量子相干

量子相干不仅是量子力学中的一个基本概念,同时也是重要的量子信息处理的物理资源.随着基于资源理论框架的量子相干度量方案的提出,量子相干度的量化研究成为近年来人们关注的一个热点问题.量子相干作为一种物理资源也十分脆弱,极容易受到环境噪声的影响而产生退相干,因此开放系统中的量子相干演化和保持也是人们广泛关注的课题.另外,量子相干在量子多体系统、量子热动力学、量子生物学等领域也有着潜在的应用价值.本文介绍量子相干度量的资源理论框架和基于该框架定义的相对熵相干性、l1范数相干性、基于量子纠缠的相干性、基于凸顶结构的相干性和相干鲁棒性等量子相干度量函数,概述开放系统中量子相干演化的动力学行为、典型信道的量子相干产生和破坏能力以及量子相干的冻结等现象,同时例举量子相干在Deutsch-Jozsa算法、Grover算法以及量子多体系统相变问题研究等方面的重要应用.量子相干研究仍处于快速发展之中,期望本综述能为该领域的发展带来启示.     

Geometric phase of an accelerated two-level atom in the presence of a perfectly reflecting plane boundary

We study the geometric phase of a uniformly accelerated two-level atom coupled with vacuum fluctuations of electromagnetic fields in the presence of a perfectly reflecting plane. We find that the geometric phase difference between the accelerated and inertial atoms which can be observed by atom interferometry crucially depends on the polarizability of the atom and the distance to the boundary and it can be dramatically manipulated with anisotropically polarizable atoms. In particular, extremely close to the boundary, the phase difference can be increased by two times as compared to the case without any boundary. So, the detectability of the effects associated with acceleration using an atom interferometer can be significantly increased by the presence of a boundary using atoms with anisotropic polarizability.     

三能级电磁感应透明中辐射场的量子统计特性

对Λ型三能级原子电磁感应透明（EIT）过程中辐射场的二阶相干度进行了研究。理论分析表明，在电磁感应透明系统中，由于原子的相干效应导致其上能级共振荧光场的二阶相干度将呈现单光子场的量子统计特性。并对其随耦合场强度和探测光失谐的变化进行了详细的分析和讨论，结果发现：在｜Ω｜〉（Γ2＋Γ3）／2情况下．采用较弱的耦合光功率（由托比频率Ω表征）及较大的探测光失谐，在较长时间延迟范围内，二阶相十度保持小于1，更利于实现非经典场的量子统计行为；相反，在｜Ω｜≤（Γ2＋Γ3）／2情况下，探测光的失谐量越小，越利于获得二阶相干度小于1的量子统计光场。南此可见选取合适的参量可优化电磁感应透明过程中单光子场的量子统计特性。     

Heisenberg-Type Quantum Steering by Continuous Weak Measurement in Circuit QED *

Quantum steering has attracted great interest in the last decade, especially in the celebrated optomechanical, cold atom, and quantum optical systems. However, there is still a lack of studies on quantum steering in circuit quantum electrodynamics (QED), which provides a useful experimental platform for revealing novel quantum phenomena. In this work, we investigate the steering of qubit by continuous weak measurement in a circuit QED system and establish a set of multiplicative steering inequalities based on the Heisenberg uncertainty principle. Different from the widely studied systems mentioned above, multiplicative steering inequalities in the circuit QED system are in various forms. We find that only a portion of them can be used to show the detection dependence of the qubit state and we also analyze the reason. Furthermore, we discuss several conditions for the violation of a typical steering inequality, including the measurement strength and methods in detecting the cavity field as well as the quantum efficiency of the detector. This preliminary work could be helpful to quantum steering experiments in circuit QED systems.     

Quantum coherence preservation of atom with a classical driving field under non-Markovian environment

The exact dynamics of an open quantum system consisting of one qubit driven by a classical driving field is investigated. Our attention is focused on the influences of single-and two-photon excitations on the dynamics of quantum coherence and quantum entanglement. It is shown that the atomic coherence can be improved or even maintained by the classical driving field, the non-Markovian effect, and the atom-reservoir detuning. The interconversion between the atomic coherence and the atom-reservoir entanglement exists and can be controlled by the appropriate conditions. The conservation of coherence for different partitions is explored, and the dynamics of a system with two-photon excitations is different from the case of single-photon excitation.     

非对称自旋-轨道耦合系统的多体量子相干含时演化

本文以具有非对称性自旋相互作用的三体自旋系统为研究对象,重点研究了三体量子相干含时演化规律.采用精确量子对角化和基于量子主方程的数值模拟方法,讨论了三体量子系统中多种量子相干组分及其退相干.研究发现,量子相干组分的含时演化与整个系统的初态量子特性紧密相关.当初态为可分离纯态时,在较短时间内,非对称相互作用有利于增加多体量子相干度.这些量子相干度因受噪声影响而逐渐衰减.当初态为类Werner态时,量子相干度的分布满足加和性,即三体量子相干度等于所有两体量子相干度之和.自旋之间非对称相互作用和环境噪声都会引起三体量子相干度大于所有两体量子相干度之和.这些结论有助于多体量子资源的制备.     

微波场驱动下V型三能级原子的吸收和色散特性

当用微波场作用到V型三能级原子的两个激发态能级时,系统跃迁路径之间发生交叉耦合导致了量子相干效应.通过调节微波场的强度,可实现对原子吸收和色散性质的改变,并呈现零吸收高折射率现象.此外,微波场诱导的量子相干也可实现相对相位对探测光增益的控制.     

微波场驱动下V型三能级原子的吸收和色散特性

当用微波场作用到V型三能级原子的两个激发态能级时,系统跃迁路径之间发生交叉耦合导致了量子相干效应.通过调节微波场的强度,可实现对原子吸收和色散性质的改变,并呈现零吸收高折射率现象.此外,微波场诱导的量子相干也可实现相对相位对探测光增益的控制.     

从离散Wigner函数的角度探讨量子相干性度量

量子相干性是量子信息处理的基本要素,在量子计算中扮演着重要的角色.为了便于讨论量子相干性在量子计算中的作用,本文从离散Wigner函数角度对量子相干性进行了探讨.首先对奇素数维量子系统的离散Wigner函数进行了分析,分离出表征相干性的部分,提出了一种可能的基于离散Wigner函数的量子相干性度量方法,并对其进行了量子相干性度量规范的分析;同时也比较了该度量与l_1范数相干性度量之间的关系.重要的是,这种度量方法能够明确给出量子相干性程度与衡量量子态量子计算加速能力的负性和之间不等式关系,由此可以解析地解释量子相干性仅是量子计算加速的必要条件.     

光场诱导的原子激光的量子相干性

基于伞量子理论,分别研究了几种重要的光场作用下,从原子玻色-爱因斯坦凝聚(BEC)体耦合输出的原子激光的量子相干特性.结果表明,粒子数态光场诱导的原子激光总是反聚束的,相干态光场诱导的原子激光是任意阶相干的,而压缩相干态光场诱导的原子激光总是聚束的.表明用光场诱导产生的原子激光具有与初始光场完全相间的量子相干性质.     

Models of the Dynamics of Spatially Separated Broadband Electromagnetic Fields Interacting with Resonant Atoms

The Markov model of spontaneous emission of an atom localized in a spatial region with a broadband electromagnetic field with zero photon density is considered in the conditions of coupling of the electromagnetic field with the broadband field of a neighboring space. The evolution operator of the system and the kinetic equation for the atom are obtained. It is shown that the field coupling constant affects the rate of spontaneous emission of the atom, but is not manifested in the atomic frequency shift. The analytic expression for the radiative decay constant for the atom is found to be analogous in a certain sense to the expression for the decay constant for a singly excited localized ensemble of identical atoms in the conditions when the effect of stabilization of its excited state by the Stark interaction with the vacuum broadband electromagnetic field is manifested. The model is formulated based on quantum stochastic differential equations of the non- Wiener type and the generalized algebra of the Ito differential of quantum random processes.     

Quantifying coherence with dynamical discord

Quantum coherence and discord are two kinds of manifestations of nonclassicality. By calculating the coherence and discord in the specific bipartite quantum systems, we show quantitative connections between the coherence and the discord in the bipartite quantum systems created from local systems with the help of incoherent operations. We show that the coherence bounds the dynamical discord, and under particular conditions of the initial quantum states, the coherence of single systems is equal to the dynamical discord. We extend these results to the multipartite quantum systems.     

Quantum Teleportation in Thermal Fluctuating Electromagnetic Field

In this paper, we study the quantum teleportation protocol in fluctuating electromagnetic field. The noisy model of quantum teleportation is constructed and the master equation that governs the evolution is solved. We analyze the effect of temperature and noisy parameter on fidelity and quantum coherence, which give us more freedom in controlling the quantum teleportation. We find that the fidelity has some relations with quantum coherence. Fidelity decay rate is dependent on the atom spontaneous emission rate and temperature. When teleporting a non-maximally coherent state, for different ranges of noisy parameter, fidelity has different variations with temperature, and evolves to different values, higher temperature leading to higher fidelity at last; when teleporting a maximally coherent state, fidelity decays to a fixed value with increasing noisy parameter and temperature.

     

q形变光场与级联型三能级原子相互作用中场的压缩效应

建立了q形变光场与级联型三能级原子相互作用的非线性理论,求得相互作用绘景中薛定谔方程的形式解,利用数值计算讨论了q形变对相互作用中场压缩效应的影响并与q-偶相干光场的情形做了对比分析。研究发现,q偏离1的程度越大,q形变对场压缩效应的调控能力越强,反映出q形变的非线性行为对量子相干性的干扰以及对量子特性的影响。当q→1时,恢复为普通线性理论。和q-偶相干光场的情形做的对比分析,反映出q-奇相干光场不仅不存在压缩效应,还会弱化压缩效应。这与q-偶相干光场和q-奇相干光场存在不同的量子效应的结论完全一致。     

Dynamics and quantum entanglement of two-level atoms in de Sitter spacetime

In the framework of open quantum systems, we study the internal dynamics of both freely falling and static two-level atoms interacting with quantized conformally coupled massless scalar field in de Sitter spacetime. We find that the atomic transition rates depend on both the nature of de Sitter spacetime and the motion of atoms, interestingly the steady states for both cases are always driven to being purely thermal, regardless of the atomic initial states. This thermalization phenomenon is structurally similar to what happens to an elementary quantum system immersed in a thermal field, and thus reveals the thermal nature of de Sitter spacetime. Besides, we find that the thermal baths will drive the entanglement shared by the freely falling atom (the static atom) and its auxiliary partner, a same two-level atom which is isolated from external fields, to being sudden death, and the proper time for the entanglement to be extinguished is computed. We also analyze that such thermalization and disentanglement phenomena, in principle, could be understood from the perspective of table-top simulation experiment.     

Remote Three-Party Quantum State Sharing Based on Three-Atom Entangled States Assisted by Cavity QED and Flying Qubits

We present a remote three-party quantum state sharing (QSTS) schemewith three-atom Greenberger-Horne-Zeilinger (GHZ) states assisted bycavity QED and flying qubits. It exploits some photons to act as the flying qubits for setting up the quantum channel securely with three-atom systems in a GHZ state, which maybe make this remote QSTS scheme more practical than some other schemes based on atom systems only or ion-trap systems as photons interact with their environments weakly. The coherence of the stationary atom qubits in cavities provides the convenience for the parties in QSTS to check eavesdropping, different from entangled photon systems. Moreover, the present scheme works in a collective-noise condition and it may be more practical than others in applications in future.     

Enhancement of charging performance of quantum battery via quantum coherence of bath

An open quantum battery (QB) model of a single qubit system charging in a coherent auxiliary bath (CAB) consisting of a series of independent coherent ancillae is considered. According to the collision charging protocol we derive a quantum master equation and obtain the analytical solution of QB in a steady state. We find that the full charging capacity (or the maximal extractable work (MEW)) of QB, in the weak QB-ancilla coupling limit, is positively correlated with the coherence magnitude of ancilla. Combining with the numerical simulations we compare with the charging properties of QB at finite coupling strength, such as the MEW, average charging power and the charging efficiency, when considering the bath to be a thermal auxiliary bath (TAB) and a CAB, respectively. We find that when the QB with CAB, in the weak coupling regime, is in fully charging, both its capacity and charging efficiency can go beyond its classical counterpart, and they increase with the increase of coherence magnitude of ancilla. In addition, the MEW of QB in the regime of relative strong coupling and strong coherent magnitude shows the oscillatory behavior with the charging time increasing, and the first peak value can even be larger than the full charging MEW of QB. This also leads to a much larger average charging power than that of QB with TAB in a short-time charging process. These features suggest that with the help of quantum coherence of CAB it becomes feasible to switch the charging schemes between the long-time slow charging protocol with large capacity and high efficiency and the short-time rapid charging protocol with highly charging power only by adjusting the coupling strength of QB-ancilla. This work clearly demonstrates that the quantum coherence of bath can not only serve as the role of "fuel" of QB to be utilized to improve the QB's charging performance but also provide an alternative way to integrate the different charging protocols into a single QB.     

High Intensity Generation of Entangled Photons in a Two‐Mode Electromagnetic Field

At present, the sources of entangled photons have a low rate of photon generation. This limitation is a key component of quantum informatics for the realization of such functions as linear quantum computation and quantum teleportation. In this paper, we propose a method for high intensity generation of entangled photons in a two‐mode electromagnetic field. On the basis of exact solutions of the Schrödinger equation, when electrons interact in an atom with a strong two‐mode electromagnetic field, it is shown that there may be large quantum entanglement between photons. The quantum entanglement is analyzed on the basis of the Schmidt parameter. It is shown that the Schmidt parameter can reach very high values depending on the choice of characteristics of the two‐mode fields. We find the Wigner function for the considered case. Violation of Bell's inequalities for continuous variables is demonstrated.     

Radiation Reaction of a Nonrelativistic Quantum Charged Particle

An alternative approach to analyze the nonrelativistic quantum dynamics of a rigid and extended charged particle taking into account the radiation reaction is discussed with detail. Interpretation of the field operators as annihilation and creation ones, theory of perturbations and renormalization are not used. The analysis is carried out in the Heisenberg picture with the electromagnetic field expanded in a complete orthogonal basis set of functions which allows the electromagnetic field to satisfy arbitrary boundary conditions. The corresponding coefficients are the field operators which satisfy the usual commutation relations. A nonlinear equation of motion for the charged particle is obtained. A careful consideration of the quantum effects allows the derivation of a linear equation of motion which is free of both runaway solutions and preacceleration, even for a point charge. Also, the electromagnetic mass, which is defined as the coefficient of the acceleration operator, vanishes for a point particle. However, this does not mean that the results are free of ambiguities which are exhibited and discussed.