Entanglement fidelity of the standard quantum teleportation channel

We consider the standard quantum teleportation protocol where a general bipartite state is used as entanglement resource. We use the entanglement fidelity to describe how well the standard quantum teleportation channel transmits quantum entanglement and give a simple expression for the entanglement fidelity when it is averaged on all input states.     

振幅阻尼信道量子隐形传态保真度恢复机理

在振幅阻尼信道上进行量子隐形传态的过程中, 量子Bell纠缠态将发生退相干, 导致隐形传态质量下降甚至通信失败. 为克服该影响, 本文提出了一种Bell纠缠态补偿方法. 在估计振幅阻尼信道参数的基础上, 将对纠缠态的补偿分为纠缠退相干发生之前的预补偿以及 经历量子振幅阻尼信道之后的匹配补偿两部分. 前者在纠缠源处进行, 后者在两个量子通信 用户处进行, 预补偿及匹配补偿参数与信道特性参数相关. 纠缠补偿完成后, 再进行隐形传态. 理论推导与性能分析结果表明, 相比于不进行纠缠补偿及仅在发生退相干之后进行的纠缠补偿, 本方法能够获得更高的隐形传态保真度, 适当调整补偿参数, 可使保真度接近于1, 对克服纠缠 退相干带来的隐形传态质量下降问题具有一定的意义.     

Degenerate asymmetric quantum concatenated codes for correcting biased quantum errors

In most practical quantum mechanical systems, quantum noise due to decoherence is highly biased towards dephasing. The quantum state suffers from phase flip noise much more seriously than from the bit flip noise. In this work, we construct new families of asymmetric quantum concatenated codes (AQCCs) to deal with such biased quantum noise. Our construction is based on a novel concatenation scheme for constructing AQCCs with large asymmetries, in which classical tensor product codes and concatenated codes are utilized to correct phase flip noise and bit flip noise, respectively. We generalize the original concatenation scheme to a more general case for better correcting degenerate errors. Moreover, we focus on constructing nonbinary AQCCs that are highly degenerate. Compared to previous literatures, AQCCs constructed in this paper show much better parameter performance than existed ones. Furthermore, we design the specific encoding circuit of the AQCCs. It is shown that our codes can be encoded more efficiently than standard quantum codes.     

Bidirectional multi-qubit quantum teleportation in noisy channel aided with weak measurement

Recently,bidirectional quantum teleportation has attracted a great deal of research attention.However,existing bidirectional teleportation schemes are normally discussed on the basis of perfect quantum environments.In this paper,we first put forward a bidirectional teleportation scheme to transport three-qubit Greenberger-Horne-Zeilinger(GHZ) states based on controled-not(CNOT) operation and single-qubit measurement.Then,we generalize it to the teleportation of multi-qubit GHZ states.Further,we discuss the influence of quantum noise on our scheme by the example of an amplitude damping channel,then we obtain the fidelity of the teleportation.Finally,we utilize the weak measurement and the corresponding reversing measurement to protect the quantum entanglement,which shows an effective enhancement of the teleportation fidelity.     

大失谐J-C模型中腔耗散对量子态保真度的影响

利用全量子理论研究了存在腔耗散的大失谐 Jaynes-Cumming(J-C) 模型中量子态保真度,讨论了初始平均光子数一定的情况下腔的耗散系数对量子态保真度的影响,以及当腔的耗散一定的情况下初始光场的平均光子数对量子态保真度的影响.结果表明:腔的耗散和光场的初始平均光子数对原子和原子---光场系统的保真度的影响都很明显.     

Entanglement fidelity of coherent-state teleportation with asymmetric quantum channel

A strategy for teleporting coherent states with the entanglement fidelity is considered in the general case of an asymmetric teleportation scheme. It is shown that the nonbalanced homodyne detection with the subsequent coherent displacement is required to provide the average teleportation fidelity of entanglement.     

Alternative non-Gaussianity measures for quantum states based on quantum fidelity

We propose three alternative measures for non-Gaussianity of quantum states: sine distance, Bures angle, and Bures distance, which are based on quantum fidelity introduced by Wang [Phys. Lett. A 373 58 (2008)]. Using them, we evaluate the non-Gaussianity of some relevant single-mode and two-mode non-Gaussian states and find a good consistency of the three examined measures. In addition, we show that such metrics can exactly quantify the degree of Gaussianity of even Schrödinger-cat-like states of small amplitudes that can not be measured by other known non-Gaussianity measures such as the Hilbert—Schmidt metric and the relative entropy metric. We make a comparative study between all existing non-Gaussianity measures according to the metric axioms and point out that the sine distance is the best candidate among them.     

Quantum secret sharing based on quantum error-correcting codes

Quantum secret sharing(QSS) is a procedure of sharing classical information or quantum information by using quantum states.This paper presents how to use a [2k-1,1,k] quantum error-correcting code(QECC) to implement a quantum(k,2k 1) threshold scheme.It also takes advantage of classical enhancement of the [2k-1,1,k] QECC to establish a QSS scheme which can share classical information and quantum information simultaneously.Because information is encoded into QECC,these schemes can prevent intercept-resend attacks and be implemented on some noisy channels.     

量子纠缠信令网Poisson生存模型及保真度分析

量子信令态在传输过程中,由于环境影响产生退相干,造成信令损伤,从而会对构建高生存性的量子纠缠信令网产生影响. 为分析所造成的影响,建立了自然灾害下的量子信令网Poisson损伤模型. 首先,根据信令保真度定义了灾害级数;其次,提出信令态平均损伤量子比特数,并给出信令网生存函数;最后,研究了信令损伤的修复策略并进行仿真. 仿真结果表明,灾害级数的增加会大大降低信令网生存性,而增加信令转接点数和控制信令损伤上限可改善生存性,且该修复策略循环次数少,并可将信令态的保真度由0.6快速提高到0.9,信令网的生存函数由0.4提高到0.9. 关键词： 量子通信 量子信令网 保真度 生存函数     

与光场依赖强度耦合多光子通道中原子周期量子回声的产生和控制

研究了与光场依赖强度耦合多光子通道中原子态保真度演化,探讨了原子周期量子回声的产生和控制。通过分别考察原子相干分布角、光场平均光子数以及原子跃迁时吸收（或发射）的光子数对原子态保真度演化的影响,获得了产生和控制原子周期量子回声的系统参数,并揭示了原子态高保真输出的物理实质。     

与光场依赖强度耦合多光子通道中原子周期量子回声的产生和控制

研究了与光场依赖强度耦合多光子通道中原子态保真度演化,探讨了原子周期量子回声的产生和控制。通过分别考察原子相干分布角、光场平均光子数以及原子跃迁时吸收（或发射）的光子数对原子态保真度演化的影响,获得了产生和控制原子周期量子回声的系统参数,并揭示了原子态高保真输出的物理实质。     

Preserving entanglement and the fidelity of three-qubit quantum states undergoing decoherence using weak measurement

We demonstrate a method to preserve entanglement and improve fidelity of three-qubit quantum states undergoing amplitude-damping decoherence using weak measurement and quantum measurement reversal. It is shown that we are able to enhance entanglement to the greatest extent, and to circumvent entanglement sudden death by increasing the weak measurement strength both for the GHZ state and the W state. The weak measurement technique can also enhance the fidelity to the quantum region and even close to 1 for the whole range of the decoherence parameter in both of the two cases. In addition, the W state can maintain more fidelity than the GHZ state in the protection protocol. However, the GHZ state has a higher success probability than the W state.     

基于部分测量增强量子隐形传态过程的量子Fisher信息

量子Fisher信息(QFI)是量子度量学中的一个重要物理量,可给出预估参数精度的最优值.本文研究如何引入弱测量和测量反转操作,来提高有限温环境下以Greenberger-Horne-Zeilinger态作为量子通道的隐形传态过程中的QFI.依据隐形传态过程中量子比特的传输情形,考虑了三种不同方案相应的QFI.首先,通过构造每种量子隐形传态方案的量子线路图,分析了QFI与推广振幅衰减噪声参数的变化关系.随后对各种方案中的受噪声粒子施加弱测量和测量反转操作,并对相应的部分测量参数进行优化,着重探讨了施加最优部分测量操作后QFI的改进量.结果表明,经过优化后的部分测量操作能有效提高有限温环境下量子隐形传态过程输出态的QFI;而且量子系统所处的环境温度越低,QFI的提高效果可越显著.     

PM2.5大气污染对自由空间量子通信性能的影响

近年来, PM2.5大气污染日益严重, 不仅影响空气质量与大气能见度, 而且还会对自由空间量子光信号的传输造成影响. 然而, 有关PM2.5与自由空间量子通信信道参数关系的研究, 迄今尚未展开. 本文根据PM2.5粒度谱分布函数及其化学成分的消光份额, 提出了PM2.5指数、大气湿度与自由空间量子信道衰减的关系; 针对幅值阻尼信道和退极化信道, 分别建立了PM2.5污染程度与信道容量、信道平均保真度、信道误码率的定量关系. 仿真结果表明, 当大气湿度为30%, PM2.5指数分别为50和300时, 自由空间量子通信信道容量、信道平均保真度、信道误码率分别依次为0.83和0.21, 0.91和0.56, 0.0048和0.0192. 由此可见, PM2.5污染程度对自由空间量子通信性能有显著的影响. 因此, 为了提高自由空间量子通信的可靠性, 应根据PM2.5大气污染状况, 自适应调整系统的各项参数.     

有噪量子信道生存函数研究及其仿真

为了构建高生存性的量子信令网, 基于保真度概念, 提出量子信道生存函数和信令网生存性的概念. 分析光纤和自由空间生存系数, 计算三种消相干机理下有噪量子信道保真度, 给出生存函数测试模型. 仿真分析表明, 通过降低量子态跃迁率, 控制光纤衰减损耗和大气衰减系数, 减小量子态演化时间, 设置量子中继, 可以建立高生存性量子信道, 为量子纠缠信令网标准的制定提供理论基础. 关键词： 量子通信 量子信令网 保真度 生存函数     

Quantum secure direct communication over the collective amplitude damping channel

An efficient quantum secure direct communication protocol is presented over the amplitude damping channel. The protocol encodes logical bits in two-qubit noiseless states, and so it can function over a quantum channel subjected to collective amplitude damping. The feature of this protocol is that the sender encodes the secret directly on the quantum states, the receiver decodes the secret by performing determinate measurements, and there is no basis mismatch. The transmission’s safety is ensured by the nonorthogonality of the noiseless states traveling forward and backward on the quantum channel. Moreover, we construct the efficient quantum circuits to implement channel encoding and information encoding by means of primitive operations in quantum computation. Supported by the National Natural Science Foundation of China (Grant Nos. 60873191 and 60821001), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 200800131016), the Natural Science Foundation of Beijing (Grant No. 4072020), the National Laboratory for Modern Communications Science Foundation of China (Grant No. 9140C1101010601), and the ISN Open Foundation     

二维３态Potts量子系统的保真度与序参量

二维无限正方格子上的量子3态Potts模型是发生一级相变还是二级相变?通过运用无限纠缠投影对态算法(iPEPS),在进行数值模拟时任意选取初态,能得到二维无限正方格子上的3态Potts模型的三个不同的简并基态波函数,这些简并的情况是由自发对称性破缺引起的.首先,揭示了在二维系统中自发对称性破缺引起的相变可以运用单点基态保真度的分叉来研究,也反映了在二维系统中约化保真度同样有一个分叉行为;再者,还开创性提出了二维系统的普适序参量以及多分量的复数局域序参量的行为来尝试研究二维3态Potts模型,共同确定系统发生的量子相变的临界点及其类型.即基于iPEPS算法,从单点基态保真度、约化保真度、普适序参量以及局域序参量的角度,来研究3态Potts模型的量子相变,其为一级相变.     

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.     

应用约化密度保真度确定自旋为1的一维量子Blume—Capel模型的基态相图

约化密度保真度（reducedensityfidelity）可以用来描述量子多体系统的量子相变,其是两个约化密度矩阵距离的度量．本文应用MERA（multi—scaleentanglementreorganizationansatz）算法,模拟自旋为1的一维量子Blume-Capel模型,并通过对约化密度保真度的计算,确定出其基态相图．单点和两点约化密度矩阵所包含的至关重要的信息的量是不同的,其会体现在约化密度保真度上．另外,本文还从局域序参量和系统能隙的角度,来探讨量子多体系统的相变．     

Secure deterministic communication in a quantum loss channel using quantum error correction code

The loss of a quantum channel leads to an irretrievable particle loss as well as information. In this paper, the loss of quantum channel is analysed and a method is put forward to recover the particle and information loss effectively using universal quantum error correction. Then a secure direct communication scheme is proposed, such that in a loss channel the information that an eavesdropper can obtain would be limited to arbitrarily small when the code is properly chosen and the correction operation is properly arranged.