单模费米热噪声信道量子容量的估计

从量子信道的算符求和表象出发,对单模费米系统量子信道进行了参数化.参数化的结果得到另一类费米热噪声量子信道.给出了两类费米热噪声量子信道上相干信息的最大值,以估计信道的量子容量. 关键词： 量子容量 相干信息 费米热噪声信道     

量子信道容量的数值计算方法

无论在经典信息论中,还是在量子信息论中,信道容量的确都是一个重要的问题.根据量子信息辅助资源的选择,传输协议的选择,及信号携带的是经典信息还是量子信息,一个量子信道上面可以定义多种容量.比如量子信道量子容量、量子信道经典容量、纠缠辅助的量子信道容量和经典辅助量子容量.其中量子信道的经典容量和纠缠辅助的量子信道容量的计算公式已经给出.但是,利用这些公式并不能容量地算出信道容量,这就要用到数值计算的方法.     

量子多址高斯信道的信息容量研究

与经典通信相类似,量子高斯噪声是一种重要的量子噪声模型.这里,"经典"是相对于"量子"而言的.讨论量子高斯信道传送经典信息时的信息容量,也称量子信道的经典容量,是量子通信的热点问题之一.文中在量子高斯态、高斯熵性质和Holevo界基础上,给出单用户量子高斯信道的经典容量,借助多址量子信道的经典容量区域定理,通过坐标系变换方法,从理论上推导得到多用户量子高斯信道的经典容量区域.为了计算简便且不失一般性,计算过程将采用两输入、单输出的量子多址信道模型进行说明,结论可类推到n个输入、单输出的多址信道.     

量子相干破坏信道的研究

量子相干态和量子纠缠态在量子计算中具有同等重要的地位,都是量子计算中重要的物理资源。量子相干态可用来制备量子纠缠态,一个量子相干态和一个辅助量子不相干态通过不相干操作可产生量子纠缠态。根据纠缠破坏信道原理提出输入分别是单体最大相干态和二体最大相干态的相干破坏信道,该信道将输入的相干态转化为不相干态,并简单验证了提出的相干破坏信道是否真正破坏了输入的量子相干态的相干性。同时提出量子相干破坏信道可作为检测量子态的相干性一种新的方法。     

级联动力学退耦合与无消相干子空间实现量子计算

量子计算与量子信息过程中一个最主要的障碍量子消相干问题.克服量子消相干的策略,一般可以分为两大类方式:量子编码和动力学控制.近年来,无消相干子空间的量子编码以其能够有效的避免消相干过程得到了人们的广泛关注,在理论和实验上都取得了较大的进展.另外,利用强快脉冲实现量子系统与环境的动力学退耦合也可以有效的压缩消相干过程,动力学退耦合的噪声控制形式也得到了广泛研究.本文利用强快脉冲实现量子比特与环境的部分退耦合,然后用两个量子比特编码到无消相干子空间,从而可以克服系统与环境的一般耦合带来的消相干.另外,我们考虑了量子比特之间的相互作用,两个量子比特的耦合会导致量子信息在量子比特之间的非控制流动,使量子计算失败.我们利用对两个量子比特实行非同步的脉冲控制,使编码的量子态在比特间固有相互作用影响下保持稳定.我们给出了一维比特串结构的量子计算机模型,每个计算单元内包括三个逻辑比特,每个逻辑比特采用不同的脉冲控制保护量子比特态的稳定.通过调节控制脉冲,有顺序的"打开"不同的相互作用,可以得到普适的逻辑门组,从而实现了可扩展、容错的普适量子计算机.     

Dzyaloshinskii-Moriya相互作用和内禀消相干对基于两量子比特Heisenberg自旋系统的量子密集编码的影响

通过求解Milburn方程, 研究了内禀消相干条件下包含Dzyaloshinskii-Moriya (DM) 相互作用的两量子比特Heisenberg自旋系统实现的量子密集编码最佳传输容量的演化特性, 分析了不同方向DM相互作用、不同初态、各向异性以及内禀消相干因子等参数对最佳编码容量的影响. 研究表明: 初态的选择对系统密集编码最佳传输容量的影响很大, 不同类型初态下密集编码容量的依赖参数不完全相同; 当系统初态处于c|01>+ d|10> 形式的非最大纠缠时, 引入较弱的DM相互作用z分量可提高最佳编码容量; 相位消相干可抑制最佳编码容量的涨落并使其在长时间演化下趋于稳定. 研究还发现: 内禀消相干下, 通过选取合适的最大纠缠初态, 系统密集编码的最佳传输容量能够保持理想极大值2; 而且无论引入哪个方向的DM相互作用, 基于两量子比特Heisenberg自旋系统的最佳编码容量总可优于经典通信的传输容量.     

基于轨道角动量的循环差分相移量子密钥分发

提出了一种基于标记配对相干态(HPCS)和轨道角动量的循环差分相移量子密钥分发(RRDPS-QKD)方案,以光子轨道角动量作为信息载体,使用多种不同拓扑荷的轨道角动量的叠加态进行RRDPS-QKD的信息编码,显著提高了密钥生成率。使用HPCS作为量子光源,有效减小了空脉冲和多光子脉冲的比例,提高了密钥生成率。分析了光源在湍流大气信道中的传输特性,考虑了信道衰减和大气湍流对系统性能的影响。     

基于标记配对相干态和轨道角动量的量子密钥分配

针对标记配对相干态(HPCS)下量子密钥分配协议采用极化编码和相位编码带来基的依赖性问题,研究了基于HPCS和轨道角动量(OAM)的非对称信道测量设备无关的量子密钥分配协议。分析了该协议在不同距离比率下的平均光子数、误码率、密钥生成率与信道传输损耗的关系。在HPCS和OAM下,对比了对称信道和非对称信道测量设备无关的量子密钥协议的性能优劣。仿真结果表明:采用HPCS弥补了弱相干光源和标记单光子源的不足,大大减少真空脉冲并增加了单光子脉冲;随着信道传输损耗的增大,密钥生成率和安全传输距离逐渐减小,但非对称信道的性能仍优于对称信道的。     

量子相干

量子相干不仅是量子力学中的一个基本概念,同时也是重要的量子信息处理的物理资源.随着基于资源理论框架的量子相干度量方案的提出,量子相干度的量化研究成为近年来人们关注的一个热点问题.量子相干作为一种物理资源也十分脆弱,极容易受到环境噪声的影响而产生退相干,因此开放系统中的量子相干演化和保持也是人们广泛关注的课题.另外,量子相干在量子多体系统、量子热动力学、量子生物学等领域也有着潜在的应用价值.本文介绍量子相干度量的资源理论框架和基于该框架定义的相对熵相干性、l1范数相干性、基于量子纠缠的相干性、基于凸顶结构的相干性和相干鲁棒性等量子相干度量函数,概述开放系统中量子相干演化的动力学行为、典型信道的量子相干产生和破坏能力以及量子相干的冻结等现象,同时例举量子相干在Deutsch-Jozsa算法、Grover算法以及量子多体系统相变问题研究等方面的重要应用.量子相干研究仍处于快速发展之中,期望本综述能为该领域的发展带来启示.     

可扩展的容错量子计算

消相干是量子计算机的物理实现所面临的一个主要的障碍.在现存的消相干物理因素无法消除的情况下,采用量子编码可以部分地克服这一障碍.但是,为了实现大规模的可扩展的量子计算,用于做容错计算的编码空间也应相应地能够被级联、扩展.我们考虑在某些具有局域噪声行为的系统中,如何级联它们的编码空间,实施容错的量子计算.具体而言,我们提出了"无相互作用子空间"的编码思想,将该思想与"无消相干子空间"量子编码的思想相结合,我们提出了不需要逻辑开关的可扩展的容错量子计算方案.另外,针对实际的物理系统,我们构造了在约瑟芬结电荷qubit系统的可扩展容错量子计算方案.     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.     

A stereo matching algorithm using multi-peak candidate matches and geometric constraints

Gray cross correlation matching technique is adopted to extract candidate matches with gray cross correla- tion coefficients less than some certain range of maximal correlation coefficient called multi-peak candidate matches. Multi-peak candidates are extracted corresponding to three closest feature points at first. The corresponding multi-peak candidate matches are used to construct the model polygon. Correspondence is determined based on the local geometric relations between the three feature points and the multi-peak candidates. The disparity test and the global consistency checkout are applied to eliminate the remaining ambiguous matches that are not removed by the local geometric relational test. Experimental results show that the proposed algorithm is feasible and accurate.