Entanglement and Its Dynamics with Atomic Spontaneous Decay

The atomic decay for a two-level atom interacting with a single mode of electromagnetic tield is considered. For a chosen initial state, the exact solution of the master equation is found. Therefore, effect of the atomic damping on entanglement （purity loss）, degree of entanglement by the negativity, mutual information and atomic coherence through the master equation are studied.     

Markov Quantum Feedback Control Based on Homodyne Measurement of a Two-Qubit System

We consider the system consisting of two qubits collectively damped,with the output being unit-efficiency measured and subsequently fedback to control the system state. Our primary goal in this paper is (i) to solve the feedback-modified master equation, (ii) to demonstrate the ability of feedback control based on the solutions, and (iii) to pick out different steady states by choosing different driving strengths and feedback strengths tocounteract the effects of both damping and the measurement back-action on the system. We further investigate some properties of the equilibrium steady state, its distribution probability and entanglement vs. the driving and feedback amplitudes. We find that in our feedback model feedback plays a negative role in producing entanglement.     

Numerical Study of Some Statistical Quantities for Quantum Systems Under Damping Effects

We present a model of interaction between a four-level atom and the cavity field initially prepared in the coherent state in the presence of the phase damping effect. We discuss the atom–field entanglement and statistical properties under the damping effect in view of numerical calculations. We use the Mandel parameter as a quantifier of the statistical properties of the field; moreover, we study the different effects of the collective parameters in the master equation on the dynamical behavior of the field statistical properties and the entanglement measured by the negativity. Finally, we explore the link between the entanglement and statistical properties in view of the numerical results during the time evolution.     

利用置于光腔中的非线性耦合器制备压缩和连续变量纠缠光

利用置于光腔中的非线性耦合器制备了压缩和连续变量纠缠光.通过特征函数方法解析求解了双模腔场满足的主方程.分析发现：在一定的条件下,此系统能产生稳定的双模压缩和连续变量纠缠光,且压缩和纠缠的强度与系统的耦合参量及腔场损耗系数密切相关.     

Influence of phase damping on the entanglement for the damped JC model in the pure and mixed states

In this article, we investigate the effects of phase damping on the temporal evolution of different entanglement measurements and the amount of entanglement for the damped Jaynes-Cummings model. The master equation is solved for any initial state in the two-level atom. Superpositions of two coherent states (π/2 or π out of phase) and their statistical mixture state are taken as initial states when a phase-damped cavity is taken into account.     

Protecting Distribution Entanglement by Weak Measurement and Reversal under Various Decoherence Sources

Protection of entanglement from disturbance of the environment is an essential task marion processing. We examine the validity and limitation of the weak measurement and reversal in quantum infor- （WMR） operation in the protection of distributed entanglement from various decoherence sources. Since the entanglement variation can be investigated analytically for an arbitrarily entangled bipartite pure state under three kinds of typical noisy quantum channels, we show explicitly that the WMR operation indeed helps for protecting distributed entanglement from ampli- tude damping and phase damping, but not for depolarizing. Bxperimental feasibility for testing our results is discussed using current laboratory techniques.     

多粒子纠缠的保护方案

量子纠缠是量子信息的重要物理资源. 然而当量子系统与环境相互作用时, 会不可避免地产生消相干导致纠缠下降, 因此保护纠缠不受环境的影响具有重要意义. 振幅衰减是一种典型的衰减机制. 如果探测环境保证没有激发从系统中流出, 即视为对系统的一种弱测量. 本文基于局域脉冲序列和弱测量, 提出了一种可以保护多粒子纠缠不受振幅衰减影响的有效物理方案, 保护的对象是在量子通信和量子计算中发挥重要作用的Cluster态和Maximal slice态.     

Purity and Correlation of a Cavity Field Interacting with a SC Charge Qubit with a Lossy Cavity

A single-mode microwave cavity field, coupled to its reservoir, interacting generally with a superconducting charge qubit is considered. Using a certain canonical transformation for the qubit states, the system is transformed into the usual Jaynes-Cummings model. The solution of the master equation of this system, in the case of a high-Q cavity is obtained. The temporal evolution of the population inversion is explored. The effects of cavity damping on the purity of the qubit, the field and the global system state are studied. It is found that due to the coupling between the system and environment, the purity is lost. The entanglement is compared with total correlation. It is found that, with the damping parameter, the asymptotic value of the correlation measure is not null, since the global system evolves to a classically correlated state. The negativity is used as an indicator of the degree of entanglement between the qubit and the field. The results indicate the sensitivity of these aspects to change of the damping parameter.     

Entanglement sudden death of a SC-qubit strongly coupled with a quantized mode of a lossy cavity

The dynamics of entanglement and mixedness of a superconducting qubit strongly coupled to a cavity field induced by a cavity damping governed by a master equation are investigated. It is found that, asymptotic decays as well as finite time disentanglement depend on the parameter of the dissipation, which leads to the existence of the entanglement sudden death.     

Entanglement dynamics of a moving multi-photon Jaynesben Cummings model in mixed states

Using the algebraic dynamical method, the entanglement dynamics of an atom-field bipartite system in a mixed state is investigated. The atomic center-of-mass motion and the field-mode structure are also included in this system. We find that the values of the detuning and the average photon number are larger, the amplitude of the entanglement is smaller, but its period does not increase accordingly. Moreover, with the increase of the field-mode structure parameter and the transition photon number, the amplitude of the entanglement varies slightly while the oscillation becomes more and more fast. Interestingly, a damping evolution of the entanglement appears when both the detuning and the atomic motion are considered simultaneously.     

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.     

Entanglement Dynamics Induced by a Squeezed Coherent Cavity Coupled Nonlinearly with a Qubit and Filled with a Kerr-Like Medium

An analytical solution for a master equation describing the dynamics of a qubit interacting with a nonlinear Kerr-like cavity through intensity-dependent coupling is established. A superposition of squeezed coherent states is propped as the initial cavity field. The dynamics of the entangled qubit-cavity states are explored by negativity for different deformed function of the intensity-dependent coupling. We have examined the effects of the Kerr-like nonlinearity and the qubit-cavity detuning as well as the phase cavity damping on the generated entanglement. The intensity-dependent coupling increases the sensitivity of the generated entanglement to the phase-damping. The stability and the strength of the entanglement are controlled by the Kerr-like nonlinearity, the qubit-cavity detuning, and the initial cavity non-classicality. These physical parameters enhance the robustness of the qubit-cavity entanglement against the cavity phase-damping. The high initial cavity non-classicality enhances the robustness of the qubit-cavity entanglement against the phase-damping effect.     

Generation of Three-Partite Macroscopic Entanglement in a Ring Cavity

We propose a feasible scheme to generate tripartite macroscopic entanglement with three-mode optomechanical interactions. It shows how steady-state tripartite entanglement can be generated by means of radiation pressure. More importantly, we quantify the bipartite entanglement in each field-mirror subsystem and in the mirror-mirror one. We then discuss the influence of the parameters (the frequency of the mirrors, the coupling strength, damping of the cavity and environmental temperature) on tripartite entanglement. It is found that the high intensity of tripartite entanglement between two subsystems can be achieved.     

Effect of phase-damped cavity on dynamics of tangles of a nondegenerate two-photon JC model

A two-mode cavity field coupled to a two-level atom and damped by the environment through a phase-damped process is considered. For a chosen initial state, the effects of phase damping on the purity loss of the global system and different bipartite partitions of the system (atom-two modes, mode-(atom-mode)) through the tangles are considered. In particular, the effect of phase damping on the amount of entanglement between atom and field is evaluated by the negativity.     

Separability criteria and entanglement witnesses for symmetric quantum states

We study the separability of symmetric bipartite quantum states and show that a single correlation measurement is sufficient to detect the entanglement of any bipartite symmetric state with a non-positive partial transpose. We also discuss entanglement conditions and entanglement witnesses for states with a positive partial transpose.     

Inseparability of Phase-Damped Bipartite Gaussian States

For bipartite non-Gaussian states which are prepared by applying the phase damping to Gaussian states, we use Fock subspace inseparable criterion and the Shchukin-Vogel inseparable criterion to analyze their inseparability. The lowest order of the two criteria is obtained. Fock subspace criterion is more efficient in detecting the entanglement of damped Gaussian state by numerical results. Two photon Fock subspace criterion does not improve the entanglement detecting condition.     

Non-Markovian entanglement transfer to distant atoms in a coupled superconducting resonator

We investigate the non-Markovian effects on the entanglement transfer to the distant non-interacting atom qubits,which are embedded in a coupled superconducting resonator. The master equation governing the dynamics of the system is derived by the non-Markovian quantum state diffusion(NMQSD) method. Based on the solution, we show that the memory effect of the environment can lead to higher entanglement revival and make the entanglement last for a longer time. That is to say, the non-Markovian environment can enhance the entanglement transfer. It is also found that the maximum entanglement transferred to distant atoms can be modified by appropriately selecting the frequency of the modulated intercavity coupling. Moreover, with the initial anti-correlated state, the entanglement between the cavity fields can be almost completely transferred to the separated atoms. Lastly, we show that the memory effect has a significant impact on the generation of entanglement from the initial non-entangled states.     

Cavity-mediated stationary atom–mirror entanglement in the presence of photothermal effects

We study stationary entanglement properties of an optomechanical system containing an atomic ensemble. We focus onto the case of the movable mirror strongly coupled to the cavity field through both radiation pressure and photothermal force. Exploiting a quantum Langevin equation approach we investigate the bipartite entanglement properties of various bipartite subsystems as well as stationary tripartite entanglement of the system. We particularly study robustness of the atom–mirror entanglement against temperature. We show that, even though the photothermal force is a dissipative force, it can significantly improve the cavity mediated atom–mirror entanglement.     

Locking entanglement with a single qubit

We study the loss of entanglement of a bipartite state subjected to discarding or measurement of one qubit. Examining behavior of different entanglement measures, we find that entanglement of formation, entanglement cost, logarithmic negativity, and one-way distillable entanglement are lockable measures in that they can decrease arbitrarily after measuring one qubit. We prove that any convex and asymptotically noncontinuous measure is lockable. As a consequence, all the convex-roof measures can be locked. The relative entropy of entanglement is shown to be a nonlockable measure.     

Quantitative complementarity relations in bipartite systems: Entanglement as a physical reality

We introduce a complete set of complementary quantities in bipartite, two-dimensional systems. Complementarity then relates the quantitative entanglement measure concurrence which is a bipartite property to the single-particle quantum properties predictability and visibility, for the most general quantum state of two qubits. Consequently, from an interferometric point of view, the usual wave-particle duality relation must be extended to a “triality” relation containing, in addition, the quantitative entanglement measure concurrence, which has no classical counterpart and manifests a genuine quantum aspect of bipartite systems. A generalized duality relation, that also governs possible violations of the Bell’s inequality, arises between single- and bipartite properties.