无相互作用费米量子信道直积态容量研究

用费米线性光学方法,提出无相互作用费米量子信道物理模型.用平稳量子高斯态协方差矩阵性质及Majorization不等式理论,推导出在平稳高斯输入态下费米量子信道最小输出熵的表达式.利用在n模费米系统添加一个额外模的方法,得到平稳高斯态和高斯态输出熵的关系|利用此关系式,借助在高斯输入态下费米信道最小输出熵值是可达的猜测,推导出无相互作用费米信道直积态容量的表达式.最后,用最小输出熵的迭代算法验证已推出的费米信道最小输出熵表达式正确性,数值计算结果表明:对于带噪声的无相互作用费米量子信道,已推出最小输出熵与数值计算结果的吻合度可以达到10e-9.     

量子热噪声信道在纠缠辅助下的容量

研究了量子纠缠辅助下输入功率受限时的单模热辐射噪声信道传输经典信息的容量问题，和带有放大或衰减的单模热噪声信道的相应问题。对热噪声信道，表明了容量在输入信号为热噪声信号时达到，压缩态无助于达到信道容量CE；对带有放大或衰减的单模热噪声信道，说明了容量同样在输入为热噪声态时达到。     

无相互作用费米量子信道直积态容量研究

用费米线性光学方法,提出无相互作用费米量子信道物理模型.用平稳量子高斯态协方差矩阵性质及Majorization不等式理论,推导出在平稳高斯输入态下费米量子信道最小输出熵的表达式.利用在n模费米系统添加一个额外模的方法,得到平稳高斯态和高斯态输出熵的关系;利用此关系式,借助在高斯输入态下费米信道最小输出熵值是可达的猜测...     

基于DWDM的经典-量子信息共信道同传系统噪声分析

经典-量子信息共信道同传是光纤量子密钥通信中的关键应用技术。为解决同传系统在复杂的背景噪声环境下的传输效率问题,通过剖析基于DWDM的共信道同传系统中典型噪声干扰因素,对该系统中DWDM信道串扰及自发拉曼散射噪声建立模型,仿真计算不同光纤长度及不同波长信道下噪声干扰对系统的影响。分析结果表明,在初始功率恒定的情况下,系统噪声不会随光纤长度增加而无限积累,而是趋于恒定数值;而在接收功率恒定的情况下,系统噪声随光纤长度增加不断增长。从信道的波长角度看,处于低噪声环境的经典与量子信道波长符合密集波分复用要求。根据ITU-T建议下的DWDM信道间隔,在经典-量子信道波长间距为1.6nm时,系统性能最佳,该仿真结果与经典实验的结论相吻合。     

基于信道估计的自适应连续变量量子密钥分发方法

为使连续变量量子密钥分发协议获得稳定的密钥生成率,需要根据信道变化动态调整发送光脉冲的强度.将光纤量子信道看作加性玻色量子高斯信道,给出高斯态通过玻色量子高斯信道仍得到高斯态的证明过程.通过平衡零差检测后,采用最大似然估计法得到了信道参数,进而根据估计的噪声大小自适应调整Alice发送的光脉冲的强度,从而获得稳定的密钥生成率. 关键词： 量子密钥分发 连续变量 玻色量子高斯信道 信道估计     

一般泡利信道量子容量的逼近

量子编码定理证明信道在没有辅助资源的情况下其量子容量等于规整化相干信息的最大值。一般泡利信道是最普遍使用的信道模型,其量子容量目前无法准确计算,只能用多信道相干信息去逼近。本文应用量子图态级联编码,得到一般泡利信道在该编码输入下的多信道相干信息的公式,能够有效计算一般泡利信道量子容量的逼近值和信道传输量子信息的噪声容限。计算速度比Monte Carlo算法提高三个数量级。     

MIMO量子信道的空间自由度研究

量子通信是一个量子密钥分发过程,目前采用的通信技术严重制约了量子密钥分发的比特率.将多输入多输出（MIMO）技术应用于量子通信系统,提高量子密钥分发的比特率,促进量子通信向高速大容量发展.然而,量子场本身不可避免地存在量子噪声约束容量的增长,限制了可利用空间资源,即空间自由度.文中采用光子场的量子化和满足Schrdinger方程条件的电磁场波动方程推导出MIMO量子信道的空间自由度上限,为开发稳健的MIMO量子通信空时处理算法和优化设计高性能MIMO量子通信系统提供理论基础和技术支持. 关键词： 多输入多输出 量子密钥分发 Schrdinger方程 光子场的量子化     

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     

基于量子隐形传态的量子保密通信方案

量子保密通信包括量子密钥分发、量子安全直接通信和量子秘密共享等主要形式.在量子密钥分发和秘密共享中,传输的是随机数而不是信息,要再经过一次经典通信才能完成信息的传输.在量子信道直接传输信息的量子通信形式是量子安全直接通信.基于量子隐形传态的量子通信(简称量子隐形传态通信)是否属于量子安全直接通信尚需解释.构造了一个量子隐形传态通信方案,给出了具体的操作步骤.与一般的量子隐形传态不同,量子隐形传态通信所传输的量子态是计算基矢态,大大简化了贝尔基测量和单粒子操作.分析结果表明,量子隐形传态通信等价于包含了全用型量子密钥分发和经典通信的复合过程,不是量子安全直接通信,其传输受到中间介质和距离的影响,所以不比量子密钥分发更有优势.将该方案与量子密钥分发、量子安全直接通信和经典一次性便笺密码方案进行对比,通过几个通信参数的比较给出各个方案的特点,还特别讨论了各方案在空间量子通信方面的特点.     

基于双模压缩信道的双模高斯态量子隐形传态

研究了双模高斯态的两个模经由两个双模压缩量子信道的隐形传态.结果表明，当输入态有纠缠时，若要输出态的纠缠不为零，量子信道的纠缠必须大于一确定值，其大小在输入态为纯态时依赖于输入态的纠缠度；在输入态为混合态时不仅与输入态的纠缠有关，还依赖于输入态的整体纯度. 关键词： 量子隐形传态 量子纠缠 双模高斯态     

Effect of excess noise on continuous variable entanglement sudden death and Gaussian quantum discord

A symmetric two-mode Gaussian entangled state is used to investigate the effect of excess noise on entanglement sudden death and Gaussian quantum discord with continuous variables. The results show that the excess noise in the channel can lead to entanglement sudden death of a symmetric two-mode Gaussian entangled state, while Gaussian quantum discord never vanishes. As a practical application, the security of a quantum key distribution （QKD） scheme based on a symmetric two-mode Gaussian entangled state against collective Gaussian attacks is analyzed. The calculation results show that the secret key cannot be distilled when entanglement vanishes and only quantum discord exists in such a QKD scheme.     

Correlation and Information in Quantum Channels

Quantum and classical correlations in quantum channels are investigated by means of an entangled pure state and a separable state which is closest to an entangled pure state. The coherent information and the separable information are used to estimate how much correlation is transmitted through a quantum channel. As the examples, the linear dissipative channel of qubits and the quantum erasure channel are considered.     

Upper bounds on the private capacity for bosonic Gaussian channels

Recently, there have been considerable progresses on the bounds of various quantum channel capacities for bosonic Gaussian channels. Especially, several upper bounds for the classical capacity and the quantum capacity on the bosonic Gaussian channels, via a technique known as quantum entropy power inequality, have been shed light on understanding the mysterious quantum-channel-capacity problems. However, upper bounds for the private capacity on quantum channels are still missing for the study on certain universal upper bounds. Here, we derive upper bounds on the private capacity for bosonic Gaussian channels involving a general Gaussian-noise case through the conditional quantum entropy power inequality.     

Controlled quantum teleportation of an unknown single-qutrit state in noisy channels with memory

This paper proposes a three-dimensional(3 D) controlled quantum teleportation scheme for an unknown single-qutrit state. The scheme is first introduced in an ideal environment, and its detailed implementation is described via the transformation of the quantum system. Four types of 3 D-Pauli-like noise corresponding to Weyl operators are created by Kraus operators: trit-flip, t-phase-flip, trit-phase-flip, and t-depolarizing. Then, this scheme is analyzed in terms of four types of noisy channel with memory. For each type of noise, the average fidelity is calculated as a function of memory and noise parameters, which is afterwards compared with classical fidelity. The results demonstrate that for trit-flip and t-depolarizing noises, memory will increase the average fidelity regardless of the noise parameter. However, for t-phase-flip and trit-phaseflip noises, memory may become ineffective in increasing the average fidelity above a certain noise threshold.     

On the Classical Capacity of General Quantum Gaussian Measurement

In this paper, we consider the classical capacity problem for Gaussian measurement channels. We establish Gaussianity of the average state of the optimal ensemble in the general case and discuss the Hypothesis of Gaussian Maximizers concerning the structure of the ensemble. Then, we consider the case of one mode in detail, including the dual problem of accessible information of a Gaussian ensemble. Our findings are relevant to practical situations in quantum communications where the receiver is Gaussian (say, a general-dyne detection) and concatenation of the Gaussian channel and the receiver can be considered as one Gaussian measurement channel. Our efforts in this and preceding papers are then aimed at establishing full Gaussianity of the optimal ensemble (usually taken as an assumption) in such schemes.     

A Family of Degenerate Codes for Depolarizing Channels

Quantum degenerate code may improve the hashing bound of quantum capacity. We propose a family of quantum degenerate codes derived from two-colorable graphs. The coherent information of the codes is analytically obtained as a function of the channel noise for the depolarizing channel. We find a new code which has a higher noise threshold than that of the repetition code.