelopes of wave functions of different states are determined. The results are used to simulate operations with the P ₂ ⁺ /Si-based charge qubits via exposure to laser pulses.     

Shape Reconstruction of a Perfectly Conducting Scatterer Using Differential Evolution and Particle Swarm Optimization

The shape reconstruction of a perfectly conducting 2-D scatterer by inverting transverse magnetic scattered field measurements is investigated. The reconstruction is based on evolutionary algorithms that minimize the discrepancy between measured and estimated scattered field data. A closed cubic B-spline expansion is adopted to represent the scatterer contour. Two algorithms have been examined the differential-evolution (DE) algorithm and the particle swarm optimization (PSO). Numerical results indicate that the DE algorithm outperforms the PSO in terms of reconstruction accuracy and convergence speed. Both techniques have been tested in the case of simulated measurements contaminated by additive white Gaussian noise.