基于免退纠缠态的原子态隐形传输

量子隐形传态易受噪声环境的影响。为了减小这种不利影响,基于振幅衰减噪声模型,提出一种基于免退纠缠态和量子错误避免代码技术的量子隐形传态方案。在这个方案中,一种4比特的免退纠缠态被作为量子纠缠信道,被传输的单个比特的量子信息被编码为两个比特。该方案的优点是隐形传输具有非常高的保真度,并且具有100%成功率。该项研究对改善量子通讯有重要的作用。     

LA声子对激子qubit纯退相干的影响

在一个抛物量子点中,以激子的真空态和基态作为量子比特(qubit),采用求密度矩阵元的方法,计算了由形变势下声学声子引发的激子量子比特纯退相干.找到了激子量子比特纯退相干因子对时间、温度和量子点受限长度的依赖关系.研究发现,激子量子比特的退相干因子在2.5ps的时间范围内随时间的增加而迅速增加,其纯退相干时间在ps量级;在温度即使为绝对温度0K时由LA声子引发的退相干依然存在,在温度大于3K后退相干因子随温度的增大而开始迅速增大;并同时发现量子点受限长度对退相干因子有重要影响,激子越受限退相干越快.研究结果表明,对激子量子比特使用适当大小量子点,且保持环境低温,并采用低能超快光学操作可以有效地抑制声子对激子量子比特纯退相干的影响. 关键词： 量子点 量子信息 量子比特     

量子比特的普适远程翻转和克隆

提出一种把量子隐形传态、最佳普适量子比特翻转和最佳普适量子克隆三者结合起来的量子比特普适远程翻转和克隆方案.当发送者和处于不同地点的三个接收者共享一个特定的四粒子纠缠态作为量子信道时,通过发送者的Bell基测量、经典通信和各个接收者的局域幺正变换,一个接收者能够以2/3的最佳保真度得到一份原未知量子比特的正交补态,另外两个接收者能够分别以5/6的最佳保真度得到原未知量子比特的一份拷贝.此方案用较少的量子纠缠资源同时完成了未知量子比特的普适远程翻转和克隆,且其保真度分别达到了最佳.实现此方案的关键在于构造出发送者和接收者共享的特定四粒子纠缠态作为量子信道,分析了此特殊四粒子态内在的纠缠结构.     

噪声情况下的量子网络直接通信

提出和研究了噪声情况下的量子网络直接通信. 通信过程中所有量子节点共享多粒子Greenberger-Horne-Zeilinger (GHZ)量子纠缠态; 发送节点将手中共享的GHZ态的粒子作为控制比特、传输秘密信息的粒子作为目标比特, 应用控制非门(CNOT)操作; 每个接收节点将手中共享GHZ 态的粒子作为控制比特、接收到的秘密信息粒子作为目标比特, 再次应用CNOT门操作从而获得含误码的秘密信息. 每个接收节点从秘密信息中提取部分作为检测比特串, 并将剩余的秘密信息应用奇偶校验矩阵纠正其中存在的比特翻转错误, 所有接收节点获得纠正后的秘密信息. 对协议安全、吞吐效率、通信效率等进行了分析和讨论.     

退相干条件下两比特纠缠态的量子非局域关联检验

量子纠缠态的量子非局域关联特性在当前量子信息和量子计算协议中起着重要的作用.然而,任何实际的物理系统都不可避免地与周围环境相互作用,使得在量子信道中的传输过程中,量子态会发生相干性退化,进而弱化量子态的量子非局域关联特性.本文利用一种基于Hardy-type佯谬的高概率量子非局域关联检验方案,分别研究了两比特偏振纠缠态在经过振幅阻尼信道(ADC)、相位阻尼信道(PDC)和退极化阻尼信道(DC)后的量子非局域关联检验情况.研究结果表明,DC传输信道对量子态的量子非局域关联检验特性影响较大,而PDC传输信道对量子态的量子非局域关联检验特性影响较小.最后,本文还给出了利用弱测量结合弱测量反转操作克服ADC退相干时,偏振纠缠态成功进行量子非局域关联检验的条件.结果表明,当弱测量的强度增大时,可有效地降低ADC退相干效应对偏振纠缠态成功进行量子非局域关联检验造成的影响.     

两量子比特与谐振子相耦合系统中的量子纠缠演化特性

在非旋波近似下, 利用相干态正交化展开方法, 对两量子比特与谐振子相耦合系统中的量子纠缠演化特性进行了精确计算. 讨论了在共振时, 两量子比特和谐振子耦合系统基态的性质以及量子比特和谐振子之间的纠缠与量子比特-量子比特间的纠缠的不同. 结果表明: 当不考虑外场时, 量子比特-量子比特间的纠缠随着耦合强度的增大从1迅速地减小到零, 表明了量子比特-量子比特间的纠缠对耦合强度是非常敏感的; 而量子比特和谐振子之间的纠缠随着耦合强度的增大从零迅速地增大, 但不能达到理论上的最大值2; 当初始时刻两量子比特没有纠缠时, 在弱耦合强度下, 真空场不能导致纠缠的产生; 而强的耦合非旋波效应则可以导致纠缠的突然产生现象. 关键词： 相干态正交化展开 非旋波近似 量子纠缠     

振幅阻尼信道量子隐形传态保真度恢复机理

在振幅阻尼信道上进行量子隐形传态的过程中, 量子Bell纠缠态将发生退相干, 导致隐形传态质量下降甚至通信失败. 为克服该影响, 本文提出了一种Bell纠缠态补偿方法. 在估计振幅阻尼信道参数的基础上, 将对纠缠态的补偿分为纠缠退相干发生之前的预补偿以及 经历量子振幅阻尼信道之后的匹配补偿两部分. 前者在纠缠源处进行, 后者在两个量子通信 用户处进行, 预补偿及匹配补偿参数与信道特性参数相关. 纠缠补偿完成后, 再进行隐形传态. 理论推导与性能分析结果表明, 相比于不进行纠缠补偿及仅在发生退相干之后进行的纠缠补偿, 本方法能够获得更高的隐形传态保真度, 适当调整补偿参数, 可使保真度接近于1, 对克服纠缠 退相干带来的隐形传态质量下降问题具有一定的意义.     

采用纠缠辅助量子LDPC码和检错重传策略的量子直传通信方案

针对现有量子信息直传协议在有噪音量子信道下传输效率低及可靠性差的问题,提出了一种有效利用纠缠资源的量子安全直传通信方案.通过收发双方共享纠缠粒子作为辅助比特,采用纠缠辅助量子低密度校验码对量子态信息进行前向纠错保护,以提高系统在噪音环境下的传输可靠性.同时采用自动请求重传策略对量子态信息进行检错编码保护,当因窃听或强噪...     

采用纠缠辅助量子LDPC码和检错重传策略的 量子直传通信方案

针对现有量子信息直传协议在有噪音量子信道下传输效率低及可靠性差的问题,提出了一种有效利用纠缠资源的量子安全直传通信方案.通过收发双方共享纠缠粒子作为辅助比特,采用纠缠辅助量子低密度校验码对量子态信息进行前向纠错保护,以提高系统在噪音环境下的传输可靠性.同时采用自动请求重传策略对量子态信息进行检错编码保护,当因窃听或强噪音导致译码获得的信息不正确时,则请求发端对该组信息进行编码重传操作.文中对所选用纠缠辅助量子低密度校验码在量子退极化噪音信道下的迭代译码性能进行了仿真,最后对方案的安全性进行了分析论证.     

三维传输子量子比特的退相干参数表征

超导量子比特的退相干时间是决定超导量子计算能否实现的重要指标之一. 文章以三维传输子量子比特(3D transmon)为研究对象, 在氧化硅衬底上制备了三维传输子量子比特, 并在超低温下(10 mK), 采用拉比振荡(Rabi oscillation)、能量弛豫(energy relaxation)、 拉姆齐条纹(Ramsey fringe)、自旋回波(spin echo)的方法, 对其进行了详细的退相干时间常数表征. 结果显示该量子比特的退相干时间在几百纳秒. 根据几种退相干时间的关系进行计算, 可以看出, 低频噪声目前不是影响量子比特退相干的最主要因素, 而氧化硅中的缺陷可能是样品退相干时间的主要瓶颈. 关键词： 三维传输子量子比特 拉比振荡 拉姆齐条纹 自旋回波     

Controlling transfer of quantum correlations among bi-partitions of a composite quantum system by combining different noisy environments

The correlation dynamics are investigated for various bi-partitions of a composite quantum system consisting of two qubits and two independent and non-identical noisy environments. The two qubits have no direct interaction with each other and locally interact with their environments. Classical and quantum correlations including the entanglement are initially prepared only between the two qubits. We find that contrary to the identical noisy environment case, the quantum correlation transfer direction can be controlled by combining different noisy environments. The amplitude-damping environment determines whether there exists the entanglement transfer among bi-partitions of the system. When one qubit is coupled to an amplitude-damping environment and the other one to a bit-flip one, we find a very interesting result that all the quantum and the classical correlations, and even the entanglement, originally existing between the qubits, can be completely transferred without any loss to the qubit coupled to the bit-flit environment and the amplitude-damping environment. We also notice that it is possible to distinguish the quantum correlation from the classical correlation and the entanglement by combining different noisy environments.     

Improving Quantum Metrology via Purifying and Distilling Noisy Probe States

In quantum metrology, the precision of unknown parameter estimation is studied in the quantum regime, and the choice of the probe state plays an important role in determining the precision of the parameter to be estimated. The quality of quantum metrology will be reduced in the presence of quantum noise during the memory time of probe states after preparation. Meanwhile the noisy probe state can be manipulated by different protocols such as single‐qubit purification, entanglement purification, and entanglement distillation etc. In this paper, the effects of these manipulations on the usefulness, that is, quantum Fisher information (QFI), of the noisy probe state in quantum metrology are studied. The results show that joint operations in single‐qubit purification and entanglement purification processes play positive roles in enhancing the QFI of the probe states, and local measurements in entanglement purification and entanglement distillation processes play both positive and negative roles in enhancing the QFI of the probe states. In this sense, single‐qubit purification will always be helpful in parameter estimation by using single qubits as probe, and entanglement purification process maybe more suitable for improving the estimation precision when entangled‐state probe is adopted.     

Entanglement Fidelity as a Measure of Preservation of Entanglement in Local Noisy Process

A new formula of entanglement fidelity has been introduced, which can serve as a measure of the preservation of entanglement between two initially entangled subsystems exposed to local noisy environments. For a simple model we derive analytic expressions of concurrence and entanglement fidelity and draw the relationship between them. We find that such entanglement fidelity exhibits the behavior similar to that of the concurrence in quantum evolutions.     

Exact Entanglement Dynamics in Three Interacting Qubits

Motivated by recent experimental studies on coherent dynamics transfer in three interacting atoms or electron spins [Phys. Rev. Lett 114(2015) 113002, Phys. Rev. Lett 120(2018) 243604], here we study entanglement entropy transfer in three interacting qubits. We analytically calculate time evolutions of wave function, density matrix and entanglement of the system. We find that initially entangled two qubits may alternatively transfer their entanglement entropy to other two qubit pairs. Thus dynamical evolution of three interacting qubits may produce a genuine three-partite entangled state through entanglement entropy transfers. In particular, different pairwise interactions of the three qubits endow symmetric and asymmetric evolutions of the entanglement transfer,characterized by the quantum mutual information and concurrence. Finally, we discuss an experimental proposal of three Rydberg atoms for testing the entanglement dynamics transfer of this kind.     

Noise Effects on Entangled Coherent State Generated via Atom-Field Interaction and Beam Splitter

In this paper, we introduce a controllable method for producing two and three-mode entangled coherent states (ECS’s) using atom-field interaction in cavity QED and beam splitter. The generated states play central roles in linear optics, quantum computation and teleportation. We especially focus on qubit, qutrit and qufit like ECS’s and investigate their entanglement by concurrence measure. Moreover, we illustrate decoherence properties of ECS’s due to noisy channels, using negativity measure. At the end the effect of noise on monogamy inequality is discussed.     

Entanglement Dynamics of Two-Qubit System in Different Types of Noisy Channels

In this paper, we study entanglement dynamics of a two-qubit extended Werner-like state locally, interacting with independent noisy channels, i.e., amplitude damping, phase damping, and depolarizing channels. We show that the purity of initial entangled state has direct impacts on the entanglement robustness in each noisy channel. That is, if the initial entangled state is prepared in mixed instead of pure form, the state may exhibit entanglement sudden death （ESD） and/or be decreased for the critical probability at which the entanglement disappear.     

The fully entangled fraction as an inclusive measure of entanglement applications

Characterizing entanglement in all but the simplest case of a two qubit pure state is a hard problem, even understanding the relevant experimental quantities that are related to entanglement is difficult. It may not be necessary, however, to quantify the entanglement of a state in order to quantify the quantum information processing significance of a state. It is known that the fully entangled fraction has a direct relationship to the fidelity of teleportation maximized under the actions of local unitary operations. In the case of two qubits we point out that the fully entangled fraction can also be related to the fidelities, maximized under the actions of local unitary operations, of other important quantum information tasks such as dense coding, entanglement swapping and quantum cryptography in such a way as to provide an inclusive measure of these entanglement applications. For two qubit systems the fully entangled fraction has a simple known closed-form expression and we establish lower and upper bounds of this quantity with the concurrence. This approach is readily extendable to more complicated systems.     

Quantum logic networks for probabilistic teleportation

By eans of the primitive operations consisting of single-qubit gates.two-qubit controlled-not gates,Von Neuman measurement and classically controlled operations.,we construct efficient quantum logic networks for implementing probabilistic teleportation of a single qubit,a two-particle entangled state,and an N-particle entanglement.Based on the quantum networks,we show that after the partially entangled states are concentrated into maximal entanglement,the above three kinds of probabilistic teleportation are the same as the standard teleportation using the corresponding maximally entangled states as the quantum channels.     

利用纠缠交换在理想和非理想条件下的远程态制备(英文)

提出了利用纠缠交换技术实现一个纠缠态的远程制备.考虑在量子信道没有遭受噪声影响的条件下,利用一对两粒子纠缠态作为量子信道去实现远程态制备.此外我们考察了噪声存在时的远程态的制备,研究显示不同类型的量子噪声对远程态制备过程的保真度的影响不同.对一个确定量子态,我们考察了消相干效应对其远程制备的影响.     

Entanglement Swapping in the Presence of White and Color Noise

The influence of white and color noise on the outcome of the entanglement swapping process is investigated in a four-qubit system. Critical degree of noise in initial state, that could destroy entanglement in a result state is presented. The entanglement characteristics, such as concurrence, tangle, etc. are compared. Results could be helpful for experiments regarding entanglement swapping as conditions for initial quantum entangled states, to obtain entangled result state.