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.

Application of Bipartite and Tripartite Entangled State Representations in Quantum Teleportation of Continuous Variables

We mostly investigate two schemes. One is to teleport a multi-mode W-type entangled coherent state using a peculiar bipartite entangled state as the quantum channel different from other proposals. Based on our formalism,teleporting multi-mode coherent state or squeezed state is also possible. Another is that the tripartite entangled state is used as the quantum channel of controlled teleportation of an arbitrary and unknown continuous variable in the case of three participators.

Dynamic entanglement for continuous variables in an electric field background

Quantum entanglement is a typical nonclassical correlation.Here,we use this concept to analyze quantum entanglement for continuous variables generated by the Schwinger pair production for constant and pulsed electric fields.An initial two-mode entangled state evolves into a three-mode entangled state through a Gaussian channel of the Schwinger effect,which encodes the information about the Schwinger effect.By detecting the entanglement of the output three-mode state,we obtain the optimal parameters for easier to generate particle-antiparticle pairs.We find that the generated 1→2 entanglement is more sensitive to the parameters than the generated 1→1 entanglement.Therefore,we should choose the generated 1→2 entanglement to extract information.We argue that extracting the optimal parameters from quantum entanglement may guide future experiments.

Investigation of the influence of extra noises in seed beams on continuous-variable entanglement generation

The influence of the extra classical noises in seed beams on the entanglement between the signal and the idler modes of the output fields generated by a non-degenerate optical parametric amplifier operating at deamplification is investigated theoretically and experimentally. With the increase of the extra classical noises in the seed beams, the correlation degree of the output entangled optical fields, which is scaled by the quantum noise limit, decreases rapidly. The experimental results axe in good agreement with the theoretical calculations.

Finite-size analysis of continuous-variable quantum key distribution with entanglement in the middle

Continuous-variable quantum key distribution(CVQKD) protocols with entanglement in the middle(EM) enable long maximal transmission distances for quantum communications. For the security analysis of the protocols, it is usually assumed that Eve performs collective Gaussian attacks and there is a lack of finite-size analysis of the protocols. However,in this paper we consider the finite-size regime of the EM-based CVQKD protocols by exposing the protocol to collective attacks and coherent attacks. We differentiate between the collective attacks and the coherent attacks while comparing asymptotic key rate and the key rate in the finite-size scenarios. Moreover, both symmetric and asymmetric configurations are collated in a contrastive analysis. As expected, the derived results in the finite-size scenarios are less useful than those acquired in the asymptotic regime. Nevertheless, we find that CVQKD with entanglement in the middle is capable of providing fully secure secret keys taking the finite-size effects into account with transmission distances of more than 30 km.

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.

Continuous Variable Quantum MNIST Classifiers—Classical-Quantum Hybrid Quantum Neural Networks

In this paper, classical and continuous variable (CV) quantum neural network hybrid multi-classifiers are presented using the MNIST dataset. Currently available classifiers can classify only up to two classes. The proposed architecture allows networks to classify classes up to nm classes, where n represents cutoff dimension and m the number of qumodes on photonic quantum computers. The combination of cutoff dimension and probability measurement method in the CV model allows a quantum circuit to produce output vectors of size nm. They are then interpreted as one-hot encoded labels, padded with nm - 10 zeros. The total of seven different classifiers is built using 2, 3, …, 6, and 8-qumodes on photonic quantum computing simulators, based on the binary classifier architecture proposed in “Continuous variable quantum neural networks” [1]. They are composed of a classical feed-forward neural network, a quantum data encoding circuit, and a CV quantum neural network circuit. On a truncated MNIST dataset of 600 samples, a 4-qumode hybrid classifier achieves 100% training accuracy.

Continuous-Variable Quantum Network Coding Based on Quantum Discord

Establishing entanglement is an essential task of quantum communication technology.Beyond entanglement,quantum discord,as a measure of quantum correlation,is a necessary prerequisite to the success of entanglement distribution.To realize efficient quantum communication based on quantum discord,in this paper,we consider the practical advantages of continuous variables and propose a feasible continuous-variable quantum network coding scheme based on quantum discord.By means of entanglement distribution by separable states,it can achieve quantum entanglement distribution from sources to targets in a butterfly network.Compared with the representative discrete-variable quantum network coding schemes,the proposed continuous-variable quantum network coding scheme has a higher probability of entanglement distribution and defends against eavesdropping and forgery attacks.Particularly,the deduced relationship indicates that the increase in entanglement is less than or equal to quantum discord.

Security analysis of continuous-variable quantum key distribution scheme

In this paper security of the quantum key distribution scheme using correlations of continuous variable Einstein- Podolsky-Rosen （EPR） pairs is investigated. A new approach for calculating the secret information rate △I is proposed by using the Shannon information theory. Employing an available parameter F which is associated with the entanglement of the EPR pairs, one can detect easily the eavesdropping. Results show that the proposed scheme is secure against individual bearn splitter attack strategy with a proper squeeze parameter.

Anonymous voting for multi-dimensional CV quantum system

We investigate the design of anonymous voting protocols,CV-based binary-valued ballot and CV-based multi-valued ballot with continuous variables（CV） in a multi-dimensional quantum cryptosystem to ensure the security of voting procedure and data privacy.The quantum entangled states are employed in the continuous variable quantum system to carry the voting information and assist information transmission,which takes the advantage of the GHZ-like states in terms of improving the utilization of quantum states by decreasing the number of required quantum states.It provides a potential approach to achieve the efficient quantum anonymous voting with high transmission security,especially in large-scale votes.

基于不可信纠缠源的高斯调制连续变量量子密钥分发

在实际的量子通信系统中,连续变量量子密钥分发的信源安全可能会因为器件的缺陷或隐藏的攻击而受到威胁.针对这个问题,本文提出了基于不可信纠缠源的高斯调制连续变量量子密钥分发方案,通过将高斯纠缠源置于不可信的量子信道来模拟纠缠源被攻击者所控制的场景,从而验证实际复杂环境下高斯调制连续变量量子密钥分发的安全性.本文详细分析不可信纠缠源对系统安全性的影响并引入了两种光学放大器来辅助提升所提方案的实际性能.仿真实验结果表明,本文所提方案即使在高斯纠缠源不可信的情况下仍然能够产生安全的量子密钥,同时,光学放大器也能够有效提升接收端探测器的量子效率.该工作旨在推动高斯调制连续变量量子密钥分发系统的实用化进程,为高斯调制连续变量量子密钥分发系统的实际部署和应用提供理论指导.

连续变量纠缠态光场在光纤中传输特性的实验研究

连续变量纠缠态由于其确定性产生、高效率的特点而被广泛应用于连续变量量子信息处理.在量子信息处理过程中纠缠态与量子信道发生相互作用而退相干,这是限制长距离量子信息发展的重要因素之一.光纤信道作为理想的量子信道,是目前连续变量量子信息研究关注的热点.本文利用Ⅱ类匹配的楔角极化磷酸氧钛钾晶体构成了三共振的非简并光学参量放大器,获得了8.3 dB的光通信波段1.5μm连续变量纠缠态光场.将产生的纠缠态光场注入单模光纤,其量子特性在传输距离达50 km后仍得到保持,纠缠度为0.21dB.该研究可为基于光纤的长距离连续变量量子信息研究提供有效的依据.

不同单位成本条件下库若特模型的量子化研究

使用连续变量的量子系统对不同单位成本条件下的库诺特模型进行量子化处理,构造了一个最小量子化结构方案.求出量子均衡解后对量子均衡利润的相关影响因素进行了分析,并进行数值模拟,研究结果表明:没有纠缠度时,量子博弈还原为经典博弈.当纠缠度取无限大、企业相对单位成本差取特定范围时,此量子博弈模型要优于李慧等人研究的相同成本经典库诺特模型的量子化模型.

基于散粒噪声方差实时监测的连续变量量子密钥分发系统的设计与实现

为了有效抵御窃听者对本振光的攻击,提高连续变量量子密钥分发(continuous-variable quantum key distribution,CVQKD)系统的安全性,提出了一种基于散粒噪声方差实时监测的CVQKD系统.该系统采用散粒噪声方差标定技术,在原有的CVQKD系统中加入散粒噪声方差实时监测模块,通过本振光强和散粒噪声方差的线性关系评估出实时的散粒噪声方差,再计算系统准确实时的密钥率来判断当前系统是否处于安全状态.实验上也表明了该系统能够有效抵御Eve对本振光的攻击,提高CVQKD系统的安全性.

Fock空间中基于波色双模指数二次型算符的连续变量纠缠态的量子涨落特性(英文)

利用线性量子变换理论(LQTT),导出在Fock空间中连续变量两体纠缠态的量子涨落计算的一般公式,并讨论此纠缠态的压缩特性。通过线性拟合的方法,得到当涨落度和纠缠熵分别出现极值时,态参数之间关系。结果显示当达到较大纠缠时,该态的压缩程度也较大,除此以外还得到了涨落度随参数和纠缠熵的变化关系。举例说明此公式在计算双模压缩真空态和单边双模压缩真空态的量子涨落中的应用。

自旋玻色-爱因斯坦凝聚体中多粒子的量子纠缠

通过单原子拉曼跃迁,提出了自旋为1的玻色-爱因斯坦凝聚体中多粒子的最大纠缠态的理论处理方法.并且,发现文献[1]所提出的3-模模型,经过一个简单的方法可以简化成有效的2-模模型.另外,在这个系统中,文中还提出了一种产生原子-原子间连续变量纠缠态的方案.

连续变量量子通信的研究与展望

基于量子力学原理的量子通信具有绝对安全特性,能够满足信息时代的要求,因此,受到了学术界、信息产业界以及各国政府的广泛关注。连续变量量子通信是针对离散变量量子通信中所面临的探测困难、密钥率低、缺乏简单可靠的单光子源等瓶颈而提出的一种新的量子保密通信方案。在介绍连续变量量子通信的基本原理基础上,分别介绍近年来基于压缩态、相干态以及纠缠态的连续变量量子密钥分配(CVQKD)的研究现状,讨论了目前的几类QKD协议中存在的问题以及面临的挑战。

具有实用价值的纠缠态光源的研制

为获得具有实用价值的纠缠态光源,课题组首先进行了系列高质量全固态连续单频1064 nm和532 nm激光器的研制。研制完成的全固态连续单频1 064 nm激光器的性能指标如下:输出功率达30 W,光光转换效率达37%,7 h内的输出功率波动优于±0.3%,光束质量(M2)优于1.1。同时对激光器1 064 nm/532 nm双波长输出的激光器的噪声进行了研究,其强度和位相噪声均在分析频率4 MHz处达到散粒噪声基准。而且非线性晶体还可以作为噪声抑制装置,对1 064 nm输出激光的强度噪声进行抑制,使其在低频段的驰豫振荡噪声降低40 d B。进行了光通信波段1.55μm和1.34μm全固态连续单频激光器的研制,获得了680 MW的连续单横模1.55μm激光输出以及400 m W的连续单频1.55μm激光输出,1.5 h内输出功率波动优于±1.5%,其强度噪声和位相噪声在分析频率5 MHz处均达到散粒噪声基准;获得了输出功率达9 W的连续单频1.34μm激光输出,光光转换效率达18%。激光器在4小时内的功率稳定性优于±1%,在1 min的频率漂移为8.5 MHz,输出激光的光束质量M2因子为1.03。进行了1.55μm和1.34μm光通信波段连续变量量子纠缠光源的研制,经过优化实验系统,获得了压缩度为5 d B、波长为1.55μm的压缩态光场,压缩度为6.1 d B、波长为1.3μm的压缩态光场;完成了1.08μm连续变量量子纠缠光源样机的研制,纠缠源系统的输出波长为1 080 nm,振幅和位相差起伏方差大于5 d B。设计并研制了双色可调谐连续变量纠缠态产生装置,其中,纠缠光束中的一束波长位于碱金属铷原子的吸收线波段;在实验室建立起一套基于光纤通信器件的远程连续变量量子密钥分发系统平台,实验实现了30 km的四态分离调制连续变量量子密钥分发,有效密钥速率大于1 k B/s。利用光子晶体光纤,制备了接近单模的E波段单光子,实验测得其宣布效率可达86%(修正探测效率后),强度关联函数违背经典光源极限值80倍以上。

任意多人的量子博弈

量子博弈是量子信息学最近兴起的又一分支。杜江峰等在研究连续变量量子博弈中,利用了双模压缩态实现量子纠缠并产生优于经典的结果。我们在此基础上对于有任意多个参与者并且策略为连续的量子博弈做了研究。通过利用一多模光场,该光场是由各个光模之间两两互相纠缠形成的。利用这个方案上述结论可以推广至任意多个参与者。并且,随着纠缠度的增加他们的总收益也增加,当每一对双模光场达到最大纠缠时, 他们的总收益亦将最大。

连续变量1.34 μm量子纠缠态光场的实验制备

设计研制了连续单频671 nm/1342 nm双波长激光器,并通过模式清洁器降低了激光器额外噪声.利用该低噪声连续单频激光器抽运由Ⅱ类准相位匹配晶体构成的双共振非简并光学参量放大器,实验制备出纠缠度达3 dB的光通信波段1.34μm连续变量量子纠缠态光场.该波段量子纠缠态光场在光纤中传输损耗低且相散效应小,与现有的光纤通信系统相兼容,可用于实现基于光纤的实用化连续变量量子通信.