两硬币量子游走模型中的相干动力学

量子游走是量子计算的重要模型,而多硬币量子游走模型由于在量子通讯协议中表现突出也越来越受到人们的关注.量子相干不仅可以刻画量子态的特点,也可以反映量子演化过程的性质.主要对一维圆上两硬币量子游走模型的量子相干性进行了分析.一方面,讨论了初始量子态和硬币算子的选取对量子相干的影响.当硬币算子为Hadamard算子且初态只要在位置子空间上是均衡叠加态,整个量子游走演化过程是具有周期性的,且量子相干仅依赖于步数和圆上顶点的个数;当初始态是均衡叠加态而对硬币算子没有任何限制时,量子相干的演化也极具规律性.另一方面,发现在利用量子游走实现完美状态转移(perfect state transfer)的过程中,硬币算子的选取直接影响量子相干的值.最后,探讨了2种量子游走模型之间的等价性,并基于此指出了其在量子隐形传输(quantum teleportation)中的应用和改进的可能性.     

基于相干控制的二能级量子系统退相干抑制

对于二能级开放量子系统,研究了利用相干控制抑制退相干效应的问题.首先讨论了二能级开放量子系统在相干控制下的建模问题,将退相干抑制归结为与环境噪声解耦的控制问题.然后,引入开环控制抑制退相干,并证明该控制可使系统状态中的部分分量与环境噪声渐近解耦.最后引入反馈控制,使得系统状态的相应分量可以与环境精确解耦,同时能够避免测量引入的量子噪声的影响.     

在噪声情况下远程制备四比特团簇态

本文采用团簇态作为量子信道,讨论了4种噪声对远程制备四比特团簇态的影响.首先分析在理想情况下,制备者Alice利用团簇态作为量子信道,通过构造巧妙的测量基,帮助接收者Bob获取所需制备的目标四比特团簇态;然后讨论在4种噪声影响下远程制备四比特团簇态,且利用保真度来描述输出态与输入态之间的接近程度.发现不同类型的噪声对远程量子态制备的影响程度不同.尤其相位噪声,在远程量子制备四比特团簇态的过程中,系统的保真度不受待制备量子态的相位因素的影响,然而其他3种噪声都受待制备量子态的相位因素的影响.     

双量子系统最大纠缠态制备的两种控制方法

分别采用两种脉冲序列,对由两个自旋1/2粒子组成的四能级量子系统进行最大纠缠态的制备.基于部分受激拉曼绝热通道技术,设计了半反直觉脉冲序列;同时设计了基于面积控制的π脉冲控制序列.通过系统仿真实验,在参数选取对纠缠态制备性能影响分析的基础上,详细地给出了纠缠态制备中各个参数的优化过程,包括不同参数对纠缠态制备过程中系统几率影响的分析,纠缠态制备最佳参数的选取,以及制备系统最大纠缠态或贝尔(Bell)基态控制参数值的确定.     

三能级原子系统中量子态相干保持的控制策略

研究光与物质的相互作用并利用其性质设计新型的量子器件,以实现光信息存储及其消相干抑制.分析了以单色激光场为控制场的A型三能级原子系统的主方程模型,借助于无消相干子空间的构造方法,通过改变耦合激光场Rabi频率的方式,设计了实现系统中量子态相干保持的控制策略.     

用量子广义测量控制消相干

本文探讨了把量子广义测量和无消相干子空间 (DFS) 结合起来抑制消相干的潜力. 证实了: 把量子广义子空间投影测量 (QGSPM) 和量子广义分类投影测量 (QGCPM) 与 DFS 算子条件结合起来, 可以有效增强 Markovian 和非 Markovian 量子开放系统抑制消相干的能力. 强调了量子测量可以作为操控量子态的重要手段. 本方法的优点在于可以构造性地设计相干控制哈密顿量.     

开放环境下多比特量子计算机的相干控制模型

研究了开放环境下多比特量子计算系统的相干控制建模问题.基于开放量子系统的数学模型,选取适当的矩阵基将描述多比特量子计算机的复矩阵动态控制模型转化为实向量空间上的控制模型,并给出计算相应的结构系数的方法.这些工作提供了进一步研究控制律设计的基础.     

几种量子程序终止的有效验证

基于文献巨18口提出的量子程序验证方法,讨论了单量子比特系统上比特翻转、去极化、幅值阻尼、相位阻尼等 信道刻画的量子程序的验证,通过选取不同的可观测算子对程序终止的情况进行了详细的讨论。研究表明,由这些量 子信道所描述的量子程序的终止情况不仅依赖于输入态的选取,还依赖于可观测算子的选取。     

相移控制下Ising自旋链的量子态和纠缠的传递

主要研究处于外部均匀磁场环境下,在Ising自旋二分之一链中,利用Aharonov Casher形成的相位移动如何实现量子纠缠和量子态的传递.通过计算和数值仿真发现:调整Aharonov Casher形成的相位移动就可以实现量子纠缠和量子态的传递.此外,通过计算量子态传递的平均保真度也可以看出:控制相位移动能有效地抵抗退相干的影响,增加系统的纠缠.     

基于ARM-Linux的嵌入式光学相干层析成像系统的研究

分析了相位对照光学相干层析成像的基本原理,并基于ARM9架构的嵌入式微处理器S3C2440和当前主流嵌入式的Linux多用户操作系统,提出了一套用于无损检测高分子聚合物有机复合材料的相位对照光学相干层析成像系统的设计方案。详细介绍了系统的原理及搭建过程,使用相位对照理论分析方法和表面变形测量方法并通过实验获得高分子聚合物有机复合材料内部结构的分布图。实验结果表明该嵌入式光学相干层成像系统是可行的。     

Controllability and Universal Three-qubit Quantum Computation with Trapped Electron States

We show how to control and perform universal three-qubit quantum computation with trapped electron quantum states. The three qubits are the electron spin, and the first two quantum states of the cyclotron and axial harmonic oscillators. We explicitly show how universal three-qubit gates can be performed. As an example of a quantum algorithm, we outline the implementation of the three-qubit Deutsch-Jozsa algorithm in this system.      

Creation of qutrit and one-qubit gates in atom–cavity–laser systems by adiabatic passage

We derive an adiabatic technique that implements arbitrary qutrit and one-qubit rotation gates in a quantum system composed of two tripod atoms each of which interacting with two laser pulses and a cavity field. In this method, the atomic ground states playing the role of the qutrit and qubit subspace and losses due to atomic spontaneous emissions and the cavity decay are efficiently suppressed by employing adiabatic passage technique. In order to create qutrit gates, we exploit generalized quantum Householder reflection in which each Householder reflection is implemented by two-step adiabatic passage technique in a seven-state system with two dark states. With a similar method, an arbitrary rotation gate is constructed in this system. We also implement the Fourier transform in a qutrit as an example.     

Topological phenomena in quantum walks: elementary introduction to the physics of topological phases

Discrete quantum walks are dynamical protocols for controlling a single quantum particle. Despite of its simplicity, quantum walks display rich topological phenomena and provide one of the simplest systems to study and understand topological phases. In this article, we review the physics of discrete quantum walks in one and two dimensions in light of topological phenomena and provide elementary explanations of topological phases and their physical consequence, namely the existence of boundary states. We demonstrate that quantum walks are versatile systems that simulate many topological phases whose classifications are known for static Hamiltonians. Furthermore, topological phenomena appearing in quantum walks go beyond what has been known in static systems; there are phenomena unique to quantum walks, being an example of periodically driven systems, that do not exist in static systems. Thus the quantum walks not only provide a powerful tool as a quantum simulator for static topological phases but also give unique opportunity to study topological phenomena in driven systems.     

Hierarchy,factorization law of two measurement-induced nonlocalities and their performances in quantum phase transition

There are two measurement-induced nonlocalities, which are, respectively, defined via the trace norm (MIN-1) and Hilbert–Schmidt norm (MIN-2). We investigate the hierarchy relation and factorization law of them. Their performances in quantum phase transition have also been explored. For X-shape states, a rigorous hierarchy relation is established between two MINs. When two qubits, which are initially prepared in an X-shape state, interact independently with the corresponding multimode vacuum reservoirs, the evolutions of two MINs satisfy the factorization law. With quantum renormalization group method, it is found that two MINs can signify the criticality of the spin system while the position where the derivative of MIN-1 takes the minimum value is always larger than that where the derivative of MIN-2 takes the minimum value. Therefore, MIN-1 is more suitable to identify the critical point of quantum phase transition.     

Quantum teleportation between a single-rail single-photon qubit and a coherent-state qubit using hybrid entanglement under decoherence effects

We study quantum teleportation between two different types of optical qubits using hybrid entanglement as a quantum channel under decoherence effects. One type of qubit employs the vacuum and single-photon states for the basis, called a single-rail single-photon qubit, and the other utilizes coherent states of opposite phases. We find that teleportation from a single-rail single-photon qubit to a coherent-state qubit is better than the opposite direction in terms of fidelity and success probability. We compare our results with those using a different type of hybrid entanglement between a polarized single-photon qubit and a coherent state.     

Quantum filtering for systems driven by fields in single photon states and superposition of coherent states using non-Markovian embeddings

The purpose of this paper is to determine quantum master and filter equations for systems coupled to fields in certain non-classical continuous-mode states. Specifically, we consider two types of field states (i) single photon states, and (ii) superpositions of coherent states. The system and field are described using a quantum stochastic unitary model. Master equations are derived from this model and are given in terms of systems of coupled equations. The output field carries information about the system, and is continuously monitored. The quantum filters are determined with the aid of an embedding of the system into a larger non-Markovian system, and are given by a system of coupled stochastic differential equations.     

Quantum holonomies for displaced Landau–Aharonov–Casher states

We construct displaced Fock states for a Landau–Aharonov–Casher system for neutral particles. Abelian and non-Abelian geometric phases can be obtained in an adiabatic cyclic evolution using this displaced states. Moreover, we show that a possible logical base related to the angular momenta of the neutral particle with permanent magnetic dipole moment can be defined, and then quantum holonomies for specific paths can be built and used to implement one-qubit quantum gates.     

Optimal unambiguous discrimination of pure qudits

Linearly independent pure quantum states can be discriminated unambiguously, while linearly dependent states cannot. We use a physical accessible unitary transformation to map the nonorthogonal quantum states onto a set of orthogonal ones so that measuring the output states can discriminate the initial states with the deterministic and inconclusive results. The failure states that give an inconclusive result are linearly dependent ones. In finding the optimal unambiguous discrimination (UD), we show that a main constraint condition that the determinant constructed by the complex inner products of the failure states must be zero, along with two additional conditions, can provide solutions to the problem of the optimal UD for pure qudits. For any d, we give one analytical solution as all the Berry phases being zero. We also derive the lowest bound of the total failure probability of the optimal UD.     

Polarization-entangled photon generation in a semiconductor quantum dot coupled to a cavity interacting with external fields

We theoretically investigate polarization-entangled photon generation using a semiconductor quantum dot embedded in a microcavity. The entangled states can be produced by the application of two cross-circularly polarized laser fields. The quantum dot nanostructure is considered as a four-level system (ground, two excitons and bi-exciton states), and the theoretical study relies on the dressed states scheme. The quantum correlations, reported in terms of the entanglement of formation, are extensively studied for several values of the important parameters of the quantum dot system as the bi-exciton binding energy, the decoherence times of the characteristic transitions, the quality factor of the cavity and the intensities of the applied fields.     

On Entangled Information and Quantum Capacity

The pure quantum entanglement is generalized to the case of mixed compound states to include the classical and quantum encodings as particular cases. The true quantum entanglements are characterized as transpose-CP but not CP maps. The entangled information is introduced as the relative entropy of the mutual and the input state and total information of the entangled states leads to two different types of entropy for a given quantum state: the von Neumann entropy, which is achieved as the supremum of the information over all c-entanglements, and the true quantum entropy, which is achieved at the standard entanglement. The q-capacity, defined as the supremum over all entanglements, doubles the c-capacity in the case of the simple algebra. The conditional q-entropy is positive, and q-information of a quantum channel is additive.