编码于单个中性原子的单量子比特的量子层析

量子态层析和量子过程层析是刻画量子态和量子操作过程准确度的基本工具。本文主要对编码于二能级铯原子的单量子比特及其单量子操作进行了相关实验研究,对编码在铯原子钟态的量子态■进行了量子态层析分析,得到其保真度为0.97±0.02。我们还对单量子比特的门操作R_x(π)、■、R_y(π)、■、■进行了量子过程层析测量,得到量子门操作的平均保真度为0.96±0.03。我们对影响单比特态及其操作过程的保真度的因素进行了分析。     

用约瑟夫森电荷比特系统实现一种特殊量子态的传输

构造了一种约瑟夫森电荷量子比特电路系统,并研究了四量子位态在该系统中的传输特性.对均匀与非均匀传输通道两种情形分别讨论了怎样通过控制外加磁通来实现激发态 1〉从第一量子位到第四量子位的理想传输.此外还分析了量子态在该系统中传输时的平均保真度,结果表明该系统不能传输一个任意态. 关键词： 量子态传输 约瑟夫森电荷比特     

一轴模型和横场一轴模型中的量子态保真度

针对一轴模型和横场一轴模型, 研究了自旋压缩演化过程中的量子态的量子保真度, 结果表明：量子态在一轴模型作用下, 保真度呈周期性演化.对于偶粒子数N 个qubit系统, 演化周期为T=2*pi , 在T/4 时刻, 3T/4保真度始终能保持为0.5, 在T/2 时刻,保真度为零,量子态完全失真.对于奇粒子数 N个qubit系统,演化周期为 pi,比较于偶粒子数N 的情况,在T/4,T/2, 3T/4时刻,保真度依赖于粒子数N ;在横场一轴模型中, 横场强度的增加不仅有利于自旋压缩的提高, 还有利于量子保真度的提高.     

玻色-爱因斯坦凝聚原子与Schrodinger猫态相互作用系统中的保真度

利用全量子理论及量子信息保真度理论,研究了薛定谔猫态光场与玻色爱因斯坦凝聚原子相互作用系统中,量子态保真度随时间的演化特性.讨论了光场强度和玻色爱因斯坦凝聚原子间的耦合强度对量子态保真度的影响.结果表明:光场强度可调制量子态的保真效果,而原子间的耦合常数主要影响量子态保真度的振荡频率.     

玻色-爱因斯坦凝聚原子与Schrodinger猫态相互作用系统中的保真度

利用全量子理论及量子信息保真度理论,研究了薛定谔猫态光场与玻色爱因斯坦凝聚原子相互作用系统中,量子态保真度随时间的演化特性.讨论了光场强度和玻色爱因斯坦凝聚原子间的耦合强度对量子态保真度的影响.结果表明：光场强度可调制量子态的保真效果,而原子间的耦合常数主要影响量子态保真度的振荡频率.     

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.     

量子纠缠消相干对确定型远程制备的影响

研究了两种典型的量子纠缠消相干现象对确定型量子态远程制备方案的影响.首先对该确定型远程制备方案进行了分析,得到该方案确定性和比特消耗情况; 然后通过分析制备过程中纠缠消相干现象对系统的影响得出: 在极化消相干过程中,该系统保真度与目标量子比特在Bloch球上的经度选择无关,仅与目标比特的纬度和消相干的大小有关;在相位消相干中,该系统的保真度不会受到消相干的影响,仅与目标量子态的纬度相关. 关键词： 远程制备 纠缠消相干 通信消耗 保真度     

离子阱量子计算规模化的研究进展

离子阱系统是当前实现量子计算最为领先的物理系统之一,已经在数十量子比特的规模下实现了保真度达到容错量子计算阈值的量子态制备、测量、通用量子逻辑门等基本量子操作.未来离子阱量子计算的一个重要研究方向,是在保持量子比特高性能的同时,进一步扩展量子比特的数量,最终达到解决实际问题所需的规模.本文介绍当前离子阱量子计算研究中主流的规模化方案,如离子输运方案和离子-光子量子网络方案等,以及各方案中存在的限制因素,进而探讨如二维离子阵列、双重量子比特等新的规模化方案及其前景.     

两能级原子与热辐射场Raman相互作用时原子量子态保真度

两个两能级原子置于热辐射场环境中,原子用泡利算符描述,环境用无穷的谐振子热库描述.原子与热库产生Raman相互作用,原子间存在偶极相互作用,运用密度矩阵方法,得到两能级原子密度矩阵元演化规律.针对四种不同的初始状态,分析置于热辐射场中原子量子态保真度.结果表明:两个原子初始处于不同量子叠加态,量子信息在传输过程中的保真度有明显的差异.     

与双模场依赖强度耦合下多光子通道中原子比 特周期量子回声产生和调控

旋波近似条件下,运用全量子理论研究了与双模相干光场依赖强度耦合多光子通道中原子比特周期量子回声的产生和控制. 采用数值计算的方法,讨论了双模相干光场平均光子数分布形式、分布范围及原子跃迁时吸收(或发射)的光子数k对原子比特态保真度演化的影响,获得了产生和控制原子比特周期量子回声的系统参量;根据纠缠理论,分析了原子比特态保真度演化与原子约化熵演化的关联. 结果表明:在k=1的双光子过程中,调控光场平均光子数呈对称或不对称分布,当它的取值在一定范围内,原子比特保持良好的相干性和保真度,产生周期量子回声; 对于k≥2的多光子过程,原子比特与双模相干光场始终处于最大纠缠,因此导致了原子比特始终处于部分失真状态,不产生周期量子回声. 本研究揭示了周期量子回声产生的物理实质是原子比特与光场周期性退纠缠.     

Preserving entanglement and the fidelity of three-qubit quantum states undergoing decoherence using weak measurement

We demonstrate a method to preserve entanglement and improve fidelity of three-qubit quantum states undergoing amplitude-damping decoherence using weak measurement and quantum measurement reversal. It is shown that we are able to enhance entanglement to the greatest extent, and to circumvent entanglement sudden death by increasing the weak measurement strength both for the GHZ state and the W state. The weak measurement technique can also enhance the fidelity to the quantum region and even close to 1 for the whole range of the decoherence parameter in both of the two cases. In addition, the W state can maintain more fidelity than the GHZ state in the protection protocol. However, the GHZ state has a higher success probability than the W state.     

噪声对一种三粒子量子探针态的影响

量子度量学是研究量子测量与统计推断的一门学科,主要利用量子手段来提高参数估计的精度,在量子信息处理与测量中起到关键作用.量子参数估计的一般过程包含四个步骤:探针态的制备、参数化过程、对参数化后的输出态进行测量以及根据测量结果估计待测参数.其中探针态的选取对测量精度起着至关重要的作用.然而在实际的量子探针态的制备过程中,初始探针态会受到环境噪声的影响.目前人们已经研究了W态与Greenberger-Horne-Zeilinger(GHZ)态的量子Fisher信息(QFI)在典型噪声通道下的变化行为.由于W态与GHZ态有着不同的纠缠性质,对于W态与GHZ态的叠加态的QFI动力学研究具有重要的实际意义.故此,本文主要研究典型噪声通道对这两种状态的叠加态的QFI动力学行为的影响,得出了QFI随噪声参数的变化行为.结果表明,叠加态中W态组分可明显对抗相位阻尼噪声对探针态的QFI的影响,而其中的GHZ态组分可明显对抗振幅阻尼噪声的影响,从而为在实际环境中选取高精度的参数估计过程提供参考.     

Dynamics and protection of tripartite quantum correlations in a thermal bath

We study the dynamics and protection of tripartite quantum correlations in terms of genuinely tripartite concurrence, lower bound of concurrence and tripartite geometric quantum discord in a three-qubit system interacting with independent thermal bath. By comparing the dynamics of entanglement with that of quantum discord for initial GHZ state and W state, we find that W state is more robust than GHZ state, and quantum discord performs better than entanglement against the decoherence induced by the thermal bath. When the bath temperature is low, for the initial GHZ state, combining weak measurement and measurement reversal is necessary for a successful protection of quantum correlations. But for the initial W state, the protection depends solely upon the measurement reversal. In addition, the protection cannot usually be realized irrespective of the initial states as the bath temperature increases.     

Enhancing teleportation fidelity by means of weak measurements or reversal

The enhancement of teleportation fidelity by weak measurement or quantum measurement reversal is investigated. One qubit of a maximally entangled state undergoes the amplitude damping, and the subsequent application of weak measurement or quantum measurement reversal could improve the teleportation fidelity beyond the classical region. The improvement could not be attributed to the increasing of entanglement, quantum discord, classical correlation or total correlation. We declare that it should be owed to the probabilistic nature of the method.     

Decoherence suppression for three-qubit W-like state using weak measurement and iteration method

Multi-qubit entanglement states are the key resources for various multipartite quantum communication tasks. For a class of generalized three-qubit quantum entanglement, W-like state, we demonstrate that the weak measurement and the reversal measurement are capable of suppressing the amplitude damping decoherence by reducing the initial damping factor into a smaller equivalent damping factor. Furthermore, we propose an iteration method in the weak measurement and the reversal measurement to enhance the success probability of the total measurements. Finally, we discuss how the number of the iterations influences the overall effect of decoherence suppression, and find that the "half iteration" method is a better option that has more practical value.     

利用弱测量保护两个三能级原子系统的束缚纠缠

高维量子系统的纠缠态因其诸多的优点而受到广泛的关注。目前,肖兴等人[Eur.Phys.J.D.(2013)67:204]研究了两个V-型三能级原子系统的演化动力学,得到了弱测量方案能保护系统自由纠缠的结论。本文研究了在振幅阻尼噪声中两个全同三能级原子系统束缚纠缠的演化动力学。在振幅阻尼通道中,前置弱测量和后置弱测量反转操作,实现了对系统束缚纠缠的保护。束缚纠缠保持的时间受量子态参数和弱测量强度的影响。相应的物理原因也得到了合理的解释。对于两个非全同三能级原子的情况,此结论依旧成立。另外,弱测量方案并不是对所有类型的束缚纠缠态都适用,有一定的局限性。     

Bidirectional multi-qubit quantum teleportation in noisy channel aided with weak measurement

Recently,bidirectional quantum teleportation has attracted a great deal of research attention.However,existing bidirectional teleportation schemes are normally discussed on the basis of perfect quantum environments.In this paper,we first put forward a bidirectional teleportation scheme to transport three-qubit Greenberger-Horne-Zeilinger(GHZ) states based on controled-not(CNOT) operation and single-qubit measurement.Then,we generalize it to the teleportation of multi-qubit GHZ states.Further,we discuss the influence of quantum noise on our scheme by the example of an amplitude damping channel,then we obtain the fidelity of the teleportation.Finally,we utilize the weak measurement and the corresponding reversing measurement to protect the quantum entanglement,which shows an effective enhancement of the teleportation fidelity.     

Enhancing robustness of multiparty quantum correlations using weak measurement

Multipartite quantum correlations are important resources for the development of quantum information and computation protocols. However, the resourcefulness of multipartite quantum correlations in practical settings is limited by its fragility under decoherence due to environmental interactions. Though there exist protocols to protect bipartite entanglement under decoherence, the implementation of such protocols for multipartite quantum correlations has not been sufficiently explored. Here, we study the effect of local amplitude damping channel on the generalized Greenberger–Horne–Zeilinger state, and use a protocol of optimal reversal quantum weak measurement to protect the multipartite quantum correlations. We observe that the weak measurement reversal protocol enhances the robustness of multipartite quantum correlations. Further it increases the critical damping value that corresponds to entanglement sudden death. To emphasize the efficacy of the technique in protection of multipartite quantum correlation, we investigate two proximately related quantum communication tasks, namely, quantum teleportation in a one sender, many receivers setting and multiparty quantum information splitting, through a local amplitude damping channel. We observe an increase in the average fidelity of both the quantum communication tasks under the weak measurement reversal protocol. The method may prove beneficial, for combating external interactions, in other quantum information tasks using multipartite resources.     

多粒子纠缠的保护方案

量子纠缠是量子信息的重要物理资源. 然而当量子系统与环境相互作用时, 会不可避免地产生消相干导致纠缠下降, 因此保护纠缠不受环境的影响具有重要意义. 振幅衰减是一种典型的衰减机制. 如果探测环境保证没有激发从系统中流出, 即视为对系统的一种弱测量. 本文基于局域脉冲序列和弱测量, 提出了一种可以保护多粒子纠缠不受振幅衰减影响的有效物理方案, 保护的对象是在量子通信和量子计算中发挥重要作用的Cluster态和Maximal slice态.     

Entanglement Evolution of Three-Qubit States under Local Decoherence

By using negativity as entanglement measure, we have investigated the effect of local decoherence from a non-Markovian environmenton the time evolution of entanglement of three-qubit states including the GHZ state, the W state, and the Werner state. From the results, we find that the entanglement dynamics depends not only on the coupling strengths but also on the specific states of concern. Specifically, the entanglement takes different behaviors under weak or strong coupling and it varies with the quantum states under study. The entanglement of the GHZ state and the Werner state can be destroyed completely by the local decoherence, while the entanglement of the W state can survive through the local decoherence partially.