两Jaynes-Cummings原子间的纠缠动力学和转移特性

采用两种不同的纠缠度量方法(并发度和负值度),研究了两Jaynes-Cummings原子之间的纠缠演化以及各子系统之间的纠缠转移,分析了两原子之间初始纠缠度对纠缠的影响.结果表明纠缠的幅值依赖于初始纠缠度,而解纠缠时间长度与初始纠缠度无关.制备了两个腔场之间的最大纠缠态,数值分析显示两原子之间的初始纠缠流入了其它各个子系统,导致演化过程中的纠缠突然死亡和纠缠突然产生现象.     

Entanglement evolution of a two-qubit system interacting with a quantum spin environment

We investigate the entanglement dynamics and decoherence of a two-qubit system under a quantum spin environment at finite temperature in the thermodynamics limit. For the case under study, we find different initial states will result in different entanglement evolution, what deserves mentioning here is that the state |Ψ=cosα|01+sinα|10 is most robust than other states when π/2<α<π, since the entanglement remains unchanged or increased under the spin environment. In addition, we also find the anisotropy parameter Δ can suppress the destruction of decoherence induced by the environment, and the undesirable entanglement sudden death arising from the process of entanglement evolution can be efficiently controlled by the inhomogeneous magnetic field ζ.     

三个两能级原子与数态场相互作用的纠缠特性

通过计算并发度和线性熵研究了初始处于GHZ态的三个两能级原子与数态场相互作用系统的纠缠动力学特性,讨论了场的初始光子数对并发度和线性熵的影响.结果表明,腔内两原子之间的纠缠出现突然产生现象,而且可以通过改变场的初始光子数来控制产生纠缠的阈值时间和纠缠的最大值.对腔外原子的探测导致了并发度随时间的演化呈现周期性规律,场的光子数的增加不但减小了并发度的最大值,而且缩短了并发度的演化周期.     

三个两能级原子与数态场相互作用的纠缠特性

通过计算并发度和线性熵研究了初始处于GHZ态的三个两能级原子与数态场相互作用系统的纠缠动力学特性,讨论了场的初始光子数对并发度和线性熵的影响.结果表明,腔内两原子之间的纠缠出现突然产生现象,而且可以通过改变场的初始光子数来控制产生纠缠的阈值时间和纠缠的最大值.对腔外原子的探测导致了并发度随时间的演化呈现周期性规律,场的光子数的增加不但减小了并发度的最大值,而且缩短了并发度的演化周期.     

Entanglement in Bipartite Generalized Coherent States

Entanglement of formation in a class of bipartite generalized coherent states is discussed. It is shown that a positive parameter can be associated with these bipartite states so that the states with equal value for the parameter are of equal entanglement. For the class of states considered, the maximum possible entanglement of one ebit is attained if the value of the positive parameter is . It is shown that the entanglement of formation is one ebit when the relative phase between the composing states is π in the class of bipartite generalized coherent states considered.     

双光子Jaynes-Cummings模型中的纠缠演化和热纠缠态现象

文中利用共生纠缠度研究了双光子Jaynes-Cummings模型中原子与腔纠缠的时间演化和有限温度下系统热纠缠态.结果表明,腔场中原子与腔展现出周期性的纠缠演化过程,演化周期随原子与腔耦合强度的增大而减小;在有限温度下,系统的共生纠缠度随温度的升高而降低,当趋近临界温度时,系统纠缠现象消失,这一临界温度值与原子-腔耦合强度成正比.对于典型的实验数据,临界温度大约在～10-5 K数量级.     

Quantum Entanglement in Two-Photon Tavis–Cummings Model with a Kerr Nonlinearity

The properties of quantum entanglement in the two-photon Tavis–Cummings model with a Kerr nonlinearity are studied in terms of quantum information entropy theory. The reduced quantum entropy is employed to investigate the quantum entanglement between two two-level atoms and a single-mode coherent field. The relative quantum entropy is employed to investigate the quantum entanglement between the two two-level atoms. The influences of the nonlinear interaction of the Kerr medium with the field and the atomic dipole-dipole interaction on the properties of quantum entanglement of the system are also examined. Some important results are obtained.     

Stationary State Entanglement of Two Atoms Inside an Optical Cavity under Noise

In this paper, the entanglement properties of a system of two atoms inside an optical cavity in a stochastic interaction with field are studied by the Jaynes-Cummings Model. The phase telegraph noise is considered as a noise term and an exact solution to the model is obtained. The solution reveals the resulting decoherence effects of the noise on the entanglement properties of the system. It shows that under the noise the individual atoms do not entangle with the cavity field. However, a strong atom-atom entanglement is observed in a stationary state. It is seen that a relatively strong noise is cooperative in the construction of the steady state atom-atom entanglement.     

Entanglement dynamics of two-bipartite system under the influence of dissipative environments

An experimental scheme is suggested that permits a direct measure of entanglement in a two-qubit cavity system. It is realized in the cavity-QED technology utilizing atoms as flying qubits. With this scheme we generate two different measures of entanglement, namely logarithmic negativity and concurrence. The phenomenon of sudden death entanglement (ESD) in a bipartite system subjected to dissipative environment is examined. We show that the sudden death time of the entangled states depends on the initial preparation of the entangled state and the temperature of the reservoir.     

Sudden birth of entanglement between two atoms successively passing a thermal cavity

We study the dynamics of entanglement of two initially separate atoms passing through a cavity one after another by employing the concurrence and negativity. The effects of the atomic coherence and mean photon number on the time evolution of atom-atom entanglement are examined when the field is initially in thermal field. We show that the phenomenon of sudden birth of entanglement occurs in some certain conditions and the threshold time for the creation of the entanglement can be controlled by the atomic coherence and mean photon number of the field. It is also shown that the entanglement between two atoms can be created even if the two atoms are initially in excited states.     

Relative entropy of entanglement of two-qubit ’X’ states

Two closest single-qubit states could be diagonalised by the same unitary matrix,which helps to find the relative entropy of entanglement of a two-qubit ’X’ state.We formulate two binary equations for the relative entropy of entanglement and the corresponding closest separable state of a given two-qubit ’X’ state.This approach can be applied to get the relative entropy of entanglement of many widely-discussed two-qubit states,such as pure states,Werner states,and so on.     

Entanglement, phase correlation and dephasing of two-qubit states

The relation between entanglement and phase correlation of two-qubit states is studied. Decoherence of entanglement and phase correlation caused by correlated classical noises is also investigated. It is found that the decay of the phase correlation is quite different from that of the entanglement when the finite-time disentanglement occurs.     

Entanglement between two qubits interacting with a slightly detuned thermal field

In this paper, we investigate entanglement between two two-level atoms when they simultaneously interact with a single-mode thermal field in the strong-coupling regime. We show that a slight detuning between the atomic transition frequency and the field frequency might cause high entanglement between the atoms. More interestingly, if we choose the detuning appropriately, both atoms would somehow get entangled even when both atoms are initially in the excited state.     

Effect of the time-dependent coupling on a superconducting qubit-field system under decoherence: Entanglement and Wehrl entropy

The dynamics of a superconducting (SC) qubit interacting with a field under decoherence with and without time-dependent coupling effect is analyzed. Quantum features like the collapse–revivals for the dynamics of population inversion, sudden birth and sudden death of entanglement, and statistical properties are investigated under the phase damping effect. Analytic results for certain parametric conditions are obtained. We analyze the influence of decoherence on the negativity and Wehrl entropy for different values of the physical parameters. We also explore an interesting relation between the SC-field entanglement and Wehrl entropy behavior during the time evolution. We show that the amount of SC-field entanglement can be enhanced as the field tends to be more classical. The studied model of SC-field system with the time-dependent coupling has high practical importance due to their experimental accessibility which may open new perspectives in different tasks of quantum formation processing.     

Entanglement witnesses and geometry of entanglement of two-qutrit states

We construct entanglement witnesses with regard to the geometric structure of the Hilbert-Schmidt space and investigate the geometry of entanglement. In particular, for a two-parameter family of two-qutrit states that are part of the magic simplex, we calculate the Hilbert-Schmidt measure of entanglement. We present a method to detect bound entanglement which is illustrated for a three-parameter family of states. In this way, we discover new regions of bound entangled states. Furthermore, we outline how to use our method to distinguish entangled from separable states.     

Entanglement dynamics of two-qubit systems in different quantum noises

The entanglement dynamics of two-qubit systems in different quantum noises are investigated by means of the operator-sum representation method.We find that,except for the amplitude damping and phase damping quantum noise,the sudden death of entanglement is always observed in different two-qubit systems with generalized amplitude damping and depolarizing quantum noise.     

Almost perfect transport of an entangled two-qubit state through a spin chain

We show that using a slightly modified XX model for a spin-1/2 chain, one can transmit almost perfectly a maximally entangled two-qubit state from one end of the chain to the other one. This is accomplished without external fields or modulation of the coupling constants among the qubits. We also show that this strategy works for any size of the chain and is relatively robust to imperfections in the coupling constants among the qubits belonging to the chain. Actually, under certain scenarios of small disorder, we obtain better results than those predicted by the optimal ordered and noiseless case.     

Entanglement dynamics of spatially close bipartite atomic systems in thermal environment

We investigate the entanglement dynamics in a bipartite atomic system subjected to thermal environment with arbitrary initial pure entangled states. We consider the atoms close together and study the effect of temperature of the reservoir and the interatomic distance on the evolution of entanglement for both initially entangled and unentangled states. We find that we can have long time entanglement even in thermal environment.     

Entanglement entropy for the n-sphere

We calculate the entanglement entropy for a sphere and a massless scalar field in any dimensions. The reduced density matrix is expressed in terms of the infinitesimal generator of conformal transformations keeping the sphere fixed. The problem is mapped to the one of a thermal gas in a hyperbolic space and solved by the heat kernel approach. The coefficients of the logarithmic term in the entropy for 2 and 4 spacetime dimensions are in accordance with previous numerical and analytical results. In particular, the four-dimensional result, together with the one reported by Solodukhin, gives support to the Ryu–Takayanagi holographic ansatz. We also find that there is no logarithmic contribution to the entropy for odd spacetime dimensions.