Nonlocal advantage of quantum coherence and entanglement of two spins under intrinsic decoherence

We investigate the nonlocal advantage of quantum coherence(NAQC) and entanglement for two spins coupled via the Heisenberg interaction and under the intrinsic decoherence. Solutions of this decoherence model for the initial spin-1/2 and spin-1 maximally entangled states are obtained, based on which we calculate the NAQC and entanglement. In the weak region of magnetic field, the NAQC behaves as a damped oscillation with the time evolves, while the entanglement decays exponentially(behaves as a damped oscillation) for the spin-1/2(spin-1) case. Moreover, the decay of both the NAQC and entanglement can be suppressed significantly by tuning the magnetic field and anisotropy of the spin interaction to some decoherence-rate-determined optimal values.     

相位阻尼通道下初始纠缠对纠缠演化的影响

研究了在非惯性系中相位阻尼通道下初始纠缠对纠缠演化的影响.结果发现:与振幅阻尼通道情况不同,当态参数不同而初始纠缠相同时,纠缠演化曲线重合.当Unruh单粒子态包含左右成分时,比只包含右成分时量子消相干现象更加严重,较早地出现了纠缠死亡现象,并且纠缠死亡时刻与初始纠缠无关.     

Detecting entanglement of quantum channels

Entanglement is the crucial resource for different quantum information processing tasks. While conventional studies focus on the entanglement of bipartite or multipartite quantum states, recent works have extended the scenario to the entanglement of quantum channels, an operational quantification of the channel entanglement manipulation capability. Based on the recently proposed channel entanglement resource framework, here we study a further task of resource detection—witnessing entanglement of quantum channels. We first introduce the general framework and show how channel entanglement detection is related to the Choi state of the channel, enabling channel entanglement detection via conventional state entanglement detection methods. We also consider entanglement of multipartite quantum channels and use the stabilizer formalism to construct entanglement witnesses for circuits consisting of controlled-Z gates. We study the effectiveness of the proposed detection methods and compare their performance for several typical channels. Our work paves the way for systematic theoretical studies of channel entanglement and practical benchmarking of noisy intermediate scaled quantum devices.     

Efficient entanglement purification in quantum repeaters

We present an efficient entanglement purification protocol(EPP) with controlled-not(CNOT) gates and linear optics.With the CNOT gates,our EPP can reach a higher fidelity than the conventional one.Moreover,it does not require the fidelity of the initial mixed state to satisfy F > 1/2.If the initial state is not entangled,it still can be purified.With the linear optics,this protocol can get pure maximally entangled pairs with some probabilities.Meanwhile,it can be used to purify the entanglement between the atomic ensembles in distant locations.This protocol may be useful in long-distance quantum communication.     

Dzyaloshinskii-Moriya相互作用和內禀退相干对三粒子XXZ海森堡系统的量子纠缠的影响

根据Milburn理论,研究了考虑内禀退相干情况下,三粒子XXZ海森堡系统在DzyaloshinskiiMoriya(DM)相互作用和各向异性参数的影响下的量子纠缠演化特性.分析了不同DM相互作用和內禀退相干因子等参数对量子对纠缠度的影响.研究表明:系统的对纠缠度与各向异性参数Δ无关,但DM相互作用和內禀退相干都对系统的对纠缠都有明显的影响.当无DM相互作用只存在內禀退相干时,系统的三对对纠缠度各不相同;引入DM相互作用后,系统的对纠缠度的稳定值将改变;选择合适的DM相互作用参数时,系统的对纠缠度随时间做震荡,随着时间的延长,震荡减弱至一个非零的稳定值;并且系统的对纠缠随內禀退相干增大,震荡幅度变小,震荡时间变短.因此,在內禀退相干存在时,合适的DM相互作用参数可以有效的控制对纠缠.     

Dzyaloshinskii-Moriya相互作用和內禀退相干对三粒子XXZ海森堡系统的量子纠缠的影响

根据Milburn理论, 研究了考虑内禀退相干情况下, 三粒子XXZ海森堡系统在Dzyaloshinskii-Moriya(DM)相互作用和各向异性参数的影响下的量子纠缠演化特性。 分析了不同DM相互作用和內禀退相干因子等参数对量子对纠缠度的影响。 研究表明： 系统的对纠缠度与各向异性参数Δ无关, 但DM相互作用和內禀退相干都对系统的对纠缠都有明显的影响。 当无DM相互作用只存在內禀退相干时, 系统的三对对纠缠度各不相同; 引入DM相互作用后, 系统的对纠缠度的稳定值将改变; 选择合适的DM相互作用参数时, 系统的对纠缠度随时间做震荡, 随着时间的延长, 震荡减弱至一个非零的稳定值; 并且系统的对纠缠随內禀退相干增大, 震荡幅度变小, 震荡时间变短。 因此, 在內禀退相干存在时, 合适的DM相互作用参数可以有效的控制对纠缠。     

Efficient entanglement purification in quantum repeaters

We present an efficient entanglement purification protocol (EPP) with controlled-not (CNOT) gates and linear optics. With the CNOT gates, our EPP can reach a higher fidelity than the conventional one. Moreover, it does not require the fidelity of the initial mixed state to satisfy F>1/2. If the initial state is not entangled, it still can be purified. With the linear optics, this protocol can get pure maximally entangled pairs with some probabilities. Meanwhile, it can be used to purify the entanglement between the atomic ensembles in distant locations. This protocol may be useful in long-distance quantum communication.     

腔量子电动力学系统中耦合三原子的纠缠特性

研究了三个全同二能级原子与单模腔相互作用系统中原子间的三体纠缠特性.考虑原子间存在相互耦合,并且腔场处于弱相干态的情况,通过数值计算给出了纠缠量的演化曲线,讨论了原子间耦合强度和弱相干场强度对三体纠缠的影响.研究结果表明:随弱相干场强度增强,原子间的三体纠缠增强;相反,随原子间耦合系数增大,原子间三体纠缠减弱.     

Quantum control of two interacting electrons in a coupled quantum dot

Quantum-state engineering, i.e. active manipulation over the coherent dynamics of suitable quantum-mechanical systems, has become a fascinating prospect of modern physics. Here we discuss the dynamics of two interacting electrons in a coupled quantum dot driven by an external electric field. The results show that the two quantum dots can be used to prepare a maximally entangled Bell state by changing the strength and duration of an oscillatory electric field. Different from the suggestion made by Loss \textit et al (1998 Phys. Rev. A 57 120, the present entanglement involves the spatial degree of freedom for the two electrons. We also find that the coherent tunnelling suppression discussed by Grossmann \textit et al (1991 Phys. Rev. Lett. 67 516 persists in the two-particle case: i.e. two electrons initially localized in one dot can remain dynamically localized, although the strong Coulomb repulsion prevents them from behaving so. Surprisingly, the interaction enhances the degree of localization to a large extent compared with that in the non-interacting case. This phenomenon is referred to as the Coulomb-enhanced dynamical localization.     

粒子数光场中原子纠缠的演化特性

两个初始时纠缠的原子(原子1、2)之一(原子2)和另一分离原子(原子3)在单模腔中与粒子数场发生相互作用,研究了这一系统中原子纠缠的演化特性.利用部分转置矩阵负本征值的方法,得出了纠缠度与初始时原子的状态、粒子数光场中光子数目、原子与腔场偶极相互作用的耦合常数、原子间偶极-偶极相互作用强度等的大小有很强的关系.     

多个量子节点确定性纠缠的建立

量子纠缠是一种重要的量子资源,在多个空间分离的量子存储器间建立确定性的量子纠缠,然后在用户控制的时刻将所存储的量子纠缠转移到量子信道中进行信息的分发和传送,这对于实现量子信息网络是至关重要的.本文介绍了用光学参量放大器制备与铷原子D1吸收线对应的非经典光场,而且在三个空间分离的原子系综中确定性量子纠缠的产生、存储和转移.利用电磁感应透明光和原子相互作用的原理,将制备的多组分光场纠缠态模式映射到三个远距离的原子系综以建立原子自旋波之间的纠缠.然后,存储在原子系综中的纠缠态通过三个量子通道,纠缠态的量子噪声被转移到三束空间分离的正交纠缠光场.三束释放的光场间纠缠的存在验证了该系统具有保持多组分纠缠的能力.这个方案实现了三个量子节点间的纠缠,并且可以直接扩展到具有更多节点的量子网络,为未来实现大型量子网络通信奠定了基础.     

Experimental detection of quantum entanglement

In this review, we introduce some methods for detecting or measuring entanglement. Several nonlinear entanglement witnesses are presented. We derive a series of Bell inequalities whose maximally violations for any multipartite qubit states can be calculated by using our formulas. Both the nonlinear entanglement witnesses and the Bell inequalities can be operated experimentally. Thus they supply an effective way for detecting entanglement. We also introduce some experimental methods to measure the entanglement of formation, and the lower bound of the convex-roof extension of negativity.     

Complete entanglement analysis on electron spins using quantum dot and microcavity coupled system

We present an entanglement analysis protocol on entangled electron spins using quantum dot (QD) and microcavity coupled system. Each quantum dot is placed in the microcavity and ancilla photon input-output process could be used to check the parity of the quantum dots. After the parity check process, the user only needs to measure the spin direction of the QD spin, and the state information can be readout completely. The feasibility of our scheme and the experimental challenge are discussed by considering currently available techniques.     

Influences of spin–orbit interaction on quantum speed limit and entanglement of spin qubits in coupled quantum dots

Quantum speed limit and entanglement of a two-spin Heisenberg XYZ system in an inhomogeneous external magnetic field are investigated. The physical system studied is the excess electron spin in two adjacent quantum dots. The influences of magnetic field inhomogeneity as well as spin–orbit coupling are studied. Moreover, the spin interaction with surrounding magnetic environment is investigated as a non-Markovian process. The spin–orbit interaction provides two important features: the formation of entanglement when two qubits are initially in a separated state and the degradation and rebirth of the entanglement.     

在有限温度下运用弱测量保护量子纠缠

根据单量子比特的映射, 提出一个在有限温度下运用弱测量保护两量子比特纠缠的方案. 在两个可通过局域幺正变化实现的等价的初始纠缠进入广义振幅阻尼信道前后, 对其分别进行弱测量, 并对四个弱测量的参数做全面的优化, 获取最大共生纠缠C_r 和弱测量参数m, n的解析表达式, 然后再进一步研究弱测量参数与信道参数的关系. 发现这种基于弱测量的纠缠保护方法在某些情况下可以有效地提高纠缠, 甚至可以避免纠缠的突然死亡. 当信道参数r一定时, 对不同的参数p,初始态|ψ>纠缠达到最大值时对应的弱测量参数m的取值一样, 共生纠缠关于p=0.5对称, 而初始态|φ>对应的参数m的取值不同; 在参数p一定、参数r不同、初始态|ψ>或|φ>的情况下, 当纠缠度取最大值时, 弱测量参数m的取值不变, 且随着r的增加, 纠缠度减少. 通过对信道参数的分析, 发现可以选择合适的信道参数和初始态来获得较大的纠缠.     

解说量子纠缠理论

量子纠缠态在量子信息处理,如量子隐形传态、量子密集码、量子纠错、量子保密通信、量子计算等过程中起了十分重要的作用.量子纠缠理论主要研究量子态的纠缠刻画、分类及其在量子信息处理中的应用.文章介绍了量子纠缠理论中的一些基本概念和结果,其中包括：量子力学的实在性、局域性的讨论与Bell不等式的联系,Bell不等式与量子态可分性间的关系;纯态和混合态可分性的定义及若干判别准则(包括矩阵正映照方法、部分转置判据、约化判据、重排判据、纠缠见证、协方差判据及局域测不准关系判据);部分纠缠度量的介绍(包括纠缠形成、并发度、相对熵、负度、缠结和纠缠帮助,以及纠缠度量的计算和上下界的估算).     

Quantum entanglement in a two-electron quantum dot in magnetic field

The properties of quantum entanglement of the ground state in an exactly solvable model of a two-electron QD have been investigated. It is shown that the degree of entanglement increases with enhancement of interaction between electrons, irrespective of the shape of electron confining potential in a QD. A magnetic field destroys electron entanglement. However, the entanglement in deformed QDs is more stable against magnetic field.     

远距离量子通信实验中的高维纠缠源

设计了适用于远距离量子通信实验的高维纠缠源.利用连续激光器抽运产生了极化-时间两体四维纠缠光子对,在抽运功率20 mW下测到每秒700对符合,保真度为89%±3%.相比已有的高维纠缠源,在本文中发展的源具有传输便利、相位稳定性好等优点,适用于未来远距离高维量子通信实验和量子力学基本问题实验检验,如远距离高维量子密码实验、两粒子Greenberger-Horne-Zeilinger定理检验、两粒子量子赝"心灵感应"(quantum pseudo telepathy)实验演示等. 关键词： 高维纠缠 极化纠缠 时间箱纠缠 量子通信     

Universal quantum gates between distant quantum dot spins

We propose a scheme to engineer a nonlocal two-qubit phase gate between two remote quantum-dot spins in cavities. As the effect of cavity decay is considered and strong coupling condition is not used, the scheme has good adaptability. Along with single qubit operations, one can perform in principle various types of distributed quantum information processing.