平衡零拍测量对连续变量量子密钥分配的影响

与单光子量子密钥分配采用单光子探测器不同，连续变量量子密钥分配采用平衡零拍测量技术.分析了由于参考光的真空噪声、分束器的透射率和反射率不相等引入的平衡零拍测量误差，以及平衡零拍测量探测器的电子噪声对连续变量量子密钥传输的最大安全距离的限制，给出了平衡零拍测量的探测噪声、电子噪声和密钥量之间的定量表达式.     

平衡零拍探测器的改进

平衡零拍探测技术是连续变量量子信息科学研究中测量量子态量子噪声的最佳方法之一。在平衡零拍探测系统中,需要一对性能和结构尽量相同的光电探测器,以便很好地减去经典噪声,有效探测散粒噪声基准及光场噪声。根据基尔霍夫电流定律及光电二极管串联的办法,设计并制作出一种性能优良的低噪声平衡零拍探测器。该探测器从光电二极管串联节点处取出光电流相减信号,然后对信号进行放大,在对两探测器全同性要求降低的同时提高了平衡零拍探测的性能。实验结果表明,该探测器在2MHz处共模抑制比达39dB,能很好地满足量子信息对低噪声、线性增益及高共模抑制比的要求。     

EPR纠缠光束时域量子关联的实验验证

我们利用平衡零拍探测系统和数据采集系统直接测量了EPR纠缠光束信号光场与闲置光场正交分量之间的时域量子起伏,验证了其量子关联.通过对EPR纠缠光束中的一束光场进行延迟,验证了信号光与闲置光不同步时将导致量子关联减弱乃至消失.我们的实验结果为开展连续变量量子密钥分发、非高斯态的实验制备和纠缠纯化奠定了实验基础.     

基于R-C耦合跨阻设计的1 kHz～200 MHz宽带平衡零拍探测器

通过分析平衡零拍探测器的噪声来源，采用电阻-电容（R-C）耦合跨阻电路方案并优化跨阻放大电路结构，实现了一种低噪声、高共模抑制比的宽带平衡零拍探测器，工作带宽为1 kHz～200 MHz。当光功率为8 mW的1550 nm激光入射时：在5 kHz分析频率处散粒噪声功率比电子学噪声功率高12 dB，共模抑制比达到70 dB；在100 MHz分析频率处散粒噪声功率比电子学噪声功率高20 dB，共模抑制比达到66 dB。该探测器可以为低频引力波探测、高速连续变量量子密钥分发和高速量子随机数发生器等应用提供高性能的探测工具。     

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     

用于连续变量量子存储的快速响应平衡零拍探测器

改进了连续变量量子存储的平衡零拍探测器,得到了一种可以实时测量短脉冲光信号正交分量的快速响应平衡零拍探测器。利用无电容电路,以及高量子效率、低结电容的光电二极管,获得的平衡零拍探测器的响应时间为65ns;当波长为795nm、功率大于100μW的激光入射时,在2.5 MHz处的信噪比超过12dB,相应的饱和功率为6.8mW。该平衡零拍探测器可以应用于连续变量量子存储、量子网络等研究领域中。     

压缩态光场平衡零拍探测的位相锁定

平衡零拍探测是测量量子光场的重要方法之一. 通过对相位灵敏光学参量放大器注入的信号进行位相调制, 然后利用平衡零拍探测系统测量光学参量放大器输出的压缩光. 将相位灵敏光学参量分别运转在参量放大和参量缩小, 通过观察噪声谱中的调制信号就可确定测量的量子光场是正交振幅或位相分量. 通过解调位相调制信号可获得误差信号, 实现锁定平衡零拍探测系统本底光与待测光场相对位相为零(对应于待测光场振幅噪声分量). 关键词： 平衡零拍探测 位相锁定     

明亮压缩态光场的操控及量子层析

连续变量量子态的制备与操控是进行量子通信、量子密钥分发以及量子网络构建的重要基础.本文基于二阶非线性过程,利用周期极化磷酸氧钛钾晶体构成的简并光学参量放大腔,在实验上实现了 1064 nm波段明亮压缩态光场的制备,所制备的明亮压缩态光场在泵浦光功率为310 mW、分析频率为3 MHz处的压缩度为-11.6 dB.当注入50 mW泵浦光时,实现了压缩度为-6 dB,纯度为98.5%的压缩态光场;在此基础上,利用光电调制器进行明亮压缩态光场的线性光学操控,并基于平衡零拍探测系统的直流信号准确判断压缩态光场时域信号对应的相位,之后结合极大似然估计算法实现压缩态的量子层析,得到量子态的密度矩阵及相空间的Wigner函数,从而获得量子态的光子数分布等全部信息.     

平衡零拍探测器的噪声特性分析与实验研究

平衡零拍探测器是量子光学和量子信息科学实验中的关键器件之一。本文针对典型的基于跨阻运算放大器的平衡零拍探测器,理论上详细分析了探测器的电子学噪声、散粒噪声和信噪比等输出特性;从理论分析、软件仿真和实验研究三方面分别获得了平衡零拍探测器的电子学噪声和散粒噪声功率谱,谱线线型互相吻合;实验研制的平衡零拍探测器,在分析频率2 MHz处的信噪比达到了22 dB,相应于电子学噪声引入的测量损耗约为0.6%,满足测量压缩态光场等量子光学实验的需求。     

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.     

Versatile wideband balanced detector for quantum optical homodyne tomography

We present a comprehensive theory and an easy to follow method for the design and construction of a wideband homodyne detector for time-domain quantum measurements. We show how one can evaluate the performance of a detector in a specific time-domain experiment based on the electronic spectral characteristic of that detector. We then present and characterize a high-performance detector constructed using inexpensive, commercially available components such as low-noise high-speed operational amplifiers and high-bandwidth photodiodes. Our detector shows linear behavior up to a level of over 13 dB clearance between shot noise and electronic noise, in the range from DC to 100 MHz. The detector can be used for measuring quantum optical field quadratures both in the continuous-wave and pulsed regimes with standard commercial mode-locked lasers.     

Improving continuous-variable quantum key distribution under local oscillator intensity attack using entanglement in the middle

A modified continuous-variable quantum key distribution(CVQKD) protocol is proposed by originating the entangled source from a malicious third party Eve in the middle instead of generating it from the trustworthy Alice or Bob. This method is able to enhance the efficiency of the CVQKD scheme attacked by local oscillator(LO) intensity attack in terms of the generated secret key rate in quantum communication. The other indication of the improvement is that the maximum transmission distance and the maximum loss tolerance can be increased significantly, especially for CVQKD schemes based on homodyne detection.     

Ultrasensitive pulsed, balanced homodyne detector: application to time-domain quantum measurements

A pulsed, balanced homodyne detector has been developed for precise measurement of the electric field quadratures of pulsed optical quantum states. A high level of common mode suppression (>85 dB) and low electronic noise (730 electrons per pulse) provide a signal-to-noise ratio of 14 dB for measurement of the quantum noise of individual pulses. Measurements at repetition rates as high as 1 MHz are possible. As a test, quantum tomography of the coherent state was performed, and the Wigner function and the density matrix were reconstructed with 99.5% fidelity. The detection system can be used for ultrarsensitive balanced detection in cw mode, e.g., for weak absorption measurements.     

Practical Analysis of Continuous-Variable Quantum Key Distribution Using a Nondeterministic Noiseless Linear Amplifier

We study the impact of the imperfections and the finite-size effect on the continuous-variable quantum key distribution (CVQKD) protocol with the nondeterministic noiseless linear amplifier (NLA). The imperfections of the homodyne detector and the imperfect amplification process as well as the finite-size effect on parameter estimation procedure are considered. We can see that despite the imperfections of the homodyne detector, the maximum improved transmission distance can still reach the equivalence of 20log₁₀g dB losses theoretically. Moreover, the analysis shows the imperfect amplification process of the NLA will slightly decrease the performance of the system. And we find the finite-size effect significantly influence the secret key rates of the NLA CVQKD protocol and the performance will approach the ideal asymptotic case with the increase of block size.     

Continuous variable quantum communication with bright entangled optical beams

In this paper, we briefly introduce the basic concepts and protocols of continuous variable quantum communication, and then summarize the experimental researches accomplished by our group in this field. The main features of quantum communication systems used in our experiments are: (1) The bright entangled optical beams with the anticorrelated amplitude quadratures and the correlated phase quadratures that serve as the entanglement resources and (2) The Bell-state direct detection systems are utilized in the measurements of quantum entanglement and transmitted signals instead of the usually balanced homodyne detectors.     

中纬度地区电离层偶发E层对量子卫星通信性能的影响

电离层偶发E层是指在距离地面高度80—150 km之间,在风剪切作用下,电子密度急剧增加的不规则电离薄层,它会对量子卫星光信号的传输造成极大的影响.然而,有关电离层偶发E层与星地间量子通信信道参数关系的研究,迄今尚未展开.为了研究偶发E层对量子卫星通信性能的影响,首先分析了它的形成过程,得出自由电子密度随高度变化的关系;然后建立了自由电子密度、偶发E层的厚度对量子卫星链路衰减的模型;针对振幅阻尼信道,给出自由电子密度对信道容量、纠缠保真度、误码率和安全密钥产生率的定量关系.理论分析和仿真结果表明,当偶发E层的厚度为1 km、电子密度由3×10~5cm~(-1)增加到27×10~5cm~(-1)时,信道容量由0.8304衰减到0.1319,纠缠保真度由0.9386下降到0.3606,量子误码率由0.0093增加到0.0769,安全密钥产生率由9.968×10~(-5)减小到1.91×10~(-6).由此可见,电子密度的大小和偶发E层的厚度对量子卫星通信性能有显著的影响.因此,在进行量子卫星通信时,应根据对电离层参数的探测情况,自适应调整卫星系统的各项指标,以确保量子通信的可靠性.     

基于光前置放大器的量子密钥分发融合经典通信方案

量子密钥分发融合经典通信方案将连续变量量子密钥分发和经典通信合并到了一起,为将来在现有的光网络上同时进行密钥分发和经典通信提供了一个有效的方法.然而,在量子信号上叠加一个经典信号将会给连续变量量子密钥分发系统引入过噪声从而大大降低系统的性能.本文提出基于光前置放大器的量子密钥分发融合经典通信方案,即在接收端插入光前置放大器来提升系统的性能.首先,在相同比特误码率条件下,光前置放大器对信号的放大能够降低对发送端经典信号调制振幅的要求,从而降低经典信号对量子信号的噪声影响;其次,光前置放大器能够补偿接收端探测器的不完美;再次,对于本地本振光的情形,放大器还能放大弱相位参考脉冲,从而降低参考脉冲散粒噪声带来的相位过噪声.在实际可达到的系统参数下,数值仿真结果表明本文提出的方案相比于原方案在安全密钥率和传输距离上都有很好的提升.这些结果表明本方案为量子密钥分发融合经典通信方案的进一步发展和实际应用提供了一个十分有效而实用的方法.     

Practical security analysis of continuous-variable quantum key distribution with an unbalanced heterodyne detector

When developing a practical continuous-variable quantum key distribution (CVQKD), the detector is necessary at the receiver's side. We investigate the practical security of the CVQKD system with an unbalanced heterodyne detector. The results show that unbalanced heterodyne detector introduces extra excess noise into the system and decreases the lower bound of the secret key rate without awareness of the legitimate communicators, which leaves loopholes for Eve to attack the system. In addition, we find that the secret key rate decreases more severely with the increase in the degree of imbalance and the excess noise induced by the imbalance is proportional to the intensity of the local oscillator (LO) under the same degree of imbalance. Finally, a countermeasure is proposed to resist these kinds of effects.     

Hybrid quantum private communication with continuous-variable and discrete-variable signals

The quantum key distribution(QKD) has been entering the practical application era. Subsequently, hybrid quantum private communication with discrete-variable signals, continuous-variable signals, and classic optical signals becomes inevitable in the practical scenario. In this paper, we experimentally investigated the mutual effects between the discrete-variable QKD(DVQKD) and the continuous-variable QKD(CVQKD) via a fiber channel. The experimental results show that the DVQKD will be influenced by the continuous-variable quantum signals and classic optical signals, while the CVQKD is not sensitive to the discrete-variable quantum signals.