Preparation of Arbitrary Four-Qubit W State with Atomic Ensembles via Rydberg Blockade

The generation of various entangled states is an essential task in quantum information processing. Recently, a scheme （PRA 79, 022304） has been suggested for generating Greenberger-Horne-Zeilinger state and cluster state with atomic ensembles based on the Rydberg blockade. Using similar resources as the earlier scheme, here we propose an experimentally feasible scheme of preparing arbitrary four-qubit W class of maximally and non- maximally entangled states with atomic ensembles in a single step. Moreover, we carefully analyze the realistic noises and predict that four-qubit W states can be produced with high fidelity （F - 0.994） via our scheme.     

Generation of multi-atom W states via Raman transitionin an optical cavity

A simple scheme is proposed to generate the W state of N Λ-type neutral atoms trapped in an optical cavity via Raman transition. Conditional on no photon leakage from the cavity, the N-qubit W state can be prepared perfectly by turning on a classical coupling field for an appropriate time. Compared with the previous ones, our scheme requires neither individual laser addressing of the atoms, nor demand for controlling N atoms to go through an optical cavity simultaneously with a constant velocity. We investigate the influence of cavity decay using the quantum jump approach and show that the preparation time decreases and the success probability increases with atom number because of a collective enhancement of the coupling.     

“All versus Nothing” Inseparability for 2n Observers

We present an “all versus nothing” to show Bell‘s theorem without inequalities involving 2n observers. As a by-product, we also shows that quantum mechanics can violate Bell‘s inequality by a constant instead of by an amount that grows exponentially with 2n.     

利用V型三能级原子与单模光场的非共振相互作用制备压缩相干态的叠加态

介绍了单模压缩相干态及其基本性质,根据简并V型三能级原子与单模光场的改进型有效哈密顿量,通过矩阵方法推导出原子-光场系统的态矢量.提出利用简并V型三能级原子与单模光场的非共振相互作用制备压缩相干态的叠加态的一种新方案,讨论了怎样进行控制单模压缩相干态光场中的压缩方向问题.     

General Theory of Decoy-State Quantum Cryptography with Dark Count Rate Fluctuation

The existing theory of decoy-state quantum cryptography assumes that the dark count rate is a constant, but in practice there exists fluctuation. We develop a new scheme of the decoy state, achieve a more practical key generation rate in the presence of fluctuation of the dark count rate, and compare the result with the result of the decoy-state without fluctuation. It is found that the key generation rate and maximal secure distance will be decreased under the influence of the fluctuation of the dark count rate.     

Polarization Encoded Quantum Key Distribution over Special Optical Fibres

Employing a polarization compensator, an optical fibre quantum key distribution （QKD） system based on polarization coding has been developed. To obtain the compensator setting parameters, the measurement of the laser pulse polarization is performed with one single photon detector. We obtain a sifted key bit rate of about 2kbits/s and a qubit error rate lower than 10% within 3.5h. It is shown that polarization coding can be used for QKD over optical fibres as well. At the same time, the system is simple, easy to operate, practical and user-friendly. It gains more advantages than other systems over optical fibres when used in local area quantum communications and where the functional agility is important.     

V型三能级原子与双模光场相互作用的演化及Schrodinger猫态的制备

详细研究了一个V型三能级原子与双模光场的相互作用.结果表明:在原子与场的失谐量较大的情况下,基态能级可被绝热消除,从而把这一模型简化为一个等效的二能级原子与双模场相互作用的非简并Raman耦合模型,分别得出了非简并Raman耦合模型和简并Raman耦合模型系统的改进型有效哈密顿量和波函数.在此基础上通过对腔中原子进行选态测量,提出一种由具有可控权重因子和位相的相干叠加态组成的制备Schrdinger猫态的新方案.     

Security analysis on some experimental quantum key distribution systems with imperfect optical and electrical devices

In general, quantum key distribution （QKD） has been proved unconditionally secure for perfect devices due to quantum uncertainty principle, quantum noneloning theorem and quantum nondividing principle which means that a quantum cannot be divided further. However, the practical optical and electrical devices used in the system are imperfect, which can be exploited by the eavesdropper to partially or totally spy the secret key between the legitimate parties. In this article, we first briefly review the recent work on quantum hacking on some experimental QKD systems with respect to imperfect devices carried out internationally, then we will present our recent hacking works in details, including passive faraday mirror attack, partially random phase attack, wavelength-selected photon-number-splitting attack, frequency shift attack, and single-photon-detector attack. Those quantum attack reminds people to improve the security existed in practical QKD systems due to imperfect devices by simply adding countermeasure or adopting a totally different protocol such as measurement-device independent protocol to avoid quantum hacking on the imperfection of measurement devices [Lo, et al., Phys. Rev. Lett., 2012, 108： 130503].     

光量子随机数发生器

设计制作了基于单光子量子随机性的二进制随机数发生器,通过50/50光分束器的光子流被其后的单光子探测器记录下光子路径,经高速电路和计算机数据采集系统转换为二进制随机数流,目前实验中随机数采集速率达100 Kbit/s,本文采用延迟符合计数法来克服探测器暗计数的影响.使暗计数对随机数序列的影响下降了2到3个数量级,最后应用的三种随机性测试表明所采数据随机性良好.     

Secure deterministic communication in a quantum loss channel using quantum error correction code

The loss of a quantum channel leads to an irretrievable particle loss as well as information. In this paper, the loss of quantum channel is analysed and a method is put forward to recover the particle and information loss effectively using universal quantum error correction. Then a secure direct communication scheme is proposed, such that in a loss channel the information that an eavesdropper can obtain would be limited to arbitrarily small when the code is properly chosen and the correction operation is properly arranged.