Entanglement dynamics of W-like states in an open system

<正>The pairwise entanglement dynamics in a multipartite open system consisting of three entangled cavity photons locally coupled with independent N-mode reservoirs is studied via concurrence.The initial states of cavity photons are prepared in two types of W-like states while the corresponding reservoirs are prepared in the factorable vacuum state.The result shows that all the pairwise concurrences of the total system including cavities and reservoirs undergo qualitatively different dynamical behaviors.Among the two W-like states,only one could exhibit entanglement sudden death(ESD) leading the corresponding reservoirs to exhibit entanglement sudden birth.In addition,by taking the entanglement of the corresponding reservoirs into account,entanglement invariants are constructed for the W-like state that does not undergo ESD.     

Entanglement Dynamics of Three Qubits in the Non-Markovian Environments

We consider the entanglement dynamics of three two-level atoms interacting with independent structured non-Markovian reservoirs. The atoms are initially prepared in a mixed multipartite Greenberger—Horne—Zeilinger （GHZ）-type state. Based on the purity and non-Markovian reservoirs, we show that the reservoirs＇ entanglement can exhibit not only multipartite entanglement sudden birth but also entanglement sudden death, and the revival of atom entanglement is not always accompanied by the disentanglement of reservoirs. Meanwhile, we derive the quantitative criteria for multipartite revival and death phenomena and find multipartite entanglement sudden death and entanglement sudden birth simultaneous apparition time is independent of the initial three-qubit state.     

Entanglement dynamics between an isolated atom and a moving atom in the cavity

The entanglement dynamics between an isolated atom and a moving atom interacting with a cavity field is investigated. The results show that there appears sudden death of entanglement between the isolated atom and the moving atom and that the time of entanglement sudden death (ESD) is independent of the initial state of the system. It is interesting that the isolated atom can also entangle with a cavity field, though they do not interact with each other originally, which stems from the fact that the entanglement between the isolated atom and the moving atom may turn into the entanglement between the isolated atom and the cavity.     

Entanglement Evolution of Three-Qubit States under Local Decoherence

By using negativity as entanglement measure, we have investigated the effect of local decoherence from a non-Markovian environment on the time evolution of entanglement of three-qubit states including the GHZ state, the W state, and the Werner state. From the results, we find that the entanglement dynamics depends not only on the coupling strengths but also on the specific states of concern. Specifically, the entanglement takes different behaviors under weak or strong coupling and it varies with the quantum states under study. The entanglement of the GHZ state and the Werner state can be destroyed completely by the local decoherence, while the entanglement of the W state can survive through the local decoherence partially.     

Tripartite states Bell-nonlocality sudden death in a spin environment with multisite interaction

This paper demonstrates that multipartite Bell-inequality violations can be fully destroyed in a finite time in three-qubit states coupled to a general XY spin-chain with a three-site interaction environment.The Mermin-Ardehali-Belinksii-Klyshko inequality is used to detect the degree of nonlocality,as measured by the extent of their violations.The effects of system-environment couplings,the size of degrees of freedom of the environment and the strength of the three-site interaction on the Bell-inequality violations are given.The results indicate that the Bell-inequality violations of the tripartite states will be completely destroyed by decoherence under certain conditions for the GHZ state.The decoherence-free subspaces of our model are identified and the entanglement of quantum states is also discussed.     

Ground-State Entanglement and Mixture in an XXZ Spin Chain

We study the pairwise entanglement and mixture of a three-qubit XX Z spin chain in the ground state in the presence of an external magnetic field B. The effects of the magnetic field, the anisotropy and the temperature on the entanglement and mixture are considered, and entanglement versus the mixture of all the two-spin states is investigated. We find that the maximal entangled mixed state can be obtained in the considered system by controlling the magnetic field. Our results provide another way to generate maximally entangled mixed states.     

Entanglement of two two-level atoms trapped in coupled cavities with a Kerr medium

In this paper, the entanglement dynamics of two two-level atoms trapped in coupled cavities with a Kerr medium is investigated, We find that the phenomena of entanglement sudden death （ESD） and entanglement sudden birth （ESB） appear during the evolution process. The influences of initial atomic states, Kerr medium, and cavity-cavity hopping rate on the atom-atom entanglement are discussed. The results obtained by the numerical method show that the atom- atom entanglement is strengthened and even prevented from ESD with increasing cavity-cavity hopping rate and Kerr nonlinearity.     

Preserving entanglement and the fidelity of three-qubit quantum states undergoing decoherence using weak measurement

We demonstrate a method to preserve entanglement and improve fidelity of three-qubit quantum states undergoing amplitude-damping decoherence using weak measurement and quantum measurement reversal. It is shown that we are able to enhance entanglement to the greatest extent, and to circumvent entanglement sudden death by increasing the weak measurement strength both for the GHZ state and the W state. The weak measurement technique can also enhance the fidelity to the quantum region and even close to 1 for the whole range of the decoherence parameter in both of the two cases. In addition, the W state can maintain more fidelity than the GHZ state in the protection protocol. However, the GHZ state has a higher success probability than the W state.     

Entanglement evolution of a two-qubit system with decay beyond the rotating-wave approximation

The entanglement property of two identical atoms, initially entangled in Bell states, coupled to a single-mode cavity is considered. Based on the reduced non-perturbative quantum master equation method, the entanglement evolution of the two atoms with decay is investigated beyond the conventional rotating-wave approximation. We show that the counter-rotating wave terms, usually neglected, have a great influence on the disentanglement behaviour of the system. The phenomena of entanglement sudden death and entanglement sudden birth will occur. In addition, we show that the entanglement can be strengthened by introducing the dipole--dipole interaction of the two atoms.     

Sudden birth versus sudden death of entanglement for the extended Werner-like state in a dissipative environment

In this paper, we investigate the dynamical behaviour of entanglement in terms of concurrence in a bipartite system subjected to an external magnetic field under the action of dissipative environments in the extended Werner-like initial state. The interesting phenomenon of entanglement sudden death as well as sudden birth appears during the evolution process. We analyse in detail the effect of the purity of the initial entangled state of two qubits via Heisenberg XY interaction on the apparition time of entanglement sudden death and entanglement sudden birth. Furthermore, the conditions on the conversion of entanglement sudden death and entanglement sudden birth can be generalized when the initial entangled state is not pure. In particular, a critical purity of the initial mixed entangled state exists, above which entanglement sudden birth vanishes while entanglement sudden death appears. It is also noticed that stable entanglement, which is independent of different initial states of the qubits (pure or mixed state), occurs even in the presence of decoherence. These results arising from the combination of the extended Werner-like initial state and dissipative environments suggest an approach to control and enhance the entanglement even after purity induced sudden birth, death and revival.     

Entanglement evolution in multipartite cavity-reservoir systems under local unitary operations

We analyze the entanglement evolution of two cavity photons being affected by the dissipation of two individual reservoirs. Under an arbitrary local unitary operation on the initial state, it is shown that there is only one parameter which changes the entanglement dynamics. For the bipartite subsystems, we show that the entanglement of the cavity photons is correlated with that of the reservoirs, although the local operation can delay the time at which the photon entanglement disappears and advance the time at which the reservoir entanglement appears. Furthermore, via a new defined four-qubit entanglement measure and two three-qubit entanglement measures, we study the multipartite entanglement evolution in the composite system, which allows us to analyze quantitatively both bipartite and multipartite entanglement within a unified framework. In addition, we also discuss the entanglement evolution with an arbitrary initial state.     

三量子比特Dicke模型中的两体和三体纠缠动力学

本文利用绝热近似方法和精确对角化方法研究三量子比特Dicke模型中的纠缠动力学.处于两种典型的纠缠态GHZ态和W态上的量子比特在时间演化过程中与辐射光场发生强耦合作用,在各种子系统间产生纠缠,通过分析这些纠缠的演化特性发现初始GHZ态的三体纠缠鲁棒性比W态强,这与旋波近似结论一致.与旋波近似下结果不同的是,两种态中任意一对量子比特间的纠缠都随时间演化到几乎为零,而三体纠缠随时间周期演化,且纠缠程度相对较强,说明系统中的强耦合作用通过抑制量子比特中的对纠缠来支持三体纠缠.     

Disentanglement,Bell-nonlocality violation and teleportation capacity of the decaying tripartite states

Dynamics of disentanglement as measured by the tripartite negativity and Bell nonlocality as measured by the extent of violation of the multipartite Bell-type inequalities are investigated in this work. It is shown definitively that for the initial three-qubit Greenberger–Horne–Zeilinger (GHZ) or W class state preparation, the Bell nonlocality suffers sudden death under the influence of thermal reservoirs. Moreover, all the Bell-nonlocal states are useful for nonclassical teleportation, while there are entangled states that do not violate any Bell-type inequalities, but still yield nonclassical teleportation fidelity.     

An algebraic approach to the study of multipartite entanglement

We introduce a simple algebraic approach to the study of multipartite entanglement for pure states together with a class of suitable functionals able to detect the entanglement. On this basis, we reproduce some known results. Indeed, by investigating the properties of the introduced functionals, we show that a subset of such class is strictly connected to the purity. Moreover, we provide a direct and basic solution to the problem of simultaneous maximization of three appropriate functionals for three-qubit states, confirming that the simultaneous maximization of the entanglement for all possible bipartitions is compatible only with the structure of the GHZ states.     

Entanglement Evolution of Three-Qubit States under Local Decoherence

By using negativity as entanglement measure, we have investigated the effect of local decoherence from a non-Markovian environmenton the time evolution of entanglement of three-qubit states including the GHZ state, the W state, and the Werner state. From the results, we find that the entanglement dynamics depends not only on the coupling strengths but also on the specific states of concern. Specifically, the entanglement takes different behaviors under weak or strong coupling and it varies with the quantum states under study. The entanglement of the GHZ state and the Werner state can be destroyed completely by the local decoherence, while the entanglement of the W state can survive through the local decoherence partially.     

Implement a measurement of GHZ entanglement for a multipartite system via cavity QED

We propose a scheme for implementing a measurement of GHZ entanglement for a multipartite system via cavity QED. In the scheme the atoms interact simultaneously with a highly detuned cavity mode with the assistant of a classical field. The scheme is insensitive to the cavity decay and the thermal field. A set of GHZ states can be exactly distinguished via detecting atomic state in a simple way.     

Generation of Different Classes of Multipartite Entanglement Using Cavity-QED

Using a system of cavity quantum electrodynamics(QED) we present two schemes for multipartite entanglement generation. In the first scheme, a three-level atom is interacting with three cavities successively. In the second one, two three-level atoms are interacted with a coherent optical cavity. These protocols allow us to generate the six classes of tripartite entanglement(GHZ, W, A-B-C, AB-C, C-AB, and B-AC class states) by controling the interaction time between atoms and cavities. Moreover, they allow us to generate entanglement between the cavity fields degrees of freedom(from the first scheme), and a mixed entanglement between the cavity field degrees of freedom and the atomic degrees of freedom.     

Monogamy Relations for Squared Entanglement Negativity

This paper contains two main contents. In the first part, we provide two counterexamples of monogamy inequalities for the squared entanglement negativity: one three-qutrit pure state which violates of the He-Vidal monogamy conjecture, and one four-qubit pure state which disproves the squared-negativity-based Regula-Martino-Lee-Adesso-class strong monogamy conjecture. In the second part, we investigate the sharing of the entanglement negativity in a composite cavity-reservoir system using the corresponding multipartite entanglement scores, and then we find that there is no simple dominating relation between multipartite entanglement scores and the entanglement negativity in composite cavity-reservoir systems. As a by-product, we further validate that the entanglement of two cavity photons is a decreasing function of the evolution time, and the entanglement will suddenly disappear interacting with independent reservoirs.