单模光场与二能级原子的纠缠的内禀退相干

在一个考虑了内禀退相干的单模光场与二能级原子的耦合系统中,平均光子数,激发态的概率和退相干系数如何影响纠缠随时间演化还并不清楚.Jaynes-Cummings模型被用来描述光场与原子间相互作用.用concurrence下限来计算纠缠度.模拟的结果表明纠缠度随着退相干系数的增加或平均光子数的减少而增加.当激发态的概率在0至0.1之间,纠缠度随着的概率增加而减少;当激发态的概率在0.1至1之间,纠缠度随着的概率增加而增加.无论以上参数取何值,纠缠度都随着时间而减少.     

Intrinsic decoherence of entanglement of a single quantized field interacting with a two-level atom

How the mean photon number, the probability of excited state and intrinsic decoherenee coefficient in-fluence the time evolution of entanglement is unknown, when a single-mode quantized optic field and a two-levelatom coupling system is governed by Milburn equation. The Jaynes-Cummings model is considered. A lowerhound of concurrence is proposed to calculate the entanglement. Simulation results indicate that the entanglementof system increases following the increasing of intrinsic decoherence coefficient or the decreasing of the mean photon number. Besides that, the entanglement of system decreases, while the probability of exited state increasesfrom 0 to 0.1, and increases, while the probability of exited state increases from 0.1 to 1.     

Entanglement of two atoms in two-mode Raman coupled model with intrinsic decoherence

Considering intrinsic decoherence, the two-atom two-mode Raman coupled model is investigated in this paper. Utilizing the constants of motion in this model, we obtain the analytic expressions of the density operator of the system for investigating the entanglement of two atoms. The speed of entanglement decay increases with the increasing of the coupling coefficient of one atom. The difference between the oscillation periods when the initial state parameter of atomic subsystem belongs to two intervals becomes smaller with the increasing of the coupling coefficient of one atom. The increasing of the initial photon number of the second field can hasten the vanishing of entanglement of atomic subsystem. The robustness of atomic entanglement against decoherence depends on the interval of the initial state parameter of atomic subsystem.     

双模场与原子相互作用中的量子纠缠和内禀退相干

通过求解系统的Milburn方程，研究了两能级原子与双模SU(1,1)相干态光场发生相互作用系统中，原子与场的纠缠及双模SU(1,1)相干态场的模间纠缠随时间的演化问题，讨论了内禀退相干、双模光子数差等对纠缠度的影响.结果表明，存在内禀退相干时，随着时间的演化，场-原子纠缠逐渐减小到一个确定值，而模间纠缠逐渐增大到一个确定值，两者演化的最终值只取决于双模光子数差和平均光子数，而与内禀退相干因子无关. 关键词： Milburn理论 SU(1 1)相干态 量子约化熵 量子相对熵     

存在相位退相干时原子与光场相互作用的纠缠演化和保持

应用全量子理论研究了存在相位退相干时单模相干光场与一个二能级原子相互作用系统纠缠的时间演化规律;分别讨论了原子—光场耦合常数、光场的平均光子数以及失谐量的大小对场与原子纠缠的影响.结果表明:随着原子—光场耦合常数的增大和光场平均光子数的增加,系统纠缠的振荡频率都会明显增大.不存在相位退相干时,纠缠的时间演化明显受到失谐量的影响,若选取适当的失谐量,系统的纠缠可长时间保持在最大纠缠态.若考虑相位退相干的影响,则在共振情况下系统纠缠的时间演化是一个逐渐衰减的过程,且最终衰减到零;但若存在适当的失谐量,则在初始一段时间内系统的纠缠也是一个波动幅度逐渐衰减的过程,但随着时间的演化,失谐量抵消了相位退相干的影响,使系统的纠缠不再衰减到零.如果增大失谐量,纠缠在初始一段时间内波动的幅度会相应的减小,并且纠缠趋于稳定的时间也随着失谐量的增大而缩短;当失谐量适当时,系统可保持在纠缠相对较大的状态而无消纠缠态.     

Coarse graining makes it hard to see micro-macro entanglement

Observing quantum effects such as superpositions and entanglement in macroscopic systems requires not only a system that is well protected against environmental decoherence, but also sufficient measurement precision. Motivated by recent experiments, we study the effects of coarse graining in photon number measurements on the observability of micro-macro entanglement that is created by greatly amplifying one photon from an entangled pair. We compare the results obtained for a unitary quantum cloner, which generates micro-macro entanglement, and for a measure-and-prepare cloner, which produces a separable micro-macro state. We show that the distance between the probability distributions of results for the two cloners approaches zero for a fixed moderate amount of coarse graining. Proving the presence of micro-macro entanglement therefore becomes progressively harder as the system size increases.     

Dynamics of a Moving Two-Level Atom Under the Influence of Intrinsic Decoherence

We present a general and fascinating problem of quantum entanglement (QE) that is calculated with the help of quantum Fisher information (QFI) and von Neumann entropy (VNE) for moving two-level atomic systems. We calculate numerically the temporal evolution of the state vector of the entire system under the influence of intrinsic decoherence for a moving two-level atom. We demonstrate that the phase shifts of an estimator parameter, intrinsic decoherence, and the atomic motion play an important and prominent role during the time evolution of the atomic system. We observe that there is a monotonic relation between the atomic quantum Fisher information (QFI) and quantum entanglement (QE) in the absence of atomic motion. We also show that at the revival time the local maximum values of QFI decreases gradually. A periodic behavior of QFI is observed in the presence of atomic motion, which becomes more important and remarkable for two-level atomic systems. Moreover, the atomic quantum Fisher information and entanglement demonstrate an opposite response during the time evolution in the presence of atomic motion. We show that the evolution of entanglement is more susceptible to the intrinsic decoherence; a considerable change occurs in the degree of entanglement when the intrinsic decoherence parameter increases. Intrinsic decoherence in the atom–field interaction represses the nonclassical effects of the atomic systems. Both the entanglement and the quantum Fisher information saturate to their lower levels for longer time scales in the presence of intrinsic decoherence. For larger values of intrinsic decoherence, the sudden death of entanglement is observed.     

Entanglement and Pancharatnam Phase for Two Two-Level Atoms Interacting with a Single Mode Field

A model of two 2-level atoms interacts with a single quantized electromagnetic field is considered. We study the effect of the mean photon number and the structure of the initial states of the two atoms on the dynamics of the atomic system from the separability point of view. It is found that, if we start from a product mixed atomic state, the probability of generating long living entangled states is increased as the mean photon number increases. Starting from excited atomic system in product state, one generates a more stable entangled states with high degree of entanglement. Also, the effect of the mean photon number on atomic system prepared initially in entangled states is investigated. It is found that the entangled state generated from the initially partial entangled states are more robust than those obtained from a maximum entangled state. The Pancharatnam phase for the separable and entangled states is studied under the effect of the mean photon number and the structure of the initial state. We find that for the separable states, the collapses decrease and the amplitude of the revivals is smaller than that for the entangled state, so there are long-living entangled phases. This property give us a great chances to store safely information in entangled state.     

Thermal Field and Intrinsic Decoherence Effects on the Dynamics of N-level Moving Atomic System

Dynamical behaviour of Quantum Entanglement (QE) and atomic Quantum Fisher Information (AQFI) for a moving N-level atomic system is studied in a thermal environment. Time evolution of state vector of the entire system interacting with thermal field is calculated numerically in the presence of intrinsic decoherence. It is observed that intrinsic decoherence and thermal environment play dominant role during the time evolution of the quantum system. AQFI and entanglement show an opposite behaviour during its time evolution in the presence of thermal environment. AQFI is observed to be more prone to intrinsic decoherence as compared to the entanglement in a thermal environment. AQFI is found to be more prone to intrinsic decoherence as compared to the QE in a thermal environment. QE is found decaying when the parameter of intrinsic decoherence is increased in the absence of atomic motion. The damping behaviour of QE is observed for longer time-scales. The periodic response of entanglement due to atomic motion becomes moderate under the influence of these environments. The intrinsic decoherence and thermal environment are found to suppress the nonclassical effects of the quantum system. QE and AQFI saturate to a lower level for larger time-scales under the influence of these environments. Furthermore, the dynamics of AQFI and von Neumann entropy (VNE) changes remarkably by changing the mean number of photons.

     

Influence of Intrinsic Decoherence on Entanglement of Superconducting Charge Qubit in a Resonant Cavity

Taking the intrinsic decoherence effect into account, we investigate the entanglement dynamics of a superconducting charge qubit in a single-mode optical cavity. Concurrence, as the measure of entanglement of the coupled field-junction system, is calculated. In comparison, we also consider the entanglement of the system by using the entanglement parameter based on the ratio between mutual entropy and partial Von-Neumann entropy to investigate how the intrinsic decoherence affects the entanglement of the coupling system. Our results show that the evolution of the entanglement parameter has the behaviour similar to the concurrence and it is thus the well measure of entanglement for the mixed state in such a coupling system.     

Entanglement of Two Atoms Induced by a Fock State of Radiation Field

Considering the intrinsic decoherence proposed by Milburn, we investigate the entanglement between two two-level atoms induced by a Fock state of single-mode quantized radiation field. The time-dependent reduced density matrix of the atoms system is given explicitly. Due to the intrinsic decoherence, the atoms system will approach a stationary state, where the stationary entanglement depends on the initial states of the field and the atoms.     

Decoherence of two-qubit system in a non-Markovian squeezed reservoir

The decoherence of two initially entangled qubits coupled with a squeezed vacuum cavity separately is investigated exactly. The results show that, first, in principle, the disentanglement time decreases with the increase of squeeze parameter r, due to the augmenting of average photon number of every mode in the squeezed vacuum cavity. Second, there appear entanglement revivals after the complete disentanglement for the case of even parity initial Bell state, while there occur the entanglement decrease and the entanglement revival before the complete disentanglement for the case of odd parity initial Bell state. The results are quite different from those for the case of qubits in a vacuum cavity.     

内禀退相干下薛定谔猫态光场与原子相互作用系统的场熵演化

通过求解系统的Milburn方程,研究了二能级原子与薛定谔猫态光场相互作用系统中的场熵演化特性.讨论了内禀退相干,光场强度,相干态间的相位角对场熵演化的影响.结果表明: 内禀退相干下,随着时间的演化,场熵振荡逐渐减弱,光场与原子的纠缠度逐渐趋于恒值.并且光场与原子的最大纠缠度值只取决光场强度和相干态间的相位角,与内禀退相干因子无关.光场强度较小时,奇相干态光场与原子的纠缠度最大;偶相干态光场与原子的纠缠度值为最小;Yurke-Stoler相干态光场与原子的纠缠度值介于两者之间. 当内禀退相干因子不变、光场强度较大时,分别处于Yurke-Stoler相干态、偶相干态和奇相干态的光场与原子的纠缠度值趋近于相同.     

内禀退相干下薛定谔猫态光场与原子相互作用系统的场熵演化

通过求解系统的Milburn方程,研究了二能级原子与薛定谔猫态光场相互作用系统中的场熵演化特性.讨论了内禀退相干,光场强度,相干态间的相位角对场熵演化的影响.结果表明: 内禀退相干下,随着时间的演化,场熵振荡逐渐减弱,光场与原子的纠缠度逐渐趋于恒值.并且光场与原子的最大纠缠度值只取决光场强度和相干态间的相位角,与内禀退相干因子无关.光场强度较小时,奇相干态光场与原子的纠缠度最大;偶相干态光场与原子的纠缠度值为最小;Yurke-Stoler相干态光场与原子的纠缠度值介于两者之间. 当内禀退相干因子不变、光场强度较大时,分别处于Yurke-Stoler相干态、偶相干态和奇相干态的光场与原子的纠缠度值趋近于相同.     

Entanglement Dynamics and Bell Violations of Two Atoms in Tavis-Cummings Model with Phase Decoherence

Considering the dipole-dipole coupling intensity between two atoms and the field in the Fock state, the entanglement dynamics between two atoms that are initially entangled in the system of two two-level atoms coupled to a single mode cavity in the presence of phase decoherence has been investigated. The two-atom entanglement appears with periodicity without considering phase decoherence, however, the phase decoherence causes the decay of entanglement between two atoms, with the increasing of the phase decoherence coefficient, the entanglement will quickly become a constant value, which is affected by the two-atom initial state. Meanwhile the two-atom quantum state will forever stay in the maximal entangled state when the initial state is proper even in the presence of phase decoherence. On the other hand, the Bell violation and the entanglement do not satisfy the monotonous relation, a large Bell violation implies the presence of a large amount of entanglement under certain conditions, while a large Bell violation corresponds to a little amount of entanglement in certain situations. However, the violation of Bell-CHSH inequality can reach the maximal value if two atoms are in the maximal entangled state, or vice versa.     

Geometrical quantum discord and negativity of two separable and mixed qubits

We studied quantum correlation and quantum entanglement of a quantum system in which a coherent state light field interacts with two qubits that are initially prepared in a separable and mixed state.The influence of mean photon number of the coherent field and distribution probability of the atom on the geometrical quantum discord and the negativity are discussed.Our results show that the mean photon number of light field and distribution function of the atom can regulate and control the quantum correlation and quantum entanglement.     

A study of color centers in MgO using a hierarchical model

Tripartite entanglement dynamics is studied in terms of a generalized negativity for two kinds of mixed states in the Tavis-Cummings model with intrinsic decoherence, where the arithmetic and geometric mean negativities, which have been widely used to three qubits in recent literature, are included for consistency and comparison. Analytical and numerical simulations for various states in the model without and with intrinsic decoherence as well as without rotating-wave approximation show that the dynamical correlations between those negativities are dominantly positive when the field is initially prepared in the states with more than two photons. Such correlation between the arithmetic and geometric mean negativities is positive for a suitable state. The dependence of the steady-state entanglement on initial conditions demonstrates that the generalized negativity and the arithmetic mean negativity behave in a similar way. Moreover, a hierarchy of those indicators in entanglement dynamics and the steady-state entanglement is the same for various parameters in the model and initial states. Those are useful for tripartite entanglement and quantum information.     

Partial entropy creation of a two-level atom with intrinsic decoherence

By using an algebraic dynamical approach, the atom–field bipartite system in mixed state is employed to investigate the influence of intrinsic decoherence and Stark shift on the partial entropy change and the entanglement. It is shown that, quality of the partial entropy is improved significantly by introducing the intrinsic decoherence and(or)Stark shift. In addition, by the concept of the lower bound of the concurrence, the entanglement between the atom and the field under the influences of intrinsic decoherence and Stark shift is also presented.     

克尔介质中纠缠光与三能级原子作用的光子统计

采用求解薛定谔方程和数值计算方法,研究了克尔介质中双模纠缠相干光场与三能级原子相互作用系统的光子统计性质,分析了双模纠缠相干光场的纠缠程度、克尔介质与光场的耦合强度、双模光的平均光子数和原子基态概率幅对光子统计性质的影响。结果表明：双模纠缠相干光场的纠缠程度对光子统计性质没有明显的影响;克尔效应增强使光子统计性质的时闻演化曲线的振荡频率变大、振荡幅度变小;当保持场模1的平均光子数不变而场模2的平均光子数变小时,光子统计性质的时间演化曲线的振荡幅度变大。     

Entanglement of a two-qubit anisotropic Heisenberg XYZ chain in nonuniform magnetic fields with intrinsic decoherence

Taking the intrinsic decoherence effect into account, this paper investigates the entanglement of a two-qubit anisotropic Heisenberg XYZ model in the presence of nonuniform external magnetic fields by employing the concurrence as entanglement measure. It is found that both the intrinsic decoherence and the anisotropy of the system give a significant suppression to the entanglement. Moreover it finds that the initial state of the system plays an important role in the time evolution of the entanglement, which means that the entanglement of the system is independent of the nonuniformity and uniformity of the magnetic field when the system is in the initial state ｜ψ （0）） = ｜00） and [ψ′ （0）） = m ｜01） ＋ n ｜10）, respectively.