Intrinsically stable phase-modulated polarization encoding system for quantum key distribution

We demonstrate experimentally an intrinsically stable polarization coding and decoding system composed of optical-fiber Sagnac interferometers with integrated phase modulators for quantum key distribution. An interference visibility of 98.35% can be kept longtime during the experiment without any efforts of active compensation for coding all four desired polarization states.     

Quantum cryptography using partially entangled states

We show that non-maximally entangled states can be used to build a quantum key distribution (QKD) scheme where the key is probabilistically teleported from Alice to Bob. This probabilistic aspect of the protocol ensures the security of the key without the need of non-orthogonal states to encode it, in contrast to other QKD schemes. Also, the security and key transmission rate of the present protocol is nearly equivalent to those of standard QKD schemes and these aspects can be controlled by properly harnessing the new free parameter in the present proposal, namely, the degree of partial entanglement. Furthermore, we discuss how to build a controlled QKD scheme, also based on partially entangled states, where a third party can decide whether or not Alice and Bob are allowed to share a key.     

Experimental Study on Practical Quantum Key Distribution Scheme with Time-Multiplexed Technique

We present a scheme that is capable of detecting photon numbers during the quantum key distribution (QKD) based on an improved differential phase shift (DPS) system without Trojan horse attack. A time-multiplexed detector (TMD) is set in for the photon-number resolution. Two fibre loops are used for detecting photon numbers as well as distributing keys. The long-term stabilization is guaranteed by two Faraday mirrors (FM) at Bob's site to automatically compensate for polarization defect. Our experimental study (90km QKD is completed) indicates that such a system is stable and secure which nearly reaches the performance of a single photon scheme.     

A linear optical protocol of random quantum key distribution by using polarization entangled photons

Utilized polarization entangled photons, a linear optical protocol for generating random quantum key distribution (QKD) is proposed, which is made up of EPR-source, linear optical elements, and conventional photon detectors. It is shown that total efficiency of QKD η=100% in theory.     

Experimental Decoy State Quantum Key Distribution Over 120km Fibre

Decoy state quantum key distribution （QKD）, being capable of beating PNS attack and being unconditionally secure has become attractive recently. However, in many QKD systems, disturbances of transmission channel make the quantum bit error rate （QBER） increase, which limits both security distance and key bit rate of real-world decoy state QKD systems. We demonstrate the two-intensity decoy QKD with a one-way Faraday- Michelson phase modulation system, which is free of channel disturbance and keeps an interference fringe visibility （99%） long period, over a 120 km single mode optical fibre in telecom （1550nm） wavelength. This is the longest distance fibre decoy state QKD system based on the two-intensity protocol.     

Cryptanalysis of Multiparty Quantum Secret Sharing of Quantum State Using Entangled States

Security of a quantum secret sharing of quantum state protocol proposed by Guo et al. [Chin. Phys. Lett. 25 （2008） 16] is reexamined. It is shown that an eavesdropper can obtain some of the transmitted secret information by monitoring the classical channel or the entire secret by intercepting the quantum states, and moreover, the eavesdropper can even maliciously replace the secret message with an arbitrary message without being detected. Finally, the deep reasons why an eavesdropper can attack this protocol are discussed and the modified protocol is presented to amend the security loopholes.     

基于相位调制偏振态QKD系统的量子信道校正发送方案

提出了一种基于相位调制偏振态QKD系统的量子信道校正发送的新方案—采用波导型相位调制器研制成电控连续可调光学相移器,它可随相位调制器输入电压的连续改变而产生连续的相移,从而控制输出的偏振态;在其前放置一个半波片,即可校正接收端偏振态量子态在信道中所发生的改变,从而保证信道传送密钥的可用性.通过理论推导和实验研究验证了基于相位调制偏振态QKD系统的量子信道校正发送方案的核心部件的可行性.由于本方案能实现高速调制（GHz）,为解决光纤传输的偏振编码QKD系统中偏振态漂移问题提供了一种新的途径.     

偏振稳定控制下的量子密钥分发

由于长距离单模光纤传输中存在的双折射效应会引起偏振随机抖动，光纤中利用偏振编码进行量子密钥分发一直难以实现.利用光子计数分析光纤中的偏振变化情况，并通过反馈控制的方式补偿偏振变化，从而实现了基于BB84协议的偏振编码长时间稳定的量子密钥分发实验，传输距离为100km. 关键词： 量子密钥分发 偏振反馈控制 单光子探测 偏振随机抖动     

Scalable Quantum Secret Sharing Extended from Quantum Key Distribution

We suggest a general approach for extending quantum key distribution （Q, KD） protocols possessing discrete rotational symmetry into quantum secret sharing （QSS） schemes among multiparty, under certain conditions. Only local unitary operations are required for this generalization based on the almost mature technologies of Q, KD. Theoretically, the number of the participating partners can be arbitrary high. As an application of this method, we propose a fault-tolerant QSS protocol based on a fault-tolerant QKD implementation. The 6-state protocol is also discussed.     

Schemes for preparing atomic qubit cluster states in cavity QED

Two schemes are proposed for generating atomic qubits cluster states in cavity quantum electrodynamics (QED). In the first scheme, only two-atom-cavity interactions are involved, and cluster states can be directly generated by using constructed two-qubit controlled phase gates. The second scheme needs the assistance of additional single-qubit rotations, but takes less time than the first one for two-atom operations in the cavity. In this scheme, two projective operators are constructed to prepare two-dimension or more complicated configurations of cluster states. Both schemes are insensitive to the cavity decay due to the fact that the cavity is only virtually excited during the interaction between atoms and the cavity. The idea can also be applied to the ion trap system.     

Realization of Three-Qubit Controlled-Phase Gate Operation with Atoms in Cavity QED System

We propose a scheme for realization of three-qubit controlled-phase gate via passing two three-level atoms through a high-Q optical cavity in a cavity QED system. In the presented protocol, the two stable ground states of the atoms act as the two controlling qubits and the zero- and one-photon Fock states of the cavity-field form the target qubit, and no auxiliary state or any measurement is required. The numerical simulation shows that the gate fidelities remain at a high level under the influence of the atomic spontaneous emission, the decay of the cavity mode and deviation of the coupling strength. The experimental feasibility of our proposal is also discussed.     

Optical Realization of Deterministic Entanglement Concentration of Polarized Photons

We propose a scheme for optical realization of deterministic entanglement concentration of polarized photons. To overcome the difficulty due to the lack of sufficiently strong interactions between photons, teleportation is employed to transfer the polarization states of two photons onto the path and polarization states of a third photon, which is made possible by the recent experimental realization of the deterministic and complete Bell state measurement. Then the required positive operator-valued measurement and further operations can be implemented deterministically by using a linear optical setup. All these are within the reach of current technology.     

Analysis of polarization state in quantum key distribution via single-photon two-qubit states

An improved quantum key distribution scheme via single-photon two-qubit states is proposed. The input–output model of the polarization state is established. And the influence of the interferometers to the polarization state is analyzed. Quantum bit error rate of polarization coding caused by birefringent and coordinate system difference between incident light and the fast and slow axes in fiber interferometer is simulated. Furthermore, maintaining conditions of polarization state are given on this basis.     

相位调制自由空间量子密钥分配

通过一组正对的望远镜耦合，采用以不等臂Mach-Zehnder干涉仪为基础的相位编码方式，成功地实现了实验室内12m距离的自由空间量子密钥分配. 实验表明这样的系统工作稳定可靠，与偏振编码一样，相位编码也可用于自由空间的量子密钥分配. 关键词： 不等臂Mach-Zehnder干涉仪 量子密钥     

Maximum efficiency of a linear-optical Bell-state analyzer

In a photonic realization of qubits the implementation of quantum logic is rather difficult due to the extremely weak interaction on the few photon level. On the other hand, in these systems interference is available to process the quantum states. We formalize the use of interference by the definition of a simple class of operations which include linear-optical elements, auxiliary states and conditional operations. We investigate an important subclass of these tools, namely linear-optical elements and auxiliary modes in the vacuum state. For these tools, we are able to extend a previous qualitative result, a no-go theorem for perfect Bell-state analyzer on two qubits in polarization entanglement, by a quantitative statement. We show that within this subclass it is not possible to discriminate unambiguously four equiprobable Bell states with a probability higher than 50%. Received: 18 July 2000 / Revised version: 15 September 2000 / Published online: 6 December 2000     

Interacting spin pairs in rotational magnetic fields and geometric phase

In virtue of the quantum invariant theory, we obtain the rigorous solution of the isotropic bipartite system in rotational magnetic fields, based on which the general expression of the noncyclic geometric phase is worked out and the entanglement dependence of the noncyclic geometric phase in this model is investigated. We show that the influence of the coupling on noncyclic geometric phase depends on the initial condition of the system. We also show that when the magnetic fields are stationary, there is a more general class of states existed of which the noncyclic geometric phase could be interpreted solely in terms of the solid angle enclosed by the geodesically closed curve on a two-sphere parameterized by the evolving Schmidt coefficients.     

Quantum Cryptography in Spin Networks

In this paper we propose a new scheme of long-distance quantum cryptography based on spin networks with qubits stored in electron spins of quantum dots. By＂ conditional Faraday- rotation, single photon polarization measurement, and quantum state transfer, maximal-entangled Bell states for quantum cryptography between two long-distance parties are created. Meanwhile, efficient quantum state transfer over arbitrary＂ distances is obtained in a spin chain by＂ a proper choice of coupling strengths and using spin memory- technique improved. We also analyse the security＂ of the scheme against the cloning-based attack which can be also implemented in spin network and discover that this spin network cloning coincides with the optimal fidelity- achieved by＂ an eavesdropper for entanglement-based cryptography.     

联合调制量子密钥分配系统

本文提出了一种对每一个单光子信号进行相位和偏振两种 编码调制的联合调制量子密钥分配(QKD)系统. 结合复合QKD系统的双速协议, 本文给出了在理想情形下可以通过一个信号光子生成两比特密钥的QKD协议, 明显提高了QKD协议的内禀光子利用率. 在稳定性方面, 本文发展了联合调制的Michelson型QKD系统, 从而在原理上解决了联合调制QKD系统的稳定性问题. 关键词： 量子密钥分配 双速协议 联合调制 量子密钥分配系统的稳定性     

Demonstration of Quantum Entanglement Control Using Nuclear Magnetic Resonance

With the two forms of the quantum entanglement control, the quantum entanglement swapping and preservation are demonstrated in a three-qubit nuclear magnetic resonance quantum computer. The pseudopure state is prepared to represent the quantum entangled states through macroscopic signals. Entanglement swapping is directly realized by a swap operation. By controlling the interactions between the system and its environment,we can preserve an initial entangled state for a longer time. The experimental results are in agreement with the experiment.     

Quantum key distribution by comparing Bell states

We propose a quantum key distribution (QKD) scheme based on entanglement swapping. In this scheme, the methods to form secret keys are so interesting. By comparing initial Bell state and outcome of entanglement swapping, the secret keys between Alice and Bob are generated involuntarily.