Investigations on quantum correlation of coupled qubits in squeezed vacuum reservoir

In this paper, we investigate the quantum correlation of coupled qubits which are initially in maximally entangled mixed states in squeezed vacuum reservoir. We compare and analyse the effects of squeezed parameters on quantum discord and quantum concurrence. The results show that in squeezed vacuum reservoir, the quantum discord and quantum concurrence perform completely opposite behaviors to the change of squeezed parameters. Quantum discord survives longer with the increase of squeezed amplitude parameter, but entanglement death turns faster on the contrary. The results also indicate that the classical correlation of the system is smaller than quantum discord in vacuum reservoir, while it is bigger than quantum discord in squeezed vacuum reservoir. The quantum discord and classical correlation are more robust than quantum concurrence in the two reservoir environments, which indicates that the entanglement actually is easily affected by decoherence and quantum discord has stronger ability to avoid decoherence in squeezed vacuum reservoir.     

Robustness investigations on quantum entanglement and quantum discord of coupled qubits in squeezed vacuum reservoir

We investigate the quantum discord of coupled qubits in squeezed vacuum reservoir and compare it with the quantum entanglement of system. We find that the quantum discord and entanglement perform completely oppositely with the change of squeezed parameters. Quantum discord survives longer with the increase of squeezed amplitude parameter and entanglement death faster on the contrary. Under high squeezed amplitude parameter, the quantum discord can keep nonzero which indicate that the quantum discord is more robust than entanglement. We also find that the purity reduction of the initial quantum state will lead to the decay of concurrence or quantum discord. However, the quantum discord damps remarkably more slowly and survives longer than concurrence.     

Dynamics control of geometric quantum discord for two coupling qubits in a squeezed vacuum reservoir

We investigate the dynamics of geometric quantum discord of coupled qubits in a squeezed vacuum reservoir. The results show that there is distinct difference between the dynamics of geometric quantum discord and that of quantum entanglement near (or away from) the decoherence free subspace. We also find that the squeezed vacuum reservoir with high squeezed amplitude is more suitable for geometric quantum discord to survive. The robustness of geometric quantum discord is stronger than that of quantum entanglement.     

Quantum and Classical Correlations in Isotropic XY Chain with Three-Site Interaction

The quantum phase transition in the isotropic XY chain with three-site interaction has been studied by calculating the quantum discord, classical correlation, and concurrence measuring entanglement. It is found that the quantum discord is a better choice than concurrence to signal the presence of the quantum phase transition in this model, since that for next-nearest neighbor spins the derivative of the quantum discord still exhibits singularity at the critical point while there is no more entanglement.     

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.     

压缩真空态中原子与场的量子纠缠和量子discord

在压缩真空库中,研究了原子与场的相互作用下二比特体系的量子discord和量子纠缠的动力学行为.重点分析了原子的初始态系数和压缩真空库的压缩系数对量子纠缠和量子discord的影响.通过数值分析我们得到,随着原子的初始态系数和压缩真空库的压缩系数值的增加,量子纠缠和量子discord都会减小,但量子纠缠比量子discord减小的更快一些.最后研究了在原子的初始态系数和压缩真空库的压缩系数的值相同的情况下,量子discord比量子纠缠存在的时间更长些.     

Quantum and classical correlations for a two-qubit X structure density matrix

We derive explicit expressions for quantum discord and classical correlation for an X structure density matrix. Based on the characteristics of the expressions, the quantum discord and the classical correlation are easily obtained and compared under different initial conditions using a novel analytical method. We explain the relationships among quantum discord, classical correlation, and entanglement, and further find that the quantum discord is not always larger than the entanglement measured by concurrence in a general two-qubit X state. The new method, which is different from previous approaches, has certain guiding significance for analysing quantum discord and classical correlation of a two-qubit X state, such as a mixed state.     

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.     

Comparison and control of the robustness between quantum entanglement and quantum correlation in an open quantum system

We investigate the influence of environmental decoherence on the dynamics of a coupled qubit system and quantum correlation.We analyse the relationship between concurrence and the degree of initial entanglement or the purity of initial quantum state,and also their relationship with quantum discord.The results show that the decrease of the purity of an initial quantum state can induce the attenuation of concurrence or quantum discord,but the attenuation of quantum discord is obviously slower than the concurrence’s,correspondingly the survival time of quantum discord is longer.Further investigation reveals that the robustness of quantum discord and concurrence relies on the entanglement degree of the initial quantum state.The higher the degree of entanglement,the more robust the quantum discord is than concurrence.And the reverse is equally true.Birth and death happen to quantum discord periodically and a newborn quantum discord comes into being under a certain condition,so does the concurrence.     

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。     

Modulation of entanglement and quantum discord for circuit cavity QED states

The paper investigates the dynamic evolution behaviors of entanglement and quantum discord of coupled superconducting qubits in circuit QED system. We put emphasis on the effects of cavity field quantum state on quantum entanglement and quantum correlations dynamic behaviors of coupling superconducting qubits. The results show that, (1) generally speaking, the entanglement will appear the death and new birth because of the interaction between qubits and cavity field, on the contrary, this phenomenon will not appear in quantum discord. (2) When the cavity field is in coherent state, the entanglement survival time is controlled by the average photon number. The more the average photon number is, the longer survival time of entanglement is prolonged. Thus it has the benefit of keeping quantum correlations. (3) When the cavity field is in squeezed state, the squeezed amplitude parameters have controlling effects on quantum correlations including entanglement and quantum discord. On the one hand, the increase of squeezed amplitude parameters can prolong the survival time of entanglement, on the other hand, with the increase of squeezed amplitude parameters, the robustness of quantum discord is more and more superior to concurrence and is more advantage to keep the system quantum correlations. The further study results show that the increase of the initial relative phase of coupling superconducting qubits can also keep the quantum correlations.     

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.     

Quantum correlation dynamics of three non-coupled two-level atoms in different reservoirs

Time evolution dynamics of three non-coupled two-level atoms independently interacting with their reservoirs is solved exactly by considering a damping Lorentzian spectral density.For three atoms initially prepared in Greenberger-Horne-Zeilinger-type state,quantum correlation dynamics in a Markovian reservoir is compared with that in a nonMarkovian reservoir.By increasing detuning quantity in the non-Markovian reservoir,three-atom correlation dynamics measured by negative eigenvalue presents a trapping phenomenon which provides long-time quantum entanglement.Then we compare the correlation dynamics of three atoms with that of two atoms,measured by quantum entanglement and quantum discord for an initial robuster-entangled type state.The result further confirms that quantum discord is indeed different from quantum entanglement in identifying quantum correlation of many bodies.     

Entanglement dynamics of non-interacting two-qubit system under a squeezed vacuum environment

Entanglement dynamics of two non-interacting atoms in a squeezed vacuum reservoir is studied. Several examples with different initial entangled states are investigated, and it is found that entangled atoms become disentangled faster in squeezed vacuum than in ordinary vacuum, and larger squeezing results in faster entanglement decay. The time evolution of the concurrence and the separability "distance" A can be used to explain this novel entanglement sudden death phenomenon.     

Quantum Entanglement Dynamics of Two Atoms in Quantum Light Sources

Quantum entanglement dynamics of two Tavis-Cummings atoms interacting with the quantum light sources in a cavity is investigated. The results show the phenomenon that the concurrence disappears abruptly in a finite time, which depends on the initial atomic states and the properties of squeezed states. We find that there are two decoherence-free states in squeezed vacuum fields: one is the singlet state, and the other entangled state is the state
that combines both excited states and ground states with a relative phase being equal to the phase of the squeezed state.     

Tavis-Cummings模型中的几何量子失协特性

采用几何量子失协的计算方法,通过改变两原子初始状态、腔内光子数和偶极-偶极相互作用强度,研究了Tavis-Cummings模型中的几何量子失协特性.结果表明:几何量子失协都是随时间周期性振荡的,选取适当的初态可以使两原子一直保持失协状态,增加腔内光子数和偶极相互作用对几何量子失协有积极的影响.     

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.     

Stability investigation on quantum correlations of coupled qubits in non-Markovian process

Utilizing the concurrence and the quantum discord as the measure method, in this paper we compare and investigate the dynamic evolution features of quantum correlations of coupled qubits in non-Markovian process. We focus attention on decoherence effect influences the stability of quantum correlations. The investigation results show that because of the decoherence influence between the system and environment, the concurrence always evolves with time in oscillation form in the way of deaths and survivals, however, the quantum discord time evolution does not appear the deaths and survivals. The quantum discord survives obviously longer than concurrence, which indicates that quantum discord has a stronger ability to resist decoherence than entanglement. Through regulating and controlling the purity and entanglement of the initial quantum state, we can effectively suppress the decay of the quantum correlations, which is advantaged to complete the quantum information processing.