高斯量子密钥分发数据协调的性能优化

针对高斯量子密钥分发的数据协调问题,对高斯连续变量进行了最优量化,实现了Alice和Bob之间的互信息量最大。在分层错误校正(SEC)协议和多电平编码/多级解码(MLC/MSD)协议的基础上,各级码流采用了低密度奇偶校验码(LDPC)进行错误校正,并推出了一次硬信息级间迭代更新公式参与MSD译码算法。算法实现中使用双向十字链表方式存贮LDPC码的稀疏矩阵H,并用C语言实现整个数据协调过程,极大地降低了空间复杂度,提高了协调速度。实验仿真结果表明该算法可在信道信噪比4.9dB以上实现2×105个连续变量序列的可靠协调,协调效率达91.71%,在2.4GHz CPU,32G内存服务器平台上的协调速度可达7262bit/s。     

量子高斯密钥分发的逆向数据协调

加密系统中Alice端通过光纤传送相干态量子信号给Bob端,得到的一系列相关的高斯连续变量,然后从中提取出二进制密钥,这一过程称作高斯连续变量量子密钥分发.其中的数据协调步骤,即对相关连续变量的错误校正,是本文的研究问题.本文在分层错误校正协议(SEC)和多电平编码/多级解码(MLC/MSD)协议的基础上,用基于LDPC码的Slepian-Wolf编译码器实现协调.根据连续变量的多级Tanner信息传输流图,本文推导了多级迭代的内信息和外信息置信传播(LLR-BP)更新公式,并用实验比较了MSD三种译码模式的收敛性质及性能.仿真结果表明该算法可在信道信噪比2.5 dB以上实现10000个连续变量序列的可靠协调,协调效率可达92.3％.     

基于Turbo码的量子高斯密钥分发的数据协调

为了使加密系统中两个合法用户Alice和Bob从量子信道传送的相关的高斯连续变量X和y中获取出密钥,通过一个理想公共授权信道传送部分信息进而从不一致的X和y中得到一致的二进制密钥,这一过程称为协调.本文在样条纠错(Sliced Error Correction,SEC)和多级编码/多路译码(Multi Level coding/Multi Stage Decoding,MLC/MSD)方案的基础上,构建了一套以Turbo纠错码为基础的连续变量量子密钥分发系统的反向数据协调方案.提出了以Turbo码的校验比特流为协调信息的Slepian-Wolf编译码方案.仿真结果表明可在信道信噪比7 dB以上实现20000个连续变量序列的协调.     

基于PEG算法的连续变量量子密钥分发多维数据协调

针对连续变量量子密钥分发系统数据协调过程中效率偏低而造成的密钥量不易提取的问题,本文引入PEG(Progressive-edge-growth)算法构造合适度分布的LDPC校验矩阵,译码时采用LLR-BP译码算法。仿真结果表明,相同度分布下PEG方法构造出的校验矩阵比传统Mackay方法构造出的校验矩阵在译码时,能够更快达到收敛,并且在码率为0.5,码长为10~5时,PEG协调方案协调效率达到93.4%,提取到的安全密钥量为5.41 kb/s,信息传输距离为44.5 km。     

用于多维数据协调的双边类型LDPC码

在连续变量量子密钥分发(CV-QKD)多维数据协调中,协调效率和密钥传输距离取决于低密度奇偶校验(LDPC)码的纠错性能。在本研究中构造了一种拥有重复累积(RA)码中累积结构的高码率双边类型LDPC码(TET-LDPC),这种双边类型LDPC码在多维数据协调中相比于普通LDPC码可以得到更好的协调性能。经仿真结果可知,在信噪比为1.68 dB时,本文构造的码长为2×10~5的TET-LDPC的协调效率仍然可以达到98.48%,并得到了17.35 kb/s的安全密钥率。     

连续变量量子密钥分发多维数据协调算法

数据协调是量子密钥分发的重要组成部分,特别是连续变量量子密钥分发远程化的关键环节。在Leverrier等关于多维协调安全性证明的基础上,给出了面向多维协调的低密度奇偶校验码(LDPC)错误校正算法,考查了该算法的最小收敛信噪比阈值,并估算出基于这种多维数据协调方案的量子密钥分发的最大密钥传输距离,经过协调效率的计算以及噪声分析估算出最大安全密钥量。算法仿真结果表明:Alice和Bob之间的传输距离与分层错误校正协议(SEC)相比,从30km增加到47km左右,译码速度是SEC的4倍左右,密钥传输速率可以达到8.61kb/s。     

基于OpenCL/GPU异构计算的高速数据协调系统设计

针对当前连续变量量子密钥分发时数据协调运算速度低的问题,本文提出一种采用GPU与OpenCL异构计算的多维数据协调方案,并提出了一种静态双向十字链表存储超大规模LDPC码的校验矩阵,以适应OpenCL平台特殊要求。实验仿真结果显示,当码长为2×105时,在保证有效数据协调且相同码率的前提下,GPU平均译码速率可达到CPU的4.2倍,但牺牲了部分精度。     

连续变量量子密钥分发多维数据协调算法优化

针对连续变量量子密钥分发(CVQKD)通信距离较短的问题,在多维数据协调方案的基础上,利用连续密度进化和差分进化方法,设计出优质度数分布的低密度奇偶校验(LDPC)码,并提出LDPC码码字重复方法,进一步提高多维数据协调的效率,有效地降低了收敛信噪比,延长了信息传输距离。实验仿真结果表明:在分组码长为106时,收敛信噪比能够降低至-6dB以下,协调效率可达90.27%,提取到的安全密钥量为0.22kb/s,信息传输距离超过80km,该方法可有效延长CVQKD系统的通信距离。     

基于双边类型低密度奇偶校验码的连续变量量子密钥分发多维数据协调

在连续变量量子密钥分发(CVQKD)多维数据协调过程中,低密度奇偶校验码(LDPC)的纠错性能直接影响协调效率和传输距离。构造了一种双边类型的低密度奇偶校验码(TET-LDPC),引入了类似于重复累积码中的累积结构以提高其纠错性能,在多维数据协调算法中得到了更小的收敛信噪比、更高的协调效率以及更远的传输距离。仿真结果表明:当TET-LDPC的码率为0.5,分组码长为2×10~5时,收敛信噪比降至1.02dB,协调效率达到了98.58%,安全密钥率达到17.61kb/s,CVQKD系统的传输距离提高为44.9km。     

低密度奇偶校验码在光通信中的性能

低密度奇偶校验码(LDPC)不仅有逼近香农限的良好性能, 而且译码复杂度较低, 结构灵活, 是近年信道编码领域的研究热点, 在光通信系统中有广泛的应用前景。针对非规则Tanner图上构造的低密度奇偶校验长码具有良好的性能, 以及其在光通信系统中的应用, 构造了几种LDPC码, 并采用置信传播(BP)译码, 在加性高期白噪声(AWGN)信道、二相移相键空(BPSK)调制下进行了计算机仿真。根据规则LDPC码和非规则LDPC码的误码率(BER)和FER曲线, 对规则码和非规则LDPC码在编译码方面以及性能方面作了分析和比较, 进而对构造在光通信系统中具有实用价值的LDPC码提出了看法。通过理论分析与仿真结果表明LDPC 码型具有良好的性能, 可以节省硬件开销, 比较适用于光通信系统中, 可作为超强前向纠错码型的候选码。     

Reverse reconciliation for continuous variable quantum key distribution

An efficient and practical post-processing technique based on reverse reconciliation for continuous variable quantum key distribution is proposed and simulated with low-density parity check (LDPC) codes. MultiLevel Coding/ MultiStage Decoding, which fully utilizes optimization technique such as vector quantization and iterative decoding and the optimal channel coding most close to the Shannon limit, was used to realize efficient reverse reconciliation algorithm. Simulation results showed that the proposed m...     

Bound on Noise of Coherent Source for Secure Continuous-Variable Quantum Key Distribution

Coherent source of continuous-variable quantum key distribution (CV QKD) system may become noisy in practical applications. The security of CV-QKD scheme with the noisy coherent source is investigated under realistic conditions of quantum channel and detector. In particular, two models are proposed to characterize the noisy coherent source through introducing a party (Fred) who induces the noise with an optical amplifier. When supposing the party Fred is untrusted, two lower security bounds to the noise of the coherent source are derived for reverse reconciliation and realistic homodyne and heterodyne detections. While supposing Fred is a neutral party, we derive two tight security bounds without knowing Fred’s exact state for ideal detections. Moreover, the simulation results show that the security of the reverse reconciliation CV-QKD protocols is very sensitive to the noise of coherent source for both the homodyne and heterodyne detections.     

Collective attacks and unconditional security in continuous variable quantum key distribution

We present here an information theoretic study of Gaussian collective attacks on the continuous variable key distribution protocols based on Gaussian modulation of coherent states. These attacks, overlooked in previous security studies, give a finite advantage to the eavesdropper in the experimentally relevant lossy channel, but are not powerful enough to reduce the range of the reverse reconciliation protocols. Secret key rates are given for the ideal case where Bob performs optimal collective measurements, as well as for the realistic cases where he performs homodyne or heterodyne measurements. We also apply the generic security proof of Christiandl et al. to obtain unconditionally secure rates for these protocols.     

Hybrid linear amplifier-involved detection for continuous variable quantum key distribution with thermal states

Continuous-variable quantum key distribution(CVQKD) can be integrated with thermal states for short-distance wireless quantum communications. However, its performance is usually restricted with the practical thermal noise. We propose a method to improve the security threshold of thermal-state(TS) CVQKD by employing a heralded hybrid linear amplifier(HLA) at the receiver. We find the effect of thermal noise on the HLA-involved scheme in near-and-mid infrared band or terahertz band for direct and reverse reconciliation. Numerical simulations show that the HLA-involved scheme can compensate for the detriment of thermal noise and hence increase the security threshold of TS-CVQKD. In near-and-mid infrared band, security threshold can be extended by 2.1 dB in channel loss for direct reconciliation and 1.6 dB for reverse reconciliation, whereas in terahertz band, security threshold can be slightly enhanced for the gain parameter less than 1 due to the rise in thermal noise.     

Practical security of continuous-variable quantum key distribution under finite-dimensional effect of multi-dimensional reconciliation

The well-known multi-dimensional reconciliation is an effective method used in the continuous-variable quantum key distribution in the long-distance and the low signal-to-noise-ratio scenarios. The virtual channel employed to exchange data is generally established by using a finite-dimensional rotation in the reconciliation procedure. In this paper, we found that the finite dimension of the multi-dimensional reconciliation inevitably leads to the mismatch of the signal-to-noise-ratio between the quantum channel and the virtual channel, which may be called the finite-dimension effect. Such an effect results in an overestimation on the secret key rate, and subsequently induces vital practical security loopholes.     

High efficiency continuous-variable quantum key distribution based on QC-LDPC codes

Seeking good error correcting codes to improve the efficiency of continuous-variable quantum key distribution(CVQKD) reconciliation is a concerning issue. Due to the introduction of multidimensional reconciliation, the error correcting techniques in the classical binary-input additive white Gaussian noise channel are applicable to CVQKD. In this Letter, we apply the quasi-cyclic low-density parity-check(QC-LDPC) codes, which are specified in 5 G protocol, to the reconciliation process. Simulation results show that the reconciliation efficiency can reach 92.6% when the code rate is 22/68 and the signal-to-noise ratio is 0.623. Such a new error correcting code points out a new direction for the development of CVQKD.     

Error Estimation at the Information Reconciliation Stage of Quantum Key Distribution

Quantum key distribution (QKD) offers a practical solution for secure communication between two distinct parties via a quantum channel and an authentic public channel. In this work, we consider different approaches to the quantum bit error rate (QBER) estimation at the information reconciliation stage of the post-processing procedure. For reconciliation schemes employing low-density parity-check (LDPC) codes, we develop a novel syndrome-based QBER estimation algorithm. The algorithm suggested is suitable for irregular LDPC codes and takes into account punctured and shortened bits. Testing our approach in a real QKD setup, we show that an approach combining the proposed algorithm with conventional QBER estimation techniques allows one to improve the accuracy of the QBER estimation.     

离散调制连续变量量子密钥分发的安全边界

对一种结合离散调制和反向协调,适用于长距离传输的连续变量量子密钥分发四态协议的安全性进行了严格证明.这种协议中Alice发送的态与高斯调制协议中的有一定差异,这种差异可以等价成信道衰减和额外噪声.另外,由于Alice不可能做到精确调制,这会导致其发送的相干态中含有噪声.把这种调制引起的噪声看作光源的噪声,并推导出了在光源噪声不能被窃听者所利用的条件下的安全码率的下界.为了避免实验上快速、随机的控制本地振荡光的相位,还将无开关协议和四态协议相结合,分析了其安全性.     

Improving Key Rate of Optical Fiber Quantum Key Distribution System Based on Channel Tomography

We investigate the key rate of polarization-coded optical fiber-based quantum key distribution system (QKD) with BB84 protocol by channel tomography. The quantum bit error rate (QBER) of the QKD system under depolarizing channel is obtained by analyzing output density operators, transmission rate of channel, transmission rate of the receiver and dark count of single photon detector. According to the estimated QBER an LDPC chosen adaptively is applied to information reconciliation and the practical final key rate is discussed. Analysis results show that compared with cascade scheme and no channel knowledge the key rate can be improved by the proposed reconciliation scheme with channel tomography.     

Controlled Remote Information Concentration via Non-Maximally Entangled GHZ-Type States

In this paper, we propose a scheme of controlled remote quantum information concentration via non-maximally entangled GHZ-type states as quantum channel, which is the reverse process of the 1→3 optimal asymmetric economical phase-covariant telecloning. The necessary measurements and operations are given in detail.