Quantum Entanglement and Quantum Discord of a Two-Qubit System in a Dissipative Cavity

The goal of this work is to investigate quantum entanglement and quantum discord of a pair of two-level atoms which is driven by an external classical field and interacts with a cavity field. After extracting density matrix of the atom-atom subsystem, it is shown that we have stronger quantum discord by increasing atom-field coupling constant for the case in which there is no entanglement. Moreover, for the atom-atom subsystem it is realized that quantum entanglement and quantum discord cannot increase, they decrease after passing some times due to cavity dissipation. Also quantum entanglement and quantum discord decrease faster by increasing atom-field coupling constant.     

Exploring Tripartite Quantum Correlations: Entanglement Witness and Quantum Discord

In this study, we explore the tripartite quantum correlations by employing the quantum relative entropy as a distance measure. First, we evaluate the explicit expression for nonlinear entanglement witness (EW) of tripartite systems in the four dimensional space that lends itself to a straightforward algorithm for finding closest separable state (CSS) to the generic state. Then using nonlinear EW with specific feasible regions (FRs), quantum discord is derived analytically for the three-qubit and tripartite systems in the four dimensional space. Furthermore, we explicitly figure out the additivity relation of quantum correlations in tripartite systems.     

Geometric Measure of Quantum Discord for Entanglement of Total Dirac Fields in Noninertial Frames

We study the geometric measure of quantum discord of total Dirac fields in noninertial frames. As a comparison, we also calculate the corresponding geometric measure of entanglement of the same system. We discuss the properties of geometric measure of quantum discord and geometric measure of entanglement for this system with acceleration parameter and the parameter describing the entangle degree of the system in detail. Our results show that from an overall perspective, two geometric measures have similar behavior with the variation of the entangle parameter and the acceleration parameter. We find that this tripartite system is monogamous for the geometric measure of quantum discord.     

Properties of Entanglement between the JC Model and Atom-Cavity-Optomechanical System

We investigate the dynamics of remote entanglement in the system consisting of a Jaynes-Cummings model and an atom-cavity-optomechanical system. Meanwhile, we find that the remote entanglement is related to the initial entanglement degree between two atoms, the coupling strength between atom and cavity. Moreover, the phenomenon of entanglement sudden death appears in the evolution of entanglement for some initial conditions. By investigating the entanglement transfer among two atoms and cavity 1, we show that the entanglement transfer can be realized at some special time. In particular, we evaluate the tripartite entanglement of the two atoms and cavity 1 through π-tangle, and prepare the larger tripartite entanglement. This implementation of tripartite entanglement might provide a potential way to realize improved information processing and distributed quantum computing.

     

Non-Classical Correlations and Transfer of Quantum Information in a Superconducting Qubit System with Dynamical Decoupling Pulses

We investigate the dynamics of non-classical correlations(entanglement and quantum discord) of the system consisting of two non-interacting superconducting qubits coupling with a common data bus, where the system is driven by the dynamical decoupling pulses. It is found that the non-classical correlations between two superconducting qubits can be increased by appling a train of dynamical decoupling pulses. Furthermore, we also explore the influence of the dynamical decoupling pulses on the information flowing between superconducting qubits and data bus by making use of the trace distance. It is shown that the dynamical decoupling pulses can protect quantum information of two superconducting qubits and force information to flow back to the superconducting qubits from the data bus.

     

Quantum correlations dynamics of three-qubit states coupled to an XY spin chain: Role of coupling strengths

We investigate the prominent impacts of coupling strengths on the evolution of entanglement and quantum discord for a three-qubit system coupled to an XY spin-chain environment. In the case of a pure W state, more robust, even larger nonzero quantum correlations can be obtained by tailoring the coupling strengths between the qubits and the environment.For a mixed state consisting of the GHZ and W states, the dynamics of entanglement and quantum discord can characterize the critical point of quantum phase transition. Remarkably, a large nonzero quantum discord is generally retained, while the nonzero entanglement can only be obtained as the system-environment coupling satisfies certain conditions. We also find that the impact of each qubit's coupling strength on the quantum correlation dynamics strongly depends on the variation schemes of the system-environment couplings.     

Multipartite distribution property of one way discord beyond measurement

We investigate the distribution property of one way discord in the multipartite system by introducing the concept of polygamy deficit for one way discord. The difference between one way discord and quantum discord is analogue to the one between entanglement of assistance and entanglement of formation. For tripartite pure states, two kinds of polygamy deficits are presented with the equivalent expressions and physical interpretations regardless of measurement. For four-partite pure states, we provide a condition which makes one way discord polygamy satisfied. In addition, we generalize these results to the case for N$N$-partite pure states. Those results can be applicable to multipartite quantum systems and are complementary to our understanding of the shareability of quantum correlations.     

量子关联与原子间相互作用距离关系的研究

本文将两个二能级原子注入一个腔中,用共生纠缠的方法来度量两原子之间的纠缠并推出它们之间的量子失协,还给出了原子与环境之间的共生纠缠与量子失协的计算公式,讨论了原子自发衰变率的变化对量子纠缠及失协的影响,及不同的初始状态情况下,纠缠及失协随原子距离的演化情况。结果表明：原子自发衰变率减小,原子与环境之间的量子纠缠及失协增加;选择不同的初态,可以控制原子间出现纠缠死亡的现象或量子失协为零的状态。     

量子关联与原子间相互作用距离关系的研究

本文将两个二能级原子注入一个腔中,用共生纠缠的方法来度量两原子之间的纠缠并推出它们之间的量子失协,还给出了原子与环境之间的共生纠缠与量子失协的计算公式,讨论了原子自发衰变率的变化对量子纠缠及失协的影响,及不同的初始状态情况下,纠缠及失协随原子距离的演化情况。结果表明：原子自发衰变率减小,原子与环境之间的量子纠缠及失协增加;选择不同的初态,可以控制原子间出现纠缠死亡的现象或量子失协为零的状态。     

量子关联与原子间相互作用距离关系的研究

本文将两个二能级原子注入一个腔中,用共生纠缠的方法来度量两原子之间的纠缠并推出它们之间的量子失协,还给出了原子与环境之间的共生纠缠与量子失协的计算公式,讨论了原子自发衰变率的变化对量子纠缠及失协的影响,及不同的初始状态情况下,纠缠及失协随原子距离的演化情况.结果表明:原子自发衰变率减小,原子与环境之间的量子纠缠及失协增加;选择不同的初态,可以控制原子间出现纠缠死亡的现象或量子失协为零的状态.     

The Dynamics of Quantum Discord and Entanglement of Three Atoms Coupled to Three Spatially Separate Cavities

In this paper, we study the dynamics of quantum discord and entanglement of three identical two-level atoms simultaneously resonantly interacting with three spatially separate single-mode of high-Q cavities respectively. Taking advantage of the depiction quantum discord and entanglement of formation (EoF), we conclude that the discord and entanglement of atoms and cavities can be mediated by changing some parameters and the maximum values of discord and entanglement are independent on the couplings of cavities and atoms. In particular, there also exists quantum discord sudden death as well as entanglement sudden death and the time interval of the former is shorter than that of the later in the proposed quantum system. It is shown that the discord and entanglement of any two atoms among three atoms can be transferred to the corresponding cavities, and there exists discord and entanglement exchanging between the atoms and the corresponding cavities.     

Correlations for a Family of Two-Ququart Quantum States

In this paper, a quantum correlations analytical treatment for a family of two-ququart states are presented. For this purpose, a family of two-ququart quantum states, as well as a set of von Neumann measurements are proposed. A quantum discord analytical treatment is derived for this family of two-ququart states. Finally, the classical mutual information, the entanglement of formation and quantum discord are illustrated in this family of two-ququart quantum states.

     

Purified discord and multipartite entanglement

We study bipartite quantum discord as a manifestation of a multipartite entanglement structure in the tripartite purified system. In particular, we find that bipartite quantum discord requires the presence of both bipartite and tripartite entanglement in the purification. This allows one to understand the asymmetry of quantum discord, D(A,B)≠D(B,A)$D(A,B)\ne D(B,A)$ in terms of entanglement monogamy. As instructive special cases, we study discord for qubits and Gaussian states in detail. As a result of this we shed new light on a counterintuitive property of Gaussian states: the presence of classical correlations necessarily requires the presence of quantum correlations. Finally, our results also shed new light on a protocol for remote activation of entanglement by a third party.     

Quantum correlations between two non-interacting atoms under the influence of a thermal environment

By considering a double Jaynes-Cummings model, we investigate the dynamics of quantum correlations, such as the quantum discord and the entanglement, for two atoms in their respective noisy environments, and study the effect of the purity and the cavity temperature on the quantum correlations. The results show that the entanglement suffers sudden death and revival, however the quantum discord can still reveal the quantum correlations between the two atoms in the region where the entanglement is zero. Moreover, when the temperature of each cavity is high the entanglement dies out in a short time, but the quantum discord still survives for quite a long time. It means that the quantum discord is more resistant to environmental disturbance than the entanglement at higher temperatures.     

The Transfer and Monogamy of Quantum Correlations for Two Qubits

In this paper the entanglement and the quantum discord (QD) dynamics of two cavities interacting with a common independent reservoir are investigated. Remarkably, it has been proved that the entanglement between two cavities can be transferred to one of the cavities and the reservoir with time evolution. Compared with the dynamics of entanglement, the QD has the similar behavior. It is found that the cavity damping rate can stabilize the entanglement and quantum discord between the cavity and reservoir. We also explore the monogamy of the entanglement and the QD during the interaction of quantum system.     

极性分子摆动态的三体量子关联

极性分子具有较长的相干时间和较强的偶极-偶极相互作用,因此它被视为量子信息处理的有效量子载体.基于分子摆动态作为量子比特,研究了处于热平衡状态下三极性分子线性链系统的三体量子关联特性,分析了三体负性熵纠缠度、测量诱导扰动和三体量子失协随与电场强度、分子电偶极矩、分子转动常数、偶极-偶极相互作用和温度等参数有关的三个无量纲变量之间的变化关系.研究表明:在其他参数给定的情况下,随电场强度的增加,三体量子关联均变小;随偶极-偶极相互作用强度的增大,三体量子关联先增加到峰值再逐渐变小;温度越高,负性熵纠缠度和三体量子失协越小,但测量诱导扰动随温度的改变在电场强度和偶极-偶极相互作用影响下呈现不同的变化趋势.此外,通过调节电场强度、偶极-偶极相互作用和温度,可改变与操控极性分子摆动态的三体量子关联.     

Quantum discord of electron spins of noninteracting electron gases

In this paper, we analyze the quantum discord of electron spins of noninteracting electron gases. Our results imply that the quantum discord depends on the relative distance between the two electrons and the temperature. By a comparison between the quantum discord and the entanglement of formation, we find that the quantum discord is more robust than the entanglement of formation in the sense that the quantum discord takes a zero value only at discrete points of the range of the relative distance whereas the entanglement of formation can disappear for a finite relative distance.     

Some New Properties of Quantum Correlations

Quantum coherence measures the correlation between different measurement results in a single-system, while entanglement and quantum discord measure the correlation among different subsystems in a multipartite system. In this paper, we focus on the relative entropy form of them, and obtain three new properties of them as follows: 1) General forms of maximally coherent states for the relative entropy coherence, 2) Linear monogamy of the relative entropy entanglement, and 3) Subadditivity of quantum discord. Here, the linear monogamy is defined as there is a small constant as the upper bound on the sum of the relative entropy entanglement in subsystems.     

Quantum communication in spin chains with three-site and Dzyaloshinsky–Moriya interactions

We investigate an arbitrary single-qubit state transfer through spin chains with three-site and Dzyaloshinsky–Moriya interactions as well as the quantum discord transfer through double independent spin chains. It is shown that both the three-site and Dzyaloshinsky–Moriya interactions can improve the transfer quality in some cases and the influence of a combination of the three-site and Dzyaloshinsky–Moriya interactions is not simply a linear addition of each contribution. We also find that the three-site coupling may enhance the maximum discord more obviously while the Dzyaloshinsky–Moriya coupling may accelerate the maximum discord more effectively. Furthermore, we show that the quantum discord can always be transferred while the quantum entanglement has a threshold of minimum entanglement to be transferred and may experience entanglement sudden birth and death or even never receive any entanglement for some initial states.     

Property of Quantum Correlations with Correlated Noise

We analyze the classical and quantum correlation properties of the standard and so-called quasiclassical depolarizing channel with correlated noise and non-Markovian dephasing channel, specifically we use the quantum discord, entanglement, and measurement-induced disturbance （MID） to measure the quantum correlations. For the depolarizing channel, we find that the memory effect has more influence on the MID and quantum discord than entanglement. For the dephasing channel, we show that the non-Markovian dephasing channel is more robust than Markovian dephasing channel against deeoherence. We also find that at first MID and quantum discord take different values, and then after a specific time they will take almost the same value and both decay monotonically in the same way.