Quantum Discord in Optical Coherent States

We study the dynamics of quantum correlation of optical coherent-state qubits affected by the environment. It consists in sending these states via a decohering quantum channel. The states used as the support of the encoding information are affected by an amplitude damping channel. The quantum discord is one of type of quantum correlations between the qubits. The discord and its dynamics of two qubits in non-Markovian environments are evaluated.     

Detecting Quantum Dissonance and Discord in Exact Dynamics of qubit Systems

In this paper, we evaluate the quantum and classical correlations in exact dynamics of qubit systems interacting with a common dephasing environment. We show the existence of a sharp transition between the classical and quantum loss of correlations during the time evolution. We show that it is possible to exploit a large class of initial states in different tasks of quantum information and processing without any perturbation of the correlations from the environment noisy for large time intervals. On the other hand, we include the dynamics of a new kind of correlation so-called quantum dissonance, which contains the rest of the nonclassical correlations. We show that the quantum dissonance can be considered as an indicator to expect the behavior of the dynamics of classical and quantum correlations in composite open quantum systems.     

Quantum correlations of helicity entangled states in non-inertial frames beyond single mode approximation

A helicity entangled tripartite state is considered in which the degree of entanglement is preserved in non-inertial frames. It is shown that Quantum Entanglement remains observer independent. As another measure of quantum correlation, Quantum Discord has been investigated. It is explicitly shown that acceleration has no effect on the degree of quantum correlation for the bipartite and tripartite helicity entangled states. Geometric Quantum Discord as a Hilbert–Schmidt distance is computed for helicity entangled states. It is shown that living in non-inertial frames does not make any influence on this distance, either. In addition, the analysis has been extended beyond single mode approximation to show that acceleration does not have any impact on the quantum features in the limit beyond the single mode. As an interesting result, while the density matrix depends on the right and left Unruh modes, the Negativity as a measure of Quantum Entanglement remains constant. Also, Quantum Discord does not change beyond single mode approximation.     

Quantum Discord for Two-Qubit System in a Symmetry-Broken Environment

In contrast with the entanglement, we study the quantum discord dynamics of the two-qubit system in a symmetry-broken environment consisting of a fermionic bath. The quantum discord decay induced by the bath is analysed. By considering the two qubits that are initially prepared in the different X-states, we find that the behaviors of quantum discord and entanglement are different, the robustness of quantum discord depends on the initial state prepared in.     

Entanglement and Quantum Discord Dynamics of Two Atoms in a Broadband Squeezed Vacuum Bath

We study the dynamics of entanglement and quantum discord between two two-level atoms that interact with a common squeezed reservoir. It is shown that the degree of entanglement and quantum discord are very sensitive to the degree of two-photon correlation for large values of the mean photon number. The squeezed vacuum environment can drive the system to a stationary state with high entanglement and quantum discord for certain X-type states. Furthermore, sudden change happens to the dynamics of quantum discord while the entanglement is remained almost unchanged.     

Classical Correlation and Quantum Discord in a Two Qubit System Under Dissipation Environments

We study the dynamics of classical correlation and quantum discord of two-qubit system interacted with the thermal reservoir. Special attention is paid to the difference between the entanglement and quantum discord when considering the influences of the nonzero mean photon number and quantum fluctuation of the vacuum. It is shown that in the same range of the physical parameters, the factors leading to the entanglement sudden death only accelerates the decay of quantum discord, and the quantum discord can last for an infinite period when the entanglement disappears. So the quantum discord is more robust than entanglement under this decoherence environment, and quantum discord is a more general measure of quantum correlation than entanglement.     

耗散环境下二比特体系的量子关联性质研究

利用Quantum Discord(QD)判据,研究了与热库相互作用的二比特体系的量子关联性质.讨论了在不同初态下体系量子关联随时间的变化,以及热库的平均光子数m,n和原子的自发辐射率γ对体系量子关联性质的影响.结果表明,在不同的初态下,体系可以得到不同性质的量子关联;而且当原子自发辐射率γ取固定值时,QD的衰减会随m,n取值的减小而减慢,热库平均光子数都为零的情况下能够得到最大范围的量子关联;此外,当热库平均光子数m,n取确定值时,随着γ取值的减小,QD的衰减也会随之减慢,此时同样会得到较大范围的量子关联.说明较小的热库平均光子数以及原子自发辐射率γ能够减弱QD的衰减,从而获得生命力较强的体系量子关联.     

Quantum Discord of two Coupled Qubits with Initial System-reservoir Correlation

Recently, the effect of initial qubit-reservoir correlation for two qubits in a common reservoir on the entanglement dynamics has been studied by Yan and Xia (Acta Sinica Quantum Optica 20, 16 2014). We extend their results and investigate how the initial qubit-reservoir correlation and dipole-dipole interaction between two qubits effect on dynamics of quantum discord in Markovian and non-Markovian regimes, respectively. The results show that in general, the larger initial qubit-reservoir correlation and dipole-dipole interaction can retard the decay of quantum discord. Besides, a combination of relatively strong dipole-dipole interaction and non-Markovian effect can efficiently protect quantum discord. Finally,thecomparisonbetweenevolutionsofquantumdiscordandentanglementisalsoconsidered.     

Dynamics of Quantum Discord in Asymmetric and Local Non-Markovian Environments

The non-Markovian decoherence of quantum and classical correlationsis analytically obtained when two qubits are asymmetrically subjected to the bit flip channel and phase flip channel. For one class of initial mixed states, quantum correlations quantified by quantum discord decay synchronously with classical correlations. The discovery that the decaying rates of quantum and classical correlations suddenly change at the characteristic time is physically interpreted by the distance from quantum state to the closest classical states. In a large time interval, quantum correlations are greater than classical correlations. The quantum and classical correlations can be preserved over a longer period of time via the kernel characterizing the environment memory effects.     

Dynamics of Quantum Discord of Two-Qubit Coupled with a Vacuum Cavity

The dynamical behaviors of quantum discord between two atoms coupled with a vacuum cavity are investigated. If the two qubits are initially prepared in two extended Werner-like states, the quantum discord and entanglement can be numerically calculated. There are remarkable differences between the time evolutions of the quantum discord and entanglement under the same conditions. These results imply that quantum discord is not zero for some unentangled states and in some regions entanglement can disappear completely. A large amount of quantum discord exists between the two-qubit. Thus, the quantum discord is more robust than entanglement for the quantum system exposed to the environment. The quantum discord shows sudden change and its existence depends on the initial state of the system. This property of quantum discord may have important implications for experimental characterization of quantum phase transitions.     

Quantum Discord in Nuclear Magnetic Resonance Systems at Room Temperature

We review the theoretical and the experimental researches aimed at quantifying or identifying quantum correlations in liquid-state nuclear magnetic resonance (NMR) systems at room temperature. We first overview, at the formal level, a method to determine the quantum discord and its classical counterpart in systems described by a deviation matrix. Next, we describe an experimental implementation of that method. Previous theoretical analysis of quantum discord decoherence had predicted the time dependence of the discord to change suddenly under the influence of phase noise. The experiment attests to the robustness of the effect, sufficient to confirm the theoretical prediction even under the additional influence of a thermal environment. Finally, we discuss an observable witness for the quantumness of correlations in two-qubit systems and its first NMR implementation. Should the nature, not the amount, of the correlation be under scrutiny, the witness offers the most attractive alternative.     

非马尔科夫环境中各向异性海森堡自旋链的量子失协

利用量子失协方法研究在非马尔科夫环境中具有时变磁场的两比特各向异性海森堡XYZ模型量子失协的动力学演化。海森堡XYZ系统的初始态为最大纠缠态 $\left|\psi_{A B}\right\rangle=(1 / \sqrt{2})(|11\rangle+|00\rangle)$ , 利用非马尔科夫量子态扩散方法解析求解非马尔科夫主方程, 得出系统的约化密度矩阵; 然后代入量子失协公式得出系统量子失协的演化动力学。讨论自旋耦合强度、环境关联系数γ和余弦磁场强度B对量子失协动力学的影响。研究发现: 当环境关联系数γ较小时, 系统的量子失协明显呈现上升趋势, 因此可以表明非马尔科夫环境具有增加系统量子失协的作用。同时较大的自旋耦合系数J和J_Z以及余弦磁场强度B也具有增加系统量子失协的作用。     

Decomposition Properties of Quantum Discord

The quantum discord was introduced by Ollivier,Zurek,Henderson,and Vedral as an indicator of the degree of quantumness of mixed states.In this paper,we provide a decomposition condition for quantum discord.Moreover,we show that under the condition,the quantum correlations between the quantum systems can be captured completely by the entanglement measure.Finally,we present examples of our conclusions.     

Steady Quantum Discord Behavior for Two Qubits Heisenberg XYZ Chain with Intrinsic Decoherence

By taking into account the intrinsic decoherence and the nonuniform magnetic field, quantum discord (QD) and steady quantum discord (SQD) behavior of a two-qubit anisotropic Heisenberg XYZ chain with different initial states are investigated. We find that properly tuning the external and self parameters not only can improve the quantum correlation and steady quantum correlation but also can weaken the effects of decoherence such as increasing anisotropic parameter Δ, decreasing B or b. When t is infinity, the SQD value and the physical about the SQD phenomenon are studied in detail, the SQD value is strongly dependent on the external and self parameters, which is increased evidently by increasing anisotropic parameter and decreasing nonuniform field. Through analyzing the physical about SQD phenomenon, the conditions about the existence of SQD phenomenon are analyzed with different initial states. These investigations can imply us more control parameters on quantum correlation and steady quantum correlation in solid state systems.     

Quantum Discord of Two-Qubit Thermal State in Hydrogen Atom

The geometric measure of quantum discord (GMQD) is used to study the quantum correlation in a two-qubit thermal state in hydrogen atom. External magnetic field can destroy the initial symmetry of the state and further change its degeneracy structure. The dependence of the quantum correlation on temperature and magnetic field is discussed.     

Quantum Discord of Two-Qubit in Dephasing Model

We evaluate the dynamics of two-qubit quantum discord(QD) under the classical phase noise. We compare the dynamics of QD with that of entanglement as measured with concurrence. The influence of mixture degree on the dynamics is also discussed. The results show that there is no simple relation between the quantum correlation and entanglement as seen that QD may be smaller or larger than entanglement, and QD is more robust than the entanglement.     

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.     

Entanglement, Quantum Discord, and Non-locality in Bell-Diagonal States

Experimental approach to characterize the non-locality, entanglement, and quantum correlation of a multiparity quantum system is one of the important subjects in quantum information theory. Here, by investigating the violations of Bell inequality (BI), we analyze the relations among the non-locality, concurrence C, and quantum discord Q typically for a family of Bell-diagonal states. It is shown that, for the optimal measurement basis the BI is always violated, if the quantum discord is larger than 0.5031 and the concurrence is larger than 0.5605. Certainly, the BI is maximally violated for the maximal entanglement and quantum discord, i.e., C=Q=1. Our generic results are demonstrated with a thermal XY model of the two-qubit system with controllable interbit couplings.