Atomic coherence control on the entanglement of two atoms in two-photon processes

Considering two identical two-level atoms interacting with a single-mode thermal field through two-photon processes, this paper studies the atomic coherence control on the entanglement between two two-level atoms, and finds that the entanglement is greatly enhanced due to the initial atomic coherence. The results show that the entanglement can be manipulated by changing the initial parameters of the system, such as the superposition coefficients and the relative phases of the initial atomic coherent state and the mean photon number of the cavity field.     

Dynamical Creation of Entanglement and Steady Entanglement Between Two Spatially Separated Λ-Type Atoms

We report possibility of generating entanglement and steady entanglement between two identical atoms in free space with a very natural way when their spatial separation is on the order of wavelength or less. We show a dynamical creation of entanglement and steady entanglement due to the radiative coupling with different separable initial atomic states and study the entanglement properties about this atomic subsystem. Not only the creation of steady state entanglement is decided by the initial atomic states, but also the magnitude of the entanglement and the steady state entanglement are found to be strongly dependent on the initial states. We derive a master equation for the atomic subspace and solve it analytically to show how the spontaneous emission from the two atoms system induces entanglement and steady entanglement, the crossing coupling terms in master equation can enhance the entanglement value.     

Majorana表象下的纠缠动力学

借助在Majorana表象下提出的纠缠度定义, 研究了对称双阱中两个玻色原子组成的系统的纠缠动力学. 得到了系统的任意态在Majorana 表象下的表示, 并考察了原子间相互作用及系统初始状态对纠缠度动力学的影响. 发现原子间相互作用会决定纠缠度振荡的频率, 而纠缠度振荡的幅度对系统的初态很敏感.     

Sudden death of entanglement in the two-mode cavity field

<正>The entanglement dynamics involving the so-called entanglement sudden death of atoms in two-photon Tavis-Cummings model is investigated.Various initial conditions that may have influences on the entanglement evolution of atoms,especially on the appearance of atomic entanglement sudden death,are studied. The appearance of entanglement sudden death is sensitive to the initial conditions of the whole system, i.e.,the concrete type of atomic initial state,the photon number in the cavity field,and the dipole-dipole interaction between atoms.It is shown that the strong dipole-dipole interaction between atoms can weaken the atomic entanglement sudden death.     

Thermal Entanglement Between a Jaynes-Cummings Atom and an Isolated Atom

We studied the entanglement of a quantum system consisting of a Jaynes-Cummings atom, thermal lossless cavity and an isolated atom. The analytical expressions of the atom-atom negativity for separable and entangled initial atomic states were obtained. The influence of a detuning between the atomic transition frequency and the field frequency and direct dipole-dipole interaction on an atom-atom entanglement is examined. We showed that for a separable initial atomic states a detuning might cause high atom-atom entanglement in the presence of the dipole-dipole interaction. We also obtained that for an entangled initial atomic state a detuning causes a stabilization of an entanglement oscillations both for model with dipole-dipole interaction and model without such interaction.     

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.     

混合态下运动原子与相干态光场相互作用的量子纠缠

利用负熵方法,研究了混合态运动原子与相干态光场相互作用系统的量子纠缠特性,讨论了原子初态、场模结构参数、相干场平均光子数、失谐量、跃迁光子数等物理参量对系统纠缠度的影响。结果表明：考虑原子运动时,系统纠缠度在整个时域范围内出现了规则的周期振荡。原子初态趋于纯态时系统纠缠度较高。随着相干场平均光子数的增大,系统纠缠度的峰值逐渐变小,规则振荡的周期不变。随着跃迁光子数的增大,系统纠缠度的峰值逐渐变大,振荡变得越来越快。随着失谐量的增大,系统纠缠度的峰值逐渐变小。     

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.     

混合态下运动原子与相干态光场相互作用的量子纠缠

利用负熵方法,研究了混合态运动原子与相干态光场相互作用系统的量子纠缠特性,讨论了原子初态、场模结构参数、相干场平均光子数、失谐量、跃迁光子数等物理参量对系统纠缠度的影响。结果表明：考虑原子运动时,系统纠缠度在整个时域范围内出现了规则的周期振荡。原子初态趋于纯态时系统纠缠度较高。随着相干场平均光子数的增大,系统纠缠度的峰值逐渐变小,规则振荡的周期不变。随着跃迁光子数的增大,系统纠缠度的峰值逐渐变大,振荡变得越来越快。随着失谐量的增大,系统纠缠度的峰值逐渐变小。     

Three-body entanglement induced by spontaneous emission in a three two-level atoms system

We study three-body entanglement induced by spontaneous emission in a three two-level atoms system by using the entanglement tensor approach. The results show that the amount of entanglement is strongly dependent on the initial state of the system and the species of atoms. The three-body entanglement is the result of the coherent superposition of the two-body entanglements. The larger the two-body entanglement is, the stronger the three-body entanglement is. On the other hand, if there exists a great difference in three two-body entanglement measures, the three-body entanglement is very weak. We also find that the maximum of the two-body entanglement obtained with nonidentical atoms is greater than that obtained with identical atoms via adjusting the difference in atomic frequency.     

Coherence-Enhanced Entanglement Induced by a Two-Mode Thermal Field

Considering two identical two-level atoms interacting with two mode thermalfield through a nondegerate two-photon process, we study the entanglement dynamics between two atoms when the atomic coherence exists. It shows that the entanglement is dependent on the initial atomic states, and is greatlyenhanced due to atomic coherence as compared with the case when the atomic coherence is ignored. The results also show that the entanglement can be controlled by changing the relative phases and the amplitudes of the polarized atoms.     

Dissipation of System and Atom in Two-Photon Jaynes–Cummings Model with Degenerate Atomic Levels

A method of perturbative expansion of master equation is employed to study the dissipative properties of system and of atom in the two-photon Jaynes–Cummings model (JCM) with degenerate atomic levels. The numerical results show that the degeneracy of atomic levels prolongs the period of entanglement between the atom and the field. The asymptotic value of atomic linear entropy is apparently increased by the degeneration. The amplitude of local entanglement and disentanglement is suppressed. The better the initial coherence property of the degenerate atom, the larger the coherence loss.     

Effects of Purity on Dynamics of Multipartite Entanglement

We study the effects of purity on entanglement dynamics of a multipartite system in cavity QED. Three two-level atoms A, B and C are initially prepared in an entangled state and locally coupled with independent cavities a, b and c, respectively. We consider the effects of purity of atomic initial state on the entanglement evolution of both the systems of atoms and cavities. It is found that depending on the purity of atomic initial state, the entanglement of atoms （cavities） may or may not exhibit the sudden death （birth） phenomenon.     

Analysis of mixed state entanglement with intrinsic decoherence

We analyze different entanglement measures for a mixed state two-level system in the presence of intrinsic decoherence. The information about entanglement is obtained by comparing the results for the atomic Wehrl entropy and negativity with the analytical results for a simple case. For the strong decoherence case we find that a similar and long-lived maximum Wehrl entropy and negativity between atom and field are shown. The results highlight the important roles played by both the decoherence parameter and the initial state setting in determining the evolution of the atomic Wehrl entropy and negativity.     

Tavis-Cummings模型中三体纠缠态纠缠量的演化特性

研究两个全同二能级原子与单模场相互作用的Tavis-Cummings模型中的量子纠缠.在场和两原子初始分别为真空场和纠缠或非纠缠状态下，得出体系状态将演化为三体近似W纠缠态.通过对纠缠量的计算得出：该三体近似W纠缠态纠缠量的演化特性将随三体所处的初始状态、原子间及原子与场间的耦合系数、失谐程度、原子反转粒子数的变化而变化.值得一提的是可得出原子间耦合作用强度对纠缠量的非线性效应，并且纠缠量展现出与原子反转粒子数一致的周期振荡现象. 关键词： W纠缠态 纠缠量 反转粒子数     

Atom-photon entanglement in the system with competing k-photon and l-photon transitions

We have investigated the evolution of the atomic quantum entropy and the entanglement of atom-photon in the system with competing k-photon and l-photon transitions by means of fully quantum theory, and examined the effects of competing photon numbers （k and l）, the relative coupling strength between the atom and the two-mode field （A/g）, and the initial photon number of the field on the atomic quantum entropy and the entanglement of atom-photon. The results show that the multiphoton competing transitions or the large relative coupling strength can lead to the strong entanglement between atoms and photons. The maximal atom-photon entanglement can be prepared via the appropriate selection of system parameters and interaction time.     

Three-mode entanglement via atomic coherence induced by strong classical field

We propose a new scheme to achieve fully three-mode entanglement based on the standard criteria [P. van Loock and A. Furusawa, Phys. Rev. A 67, 052315 (2003)] in a four-level atomic system driven by two strong classical fields. Via numerically simulating the dynamics of the system, we investigate the generation and evolution of entanglement. Based on our scheme, it is demonstrated that the three-mode continuous-variable (CV) entanglement can be achieved under different initial conditions and the entangled period will be extended by enhancing the intensity of the classical field. Moreover, our numerical results also show that the present system can be considered as a three-mode entanglement amplifier.     

Effect of mode mode-competition on atom-atom entanglement

A system consisting of two atoms interacting with a two-mode vacuum is considered, where each atom is resonant with the two cavity modes through two different competing transitions. The effect of mode--mode competition on the atom--atom entanglement is investigated. We find that the entanglement between the two atoms can be induced by the mode--mode competition. For the initial atomic state |\varPsi(0)\rangle, whether the atoms are initially separated or entangled, a large or even maximal entanglement between them can be obtained periodically by introducing the mode--mode competition. For the initial atomic state |\varPhi(0)\rangle, the strong mode--mode competition can prevent the two atoms entangled initially from suffering entanglement sudden death; besides, it makes them in a more stable and longer-lived entanglement than in the non-competition case.     

Quantum Entanglement in a System of Two Spatially Separated Atoms Coupled to the Thermal Reservoir

We study quantum entanglement between two spatially separated atoms coupled to the thermal reservoir. The influences of the initial state of the system, the atomic frequency difference and the mean number of the thermal field on the entanglement are examined. The results show that the maximum of the entanglement obtained with nonidentical atoms is greater than that obtained with identical atoms. The degree of entanglement is progressively decreased with the increase of the thermal noise. Interestingly, the two atoms can be easily entangled even when the two atoms are initially prepared in the most mixed states.     

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。