基于线性光学的多通道混合纠缠态

本文提出了一种基于线性光学的多通道混合纠缠态方案。最初的混合纠缠态是在连续变量猫态和分离变量单光子量子比特之间产生。该方案基于线性光学元件,利用分束器在初始混合纠缠态基础上产生多通道混合纠缠态(M组)。在此基础上,对每对混合纠缠态相对初始混合纠缠态的保真度进行了计算,并分析了保真度随所分份数多少及猫态大小的变化关系。这种多通道混合纠缠态将会为涉及分离变量与连续变量的多通道量子信息传输及存储、量子网络节点信息分布等提供重要的资源。     

Teleportation of three-dimensional single particle state in noninertial frames

Considering the spin degree of freedom of the Dirac field, we study the entanglement behavior of a different class of communication channel and teleportation of three-dimensional single particle state in noninertial frames. Numerical analysis shows that the communication channel in our scheme can offer enough distillable entanglement for the teleportation protocol. Moreover, the teleportation protocol could work well if Rob＇s acceleration is not very big, but the fidelity of the teleportation is still reduced due to the Unruh effect.     

多方控制的量子安全直接通信协议

基于单光子序列的顺序重排，提出了一种可应用于一些特殊的场景的多方控制的量子安全直接通信协议.协议中，接收方只有在得到所有控制方的同意之后，才能恢复出发送方的秘密消息.协议的安全性由量子不可克隆定理和单光子序列的秘密传输顺序所保证.此外，除了用于窃听检测的部分光子，所有的光子都用于编码秘密消息，而且协议的实现不需要使用纠缠态，该协议具有效率高和实现简单等特点. 关键词： 量子密码 量子安全直接通信 顺序重排 单光子     

Information transfer using a single particle path-spin hybrid entangled state

The path-spin entangled state of a single spin-1/2 particle is considered which is generated by using a beam-spitter and a spin-flipper. Using this hybrid entanglement at the level of a single particle as a resource, we formulate a protocol for transferring of the state of an unknown qubit to a distant location. Our scheme is implemented by a sequence of unitary operations along with suitable spin-measurements, as well as by using classical communication between the two spatially separated parties. This protocol, thus, demonstrates the possibility of using intraparticle entanglement as a physical resource for performing information theoretic tasks.     

Quantum communications with an anonymous receiver

A new protocol for the anonymous communication of quantum information is proposed. The anonymity of the receiver and the privacy of the quantum information are perfectly protected except with exponentially small probability in this protocol. Furthermore, this protocol uses single photons to construct anonymous entanglement instead of multipartite entangled states, and therefore it reduces quantum resources compared with the pioneering work.     

An Efficient Scheme for Entanglement Concentration of Arbitrary Four-Photon States

The maximally entanglement will become less-entangled state because of the effects of decoherence. However, maximally entangled states are usually important for most of the quantum information processing protocols. So we propose a practical entanglement concentration protocol of an arbitrary four-photon less-entangled state assisted with single photons, which greatly reduces the difficulty of the experimental realization. In our scheme, we only employ three steps, a four-photon cluster state can be acquired with a certain probability by three affiliated photons. The protocol adopts the linear optical elements and weak cross-Kerr media, which make the scheme more simple and feasible in the experiment.     

Optimal Entanglement Concentration of the Greenberger-Horne-Zeilinger States in Quantum-dot and Micro-cavity Coupled System

The distillation of the triplet Greenberger-Horne-Zeilinger (GHZ) state is demonstrated by using the entanglement concentrating process for the partially electron-spin-entangled systems. We designate an entanglement concentration protocol (ECP) in the quantum-dot (QD) and micro-cavity coupled systems based on the post-selection, from which the partially entangled state can be concentrated with an aid of the ancillary QD and single photon. This protocol can be repeated several rounds to get an optimal success probability. With the current technology, the maximally entangled electron spins can be achieved in the GHZ states after performing some suitable unitary operation locally for the long-distance quantum communications. The advantage is that during the whole process only the single photon needs to pass through the micro-cavity which increases the total success probability even if the cavity is imperfect in implementations.     

A Three-Qubit State Entanglement Concentration Protocol Assisted by Two-Qubit Systems

In this paper we present an entanglement concentration protocol for enhancement of the amount of entanglement maximally in a three qubit non-maximally entangled state. We use a Bell state for this purpose. Here the speciality is that no non-local measurement involving more than one parties is involved in the protocol. It is shown that for obtaining best probability of success a maximally entangled Bell state must be used. The probability of success in our protocol increases with an increase in the amount of entanglement in the assisting Bell state, and is zero when the entanglement vanishes.     

Gaussian transformations and distillation of entangled Gaussian states

We prove that it is impossible to distill more entanglement from a single copy of a two-mode bipartite entangled Gaussian state via local Gaussian operations and classical communication. More generally, we show that any hypothetical distillation protocol for Gaussian states involving only Gaussian operations would be a deterministic protocol. Finally, we argue that the protocol considered by Eisert et al. [preceding Letter, Phys. Rev. Lett. 89, 137903 ()]] is the optimum Gaussian distillation protocol for two copies of entangled Gaussian states.     

A scheme of quantum repeaters with single atom and cavity-QED

To implement long-distance quantum communication, quantum repeaters have been proposed. The distribution and storage of quantum entanglement are essential to implement quantum repeaters. Here, we propose a new quantum repeaters protocol which is based on single atom-cavity QED. We use simple long-life two-level atoms to store quantum entanglement unlike three-level atoms which are commonly used in other quantum repeaters proposals. The property of long life-time (T₁) and transverse decay time (T₂) between excited level and ground level, such as rare-earth atoms, may store quantum entanglement as long as possible. Modulations of cavity mode and rate of coupling between cavity mode and output mode are also key steps to our scheme. And the efficiency of our protocol is analyzed by quantum trajectory theory.     

On Extension of Optimal Entanglement Concentration of GHZ States in Quantum-Dot and Micro-Cavity Coupled System

We extend an optimal entanglement distillation of the triplet Greenberger-Horne-Zeilinger （GHZ） state via entanglement concentrating in the three-partite partially electron-spin-entangled systems. Two entanglement concen- tration protocols are similarly designed in detail with the post-selection in quantum-dot （QD） and micro-cavity coupled systems. The proposed protocol can be repeated several rounds to achieve an optimal success probability with an as- sistance of the ancillary QD, where only the single photon needs to pass through the micro-cavity for each round. It increases the total success probability of the distillation even if the implemented cavity is imperfect in practice during the whole process.     

基于分组交换的量子通信网络传输协议及性能分析

量子纠缠交换能够建立可靠的量子远程传输信道, 实现量子态的远程传输. 然而, 基于纠缠交换的量子信道要求网络高度稳定, 否则会浪费大量纠缠资源. 为节省纠缠资源, 本文根据隐形传态理论, 提出了一种基于分组交换的量子通信网络传输协议, 建立了发送量子态所需的纠缠数目与所经过的路由器数、链路错误率的定量关系, 并与纠缠交换传输协议进行了比较. 仿真结果表明, 在链路错误率为0.1% 时, 分组传输协议所使用的纠缠数目少于纠缠交换的数目, 另外, 随着错误率的升高, 分组传输协议所需的纠缠数比纠缠交换协议明显减少. 由此可见, 基于分组交换的量子通信网络传输协议在网络不稳定时, 能够节省大量纠缠资源, 适用于链路不稳定的量子通信网络.     

基于纠缠的数据链路层量子通信协议

利用量子力学中的纠缠关联性，提出了数据链路层的量子通信协议. 该协议把线路分为忙闲时段，量子纠缠态的分发预先在闲时段完成，数据经由经典链路发送，确认帧经由量子纠缠信道发送. 量子信息的传输可以是瞬时的，因此两个发送成功的数据帧之间的最小时间间隔可以大为减小. 研究表明该协议能有效地提高数据链路的最大吞吐量，改善数据链路层停止等待协议的性能. 关键词： 量子通信 纠缠 停止等待协议 数据链路     

Detecting entanglement by the mean value of spin on a quantum computer

We implement a protocol to determine the degree of entanglement between a qubit and the rest of the system on a quantum computer. The protocol is based on results obtained in paper [Frydryszak et al. (2017) [23]]. This protocol is tested on a 5-qubit superconducting quantum processor called ibmq-ourense provided by the IBM company. We determine the values of entanglement of the Schrödinger cat and the Werner states prepared on this device and compare them with the theoretical ones. In addition, a protocol for determining the entanglement of rank-2 mixed states is proposed. We apply this protocol to the mixed state which consists of two Bell states prepared on the ibmq-ourense quantum device.     

Entanglement as a precondition for secure quantum key distribution

We demonstrate that a necessary precondition for an unconditionally secure quantum key distribution is that both sender and receiver can use the available measurement results to prove the presence of entanglement in a quantum state that is effectively distributed between them. One can thus systematically search for entanglement using the class of entanglement witness operators that can be constructed from the observed data. We apply such analysis to two well-known quantum key distribution protocols, namely, the 4-state protocol and the 6-state protocol. As a special case, we show that, for some asymmetric error patterns, the presence of entanglement can be proven even for error rates above 25% (4-state protocol) and 33% (6-state protocol).     

Reliability of Digitized Quantum Annealing and the Decay of Entanglement

A banged‐digital‐analog simulation of a quantum annealing protocol is performed in a two‐qubit nuclear magnetic resonance (NMR) quantum computer. The experimental simulation employs up to 235 Trotter steps, with more than 2000 gates (pulses), and a protocol success above 80% is obtained. Given the exquisite control of the NMR quantum computer, the simulation is also performed with different noise levels. Thus, the reliability of the quantum annealing process is is analyzed, and it is related to the level of entanglement produced during the protocol. Although the presence of entanglement is not a sufficient signature for a better‐than‐classical simulation, the level of entanglement achieved relates to the fidelity of the protocol.     

Quantum Private Communication with an Anonymous Sender

We propose a new quantum private communication protocol, in which the anonymity of the sender and the privacy of the quantum information are perfectly protected except with exponentially small probability. Furthermore, this protocol uses single particles to construct anonymous entanglement instead of multipartite entangled states, and thus it reduces quantum resources compared with the previous work.     

Superdense coding with single-particle entanglement

Recent work has explored the idea that nonlocality or entanglement involving a single particle should be taken seriously and has real measurable consequences. Theoretical and experimental schemes have shown, for example, that single-particle states can violate Bell’s inequalities. Here we discuss the possibility of using single-particle entanglement for implementing a superdense coding protocol. Particle-number superselection rules restrict this scheme to being able to transmit log₂(3) bits of information. While this falls short of the two-particle limit of two bits, it still exceeds what can be achieved without entanglement.     

小世界网络中的量子纠缠渗流

本文研究了小世界复杂网络中的长程量子纠缠态的制备。提出了量子纠缠渗流优化网络量子通信的方案。通过在经典纠缠渗流协议(CEP)中引入局域化的"target-swap"量子操作,形成量子纠缠渗流协议(QEP)。研究表明QEP在长程纠缠制备的效能方面明显优于CEP,通过度分布变化的分析解释了QEP具有优势的原因。     

Controlled Simultaneous Teleportation and Dense Coding

We propose a teleportation protocol and a dense coding protocol. In these protocols, one sender intends to send quantum states or classical bits to two receivers through two GHZ entanglement channels. The entanglement channels are locked by the sender and the communication process is supervised by a controller. In order to obtain the messages, the receivers need the controller’s permission, and must collaborate to unlock the entanglement channels.