利用无噪线性光放大器增加连续变量量子密钥分发最远传输距离

如何提高实际量子密钥分发系统的安全码率和最远传输距离是量子密码学领域重要的研究课题. 本文采用量子无噪线性光放大器放大量子信号, 以改进连续变量量子密钥分发系统实际性能. 经仔细研究, 本文发现增益系数为g的线性无噪放大器可将连续变量量子密钥分发系统的最远安全传输距离提高20 log₁₀(g)/a km (a=0.2 dB/km为光纤信道的损耗系数), 并改善系统的安全码率和噪声抗性. 关键词： 量子密钥分发 连续变量 最远传输距离 线性无噪放大器     

面向CVQKD量子-经典信号共纤传输技术研究

量子-经典信号共纤传输技术对实用化量子保密通信网络建设具有重要意义。针对经典光在光纤信道中的拉曼散射（RS）、四波混频（FWM）以及交叉相位调制（XPM）等非线性效应对量子信号的噪声干扰,构建了量子-经典信号共纤传输连续变量量子密钥分发（CVQKD）系统的安全密钥率仿真模型,重点分析了经典光功率、信道间隔和探测方式对系统噪声和密钥率的影响。结果表明,在近距离传输时,FWM噪声占主导地位,在传输距离大于10 km时,XPM噪声大于RS和FWM噪声。系统总噪声与经典光功率正相关,与波分复用信道间隔反相关。零差和外差检测下,随着传输距离的增加,安全密钥率整体变化趋势接近,零差检测方式具有更大的极限传输距离。该研究工作可为实用化量子-经典信号共纤传输CVQKD系统的优化设计提供参考。     

双协议量子密钥分发系统实验研究

提出了一种改进的基于时间和相位混合编码的量子密钥分发方案, 并进行了实验研究.在以BB84协议为基础的相位编码量子密钥分发系统上, 利用了系统中原来舍弃的脉冲进行时间编码, 使成码率提高为原方案的二倍. 系统同时获得时间编码密钥和相位编码密钥, 现在可以将两组密钥组合成新密钥, 提高了成码率和监测窃听灵敏度. 同时在系统的接收端用双FM反射式干涉仪代替传统的光纤M-Z干涉仪,提高了系统的稳定性. 实验上已实现90km光纤量子密钥分发, 实验表明本系统具有安全性高,稳定性好,成本低的优点. 关键词： 量子保密通信 量子密钥分发 相位编码 时间编码     

基于峰值补偿的连续变量量子密钥分发方案

在实际的连续变量量子密钥分发系统中,接收端模数转换器的有限采样带宽会导致脉冲峰值采样结果不准确,从而使参数估计过程产生误差,给窃听者留下了安全性漏洞.针对这个问题,本文提出一种基于峰值补偿的连续变量量子密钥分发方案,利用高斯脉冲的基本特性来估计每个脉冲的最大采样值与脉冲峰值之间的偏差,从而对该采样值进行峰值补偿,使系统得到正确的采样结果.本文详细分析了有限采样带宽对系统安全性的影响,阐述了峰值补偿的具体步骤,并讨论了峰值补偿前后系统估计的过噪声差别,及其在高斯集体攻击下的安全性.仿真实验结果表明,该方案能准确找到每个脉冲的峰值,纠正系统的参数估计误差.与不采用峰值补偿的方案相比,本方案消除了系统重复频率对密钥比特率的限制,具有更长的安全传输距离和更高的密钥比特率.     

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     

基于微弱相干脉冲稳定差分相位量子密钥分发

基于差分相位量子密钥分发协议，对微弱相干光脉冲相位差进行编码，在接收端采用Faraday-Michelson系统进行解码.这种量子密钥分发系统具有密钥生成效率高、接收端干涉稳定性好、极限传输距离长等优点，同时还具有光路结构简单、易于在现有的技术条件下实现等特点，特别适用于远程光纤量子密钥分发.在实验系统中利用嵌入式微处理系统来控制量子密钥分发过程，进行了76 km的稳定光纤量子密钥分发实验，其原始密钥的误码率为5.3%. 关键词： 分相位 量子密钥分发 安全性 稳定性     

时间和相位混合编码的量子密钥分发方案

提出了一种在双马赫-曾德尔干涉仪量子保密通信系统上同时实现时间编码和相位编码的混合量子密钥分发方案.提出的方案将原来相位编码方案中丢弃的脉冲进行时间编码,因此成码率是原方案的二倍.系统同时获得时间编码密钥和相位编码密钥,既可以一组用于通信,另一组用于监视窃听,又可以将两组密钥组合成新密钥.系统具有良好的应用前景. 关键词： 量子保密通信 量子密钥分发 相位编码 时间编码     

基于硬件同步的四态离散调制连续变量量子密钥分发

在连续变量量子密钥分发系统中,同步技术是确保通信双方时钟和数据一致的关键技术.本文通过巧妙设计发送端和接收端仪器的硬件时序,采用时域差拍探测方式和峰值采集技术,实验实现了可硬件同步的四态离散调制连续变量量子密钥分发.通信双方在设计好的硬件同步时序下可实现时钟的恢复和数据的自动对齐,无需借助软件算法实现数据的对齐.本文采用了加拿大滑铁卢大学Norbert Lütkenhaus研究组提出的针对连续变量离散调制协议的安全密钥速率计算方法.该方法需计算出接收端所测各种平移热态的一阶矩和二阶(非中心)矩,以此为约束条件结合凸优化算法可计算出安全密钥速率.计算过程中无需假设信道为线性信道,无需额外噪声的估算.密钥分发系统重复频率为10 MHz,传输距离为25 km,平均安全密钥比特率为24 kbit/s.本文提出的硬件同步方法无需过采样和软件帧同步,减小了系统的复杂度和计算量,在一定程度上降低了系统所需的成本、功耗和体积,有效地增强了连续变量量子密钥分发的实用性.     

基于非高斯态区分探测的往返式离散调制连续变量量子密钥分发方案

往返式离散调制连续变量量子密钥分发,无需使用两台独立的激光器也能本地生成本振光,并且信号光与本振光均来自于同一台激光器,在有效保证系统实际安全性的同时,具有较好的同频特性.此外,该方案与高效纠错码具有良好的兼容性,即使在低信噪比情况下也能获得较高的协商效率.然而,基于非可信信源模型的往返式光路结构存在较大的过噪声,严重限制离散调制方案的最大传输距离.针对这个问题,本文提出基于非高斯态区分探测的往返式离散调制连续变量量子密钥分发方案,即在探测端部署非高斯态区分探测器,采用自适应测量方法并结合贝叶斯推论,可以在满足低于标准量子极限错误概率的情况下无条件区分出基于四态离散调制的四种非正交相干态.本文详细分析了所提出的基于非高斯态区分探测的往返式离散调制连续变量量子密钥分发方案的安全性,包括渐近情况与有限长效应情况.仿真结果表明所提出的方案相比于原始方案,即使在有信源噪声的情况下,其密钥率与最大传输距离仍然有明显的提升.这些结果表明本方案能够有效降低往返式离散调制连续变量量子密钥分发方案中非可信信源噪声对方案性能的负面影响,在保证系统实际安全性的同时,实现更高效、更远传输距离的量子密钥分发.     

连续变量量子密钥分发系统同步方案及实现

同步技术是连续变量量子密钥分发系统的核心技术之一,能实现通信双方的实时通信,确保量子信息的有效提取。本文提出了一种新型的同步方案,能有效克服量子信道噪声及误码的影响,实现发送端和接收端的准确同步。文章从理论上介绍了方案的同步机制,并对其性能进行了仿真分析。     

基于量子催化的离散调制连续变量量子密钥分发

相比于离散变量量子密钥分发,连续变量量子密钥分发虽然具备更高的安全码率等优势,但是在安全传输距离上却略有不足.尽管量子催化的运用对高斯调制连续变量量子密钥分发协议的性能,尤其在安全传输距离方面有着显著的提升,然而能否用来改善离散调制协议的性能却仍然未知.鉴于上述分析,本文提出了一种基于量子催化的离散调制协议的方案,试图在安全密钥率、安全传输距离和最大可容忍过噪声方面进一步提升协议性能.研究结果表明,在相同参数下,当优化量子催化引入的透射率T,相比于原始四态调制协议,所提方案能够有效地提升量子密钥分发的性能.特别是,对于可容忍过噪声为0.002,量子催化可将安全通信距离突破300 km,密钥率为10^-8bits/pulse,而过大的可容忍噪声会抑制量子催化对协议性能的改善效果.此外,为了彰显量子催化的优势,本文给出了点对点量子通信的最终极限Pirandola-Laurenza-Ottaviani-Banchi边界,仿真结果表明,虽然原始方案与所提方案都未能突破这种边界,但是相比于前者,后者能够在远距离通信上逼近于这种边界,这为实现全球量子安全通信的最终目标提供理论依据.     

Continuous-Variable Quantum Key Distribution with Orthogonal Frequency Division Multiplexing Modulation

We demonstrate a multicarrier QKD protocol for continuous-variables (CV) using the orthogonal frequency division multiplexing (OFDM) technique which was employed in frequency domain. The multicarrier communication formulates sub-channels from OFDM technique and physical quantum channel, each dedicated to the transmission of a subcarrier CV. In the OFDM-CVQKD scenario, the input quantum states of the legal parties are granulated into subcarrier CVs, at the receiver, the subcarriers are decoded by an unitary CV operation, which results in the recovered single-carrier CVs. Compared with the device of the multichannel parallel CVQKD protocol that utilizes N arrayed-waveguide gratings (AWG) in optical domain, this protocol shows better feasibility of implementation from both equipment and technique. We derive the formulas of the secret key rate, moreover, analyze the security in the finite-size through the OFDM-CVQKD scheme. Simulation results indicate that the OFDM-CVQKD scheme leads to improved secret key rates and higher tolerable excess noise in comparison to single-carrier CVQKD. Particularly, the secret key rate of the 64 subcarrier CVQKD has increased by roughly an order of magnitude of the single-channel CVQKD, whereas the tolerable excess noise can be controlled in a small range. The results reveal that the OFDM-CVQKD protocol provides a feasible framework for the experimental implementation of an unconditionally secure communication over standard telecommunication networks.     

Phase Estimation and Compensation for Continuous-Variable Quantum Key Distribution

Phase estimation and compensation is one of the enabling functionalities in continous-variable quantum key distribution (CVQKD). Recently, a novel CVQKD scheme has been independently proposed to combat the local oscillator (LO) side channel attacks. Furthermore, we have carried out a proof-of-principle experimental study on the feasibility of the CVQKD without sending a LO. However, this scheme contains a serious weakness: The phase noise caused by the two different lasers between the sender and the receiver would severely destroy the quantum signal and finally reduce the secure distance. In this paper, we investigate the optical phase noise and explore the optimal approach to estimate and compensate such kind of noise with appropriate data overhead. Numerical simulations show that our scheme can successfully reconstruct the phase drifts even at low signal-to-noise ratio conditions. We also suggest that a higher accuracy of phase estimation could be achieved by using the frequency division multiplexing scheme. This opens an opportunity to employ advanced pilot-aided phase estimation techniques in quantum communication system.

     

Simultaneous measurement-device-independent continuous variable quantum key distribution with realistic detector compensation

We propose a novel scheme for measurement-device-independent (MDI) continuous-variable quantum key distribution (CVQKD) by simultaneously conducting classical communication and QKD, which is called “simultaneous MDI-CVQKD” protocol. In such protocol, each sender (Alice, Bob) can superimpose random numbers for QKD on classical information by taking advantage of the same weak coherent pulse and an untrusted third party (Charlie) decodes it by using the same coherent detectors, which could be appealing in practice due to that multiple purposes can be realized by employing only single communication system. What is more, the proposed protocol is MDI, which is immune to all possible side-channel attacks on practical detectors. Security results illustrate that the simultaneous MDI-CVQKD protocol can secure against arbitrary collective attacks. In addition, we employ phasesensitive optical amplifiers to compensate the imperfection existing in practical detectors. With this technology, even common practical detectors can be used for detection through choosing a suitable optical amplifier gain. Furthermore, we also take the finite-size effect into consideration and show that the whole raw keys can be taken advantage of to generate the final secret key instead of sacrificing part of them for parameter estimation. Therefore, an enhanced performance of the simultaneous MDI-CVQKD protocol can be obtained in finite-size regime.     

Performance improvement of eight-state continuous-variable quantum key distribution with an optical amplifier

Discrete modulation is proven to be beneficial to improving the performance of continuous-variable quantum key distribution (CVQKD) in long-distance transmission. In this paper, we suggest a construct to improve the maximal generated secret key rate of discretely modulated eight-state CVQKD using an optical amplifier (OA) with a slight cost of transmission distance. In the proposed scheme, an optical amplifier is exploited to compensate imperfection of Bob's apparatus, so that the generated secret key rate of eight-state protocol is enhanced. Specifically, we investigate two types of optical amplifiers, phase-insensitive amplifier (PIA) and phase-sensitive amplifier (PSA), and thereby obtain approximately equivalent improved performance for eight-state CVQKD system when applying these two different amplifiers. Numeric simulation shows that the proposed scheme can well improve the generated secret key rate of eight-state CVQKD in both asymptotic limit and finite-size regime. We also show that the proposed scheme can achieve the relatively high-rate transmission at long-distance communication system.     

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.     

Analysis of atmospheric effects on the continuous variable quantum key distribution

Atmospheric effects have significant influence on the performance of a free-space optical continuous variable quantum key distribution (CVQKD) system. In this paper, we investigate how the transmittance, excess noise and interruption probability caused by atmospheric effects affect the secret-key rate (SKR) of the CVQKD. Three signal wavelengths, two weather conditions, two detection schemes, and two types of attacks are considered in our investigation. An expression aims at calculating the interruption probability is proposed based on the Kolmogorov spectrum model. The results show that a signal using long working wavelength can propagate much further than that of using short wavelength. Moreover, as the wavelength increases, the influence of interruption probability on the SKR becomes more significant, especially within a certain transmission distance. Therefore, interruption probability must be considered for CVQKD by using long-signal wavelengths. Furthermore, different detection schemes used by the receiver will result in different transmission distances when subjected to individual attacks and collective attacks, respectively.     

The Improvement of Performance for Continuous-Variable Quantum Key Distribution with Imperfect Gaussian Modulation

Gaussian modulation is one of the key steps for the implementation of continuous-variable quantum key distribution (CVQKD) schemes. However, imperfection in the Gaussian modulation may introduce modulation noise that can deteriorate the performance of CVQKD systems. In this paper, we mainly investigate how to improve the performance of a CVQKD system from different aspects. First, we explore the several different origins, impacts and monitoring schemes for the modulation noise in detail. Then, we discuss the practical performance of a CVQKD system with an untrusted noise model and neutral party model, respectively. These analyses indicate that the neutral party model should be reasonably regarded as a general noise model, which will passively and greatly raise the performance of the system. Further, we propose a dynamic auto-bias control scheme to actively resist the modulation noise which comes from the drift of bias point of the amplitude modulator. Together these methods contribute to the improvement of the practical performance of CVQKD systems with imperfect Gaussian modulation.

     

Optical source linewidth criteria for heterodyne communication systems with PSK modulation

The performance of an optical heterodyne communication system is analysed for the cases of phase-shift keying (PSK) with synchronous electrical demodulation and differential PSK with non-synchronous demodulation. The combined effect on the system performance from the shot noise, quantum phase noise of the optical sources and the pulse shaping are considered. From the results, criteria are given for the maximum linewidth of the transmit and local optical sources to satisfy a given degradation in the optical receiver sensitivity.     

Short-wave infrared continuous-variable quantum key distribution over satellite-to-submarine channels

The trans-media transmission of quantum pulse is one of means of free-space transmission which can be applied in continuous-variable quantum key distribution (CVQKD) system. In traditional implementations for atmospheric channels, the 1500-to-1600-nm pulse is regarded as an ideal quantum pulse carrier. However, the underwater transmission of this pulses tends to suffer from severe attenuation, which inevitably deteriorates the security of the whole CVQKD system. In this paper, we propose an alternative scheme for implementations of CVQKD over satellite-to-submarine channels. We estimate the parameters of the trans-media channels, involving atmosphere, sea surface and seawater and find that the short-wave infrared performs well in the above channels. The 450-nm pulse is used for generations of quantum signal carriers to accomplish quantum communications through atmosphere, sea surface and seawater channels. Numerical simulations show that the proposed scheme can achieve the transmission distance of 600 km. In addition, we demonstrate that non-Gaussian operations can further lengthen its maximal transmission distance, which contributes to the establishment of practical global quantum networks.