Degree of Entanglement for Some Bipartite Entangled Bosonic Systems

We calculate the degree of entanglement for some bipartite entangled states of continuous variables.These states include common two-mode squeezed vacuum state, thermal vacuum state of a free single particle (where theh fictitious tilde system is regarded as another particle), and the squeezed vacuum state of two coupling harmonic oscillators.The degree of entanglement for these quantum systems are shown clearly by using the technique of integration within an ordered product of operators.     

Entanglement and Quantum Discord Dynamics of Two Atoms in a Broadband Squeezed Vacuum Bath

We study the dynamics of entanglement and quantum discord between two two-level atoms that interact with a common squeezed reservoir. It is shown that the degree of entanglement and quantum discord are very sensitive to the degree of two-photon correlation for large values of the mean photon number. The squeezed vacuum environment can drive the system to a stationary state with high entanglement and quantum discord for certain X-type states. Furthermore, sudden change happens to the dynamics of quantum discord while the entanglement is remained almost unchanged.     

Quantum entanglement and nonlocality properties of two-mode Gaussian squeezed states

Quantum entanglement and nonlocality properties of a family of two-mode Gaussian pure states have been investigated. The results show that the entanglement of these states is determined by both the two-mode squeezing parameter and the difference of the two single-mode squeezing parameters. For the same two-mode squeezing parameter, these states show larger entanglement than the usual two-mode squeezed vacuum state. The violation of Bell inequality depends strongly on all the squeezing parameters of these states and disappears completely in the limit of large squeezing. In particular, these states can exhibit much stronger violation of local realism than two-mode squeezed vacuum state in the range of experimentally available squeezing values.     

双模压缩真空态和纠缠相干态的一维势垒散射

分析了双模压缩真空态和纠缠相干态的一维势垒散射问题,结果表明,两种散射态的纠缠度都随势垒的透射系数呈类抛物线关系,且抛物线谷底对应半反半透势垒.前者的纠缠度还随入射压缩因子呈类抛物线关系,后者随入射纠缠呈线性关系.两种散射的保真度都可以由透射系数的增加而得到提高,但前者还会随入射压缩因子的增大而降低,后者随入射相干态模的增加而降低.     

依赖强度耦合双模Jaynes-Cummings 模型的纠缠和场熵演化

应用von Neumann熵研究了依赖强度耦合双模Jaynes-Cummings模型中原子与光场的纠缠度。给出了两模初始场均处于相干态和双模压缩真空态两种情况下熵演化的数值结果。讨论了初始场强度对原子―场纠缠度的影响。发现在强场条件下，原子与光场基本稳定在最大纠缠态但伴随着周期性的脉冲式解纠缠。     

Statistical properties of photon-added and photon-subtracted two-mode squeezed vacuum state

We introduce photon-added and photon-subtracted two-mode squeezed vacuum states and examine the quantum statistical properties of these states by analytic calculating. Because of the entanglement between the modes of the field in a two-mode squeezed state, photon adding (or subtracting) for one of the modes results in interesting outcome.     

利用原子-腔场喇曼相互作用制备纠缠压缩真空态

提出了利用量子态腔场与原子的喇曼相互作用制备纠缠压缩真空态的方案.在该方案中,一个初始制备在基态的原子被依次送入几个初始制备在压缩真空态的微腔中.通过控制原子的运行速度,使原子与每一个腔具有相同的相互作用时间.当原子与腔场发生相互作用,原子与腔场产生纠缠之后,进行原子的测量.当原子被测量处于基态或激发态时,按照量子力学波包塌缩原理,腔场态将塌缩到相应的纠缠压缩真空态.对纠缠压缩真空态的纠缠性质也进行了简略的讨论.     

单-双模组合压缩热态的纠缠性质及在量子隐形传态中的应用

基于单-双模组合压缩真空态一定范围内能够获得压缩增强的效果,引入单-双模组合压缩热态(DSMST),讨论其纠缠性质.利用Weyl编序算符在相似变换下的不变性,简洁方便地导出了DSMST的纠缠度-负对数值,并给出了当热效应存在时保持纠缠的条件.研究表明:与通常的双模压缩态相比,随着参数的增加,DSMST的纠缠度增加.作为DSMST的应用,利用其实现相干态的量子隐形传输.结果表明:不同于纠缠度随压缩参数增加,保真度获得改善是有条件的,该条件恰好就是一正交分量涨落出现压缩增强的参数区域.此外,解析推导了有效隐形传输保真度(1/2)的条件.     

Quantum Entanglement Dynamics of Two Atoms in Quantum Light Sources

Quantum entanglement dynamics of two Tavis-Cummings atoms interacting with the quantum light sources in a cavity is investigated. The results show the phenomenon that the concurrence disappears abruptly in a finite time, which depends on the initial atomic states and the properties of squeezed states. We find that there are two decoherence-free states in squeezed vacuum fields: one is the singlet state, and the other entangled state is the state
that combines both excited states and ground states with a relative phase being equal to the phase of the squeezed state.     

Entanglement Swapping of Pair Coherent States with Phase Decoherence

We investigate the entanglement swapping of continuous state and the two-mode squeezed vacuum which is exposed variable using the pair coherent state as the input in a phase decoherence environment as the quantum channel. By adopting the log-negativity as the measure of entanglement, we analyze how entanglement of the two initial states and the phase decoherence environment affect the entanglement swapping quality.     

Coherence-controlled stationary entanglement between two atoms embedded in a bad cavity injected with squeezed vacuum

We investigate the entanglement between two atoms in an overdamped cavity injected with squeezed vacuum when these two atoms are initially prepared in coherent states. It is shown that the stationary entanglement exhibits a strong dependence on the initial state of the two atoms when the spontaneous emission rate of each atom is equal to the collective spontaneous emission rate, corresponding to the case where the two atoms are close together. It is found that the stationary entanglement of two atoms increases with decreasing effective atomic cooperativity parameter. The squeezed vacuum can enhance the entanglement of two atoms when the atoms are initially in coherent states. Valuably, this provides us with a feasible way to manipulate and control the entanglement, by changing the relative phases and the amplitudes of the polarized atoms and by varying the effective atomic cooperativity parameter of the system, even though the cavity is a bad one. When the spontaneous emission rate of each atom is not equal to the collective spontaneous emission rate, the steady-state entanglement of two atoms always maintains the same value, as the amplitudes of the polarized atoms varies. Moreover, the larger the degree of two-photon correlation, the stronger the steady-state entanglement between the atoms.     

Proposed Entanglement Swapping in Continuous Variable Systems via Braiding

We study entanglement swapping in continuous variable systems byusing braiding transformations. It is found that entanglement swapping in two-mode squeezed vacuum states and squeezed coherent states can be realized based on the braiding operators.     

真空场注入三态叠加MFSS光场广义电场的等幂偶次Y压缩

对真空场注入三态叠加MFSS(多模泛函叠加态)光场|ψ_0~(3)〉q广义电场分量的等幂次偶次振幅压缩特性进行了详细研究,分析了光场经典强度、经典振幅和经典相位的任意空间分布特征、以及真空场注入对该光场的广义电场分量的等幂偶次Y压缩效应其压缩幅度和压缩量的影响。结果发现:导致态|ψ_0~(3)〉q的广义电场分量存在等幂偶次2m次方Y压缩效应的根本原因在于模间的量子干涉效应、态间的量子干涉效应以及模间和态间的量子纠缠效应;真空场注入可使该光场广义电场分量的2m次方Y压缩效应增强、压缩幅度增大、压缩程度加深。     

Continuous-Variable Entanglement in a Coupled Field-Mesoscopic Josephson Junction System

Continuous-variable entanglement in the interacting system of a mesoscopic Josephson junction with a squeezed field is investigated. It is shown that when the cavity field is initially prepared in a squeezed vacuum state and the junction in its lowest energy state, the coupling system can evolve into a two-mode Caussian state. The timedependent characteristic function in the Wigner representation for the system is analytically obtained, and the squeezing degree of the initial cavity field turns out to have not only an apparent enhancing but also a weakening effect on the entanglement of the coupling system under suitable parameter conditions.     

Efficient Scheme for Macroscopic Superpositions and Entanglement of High-Order Squeezed Vacuum States in Cavity QED

We propose an etticient scheme for generating the macroscopic superpositions and the entanglement between the high-order squeezed vacuum states by considering the multi-photon interaction of N two-level atoms in a cavity with high quality factor, assisted by a strong driving field. Through specific choices of the cavity detuning, a number of multi-party entangled states between the atoms and the high-order squeezed vacuum states and among the high-order squeezed vacuum states of the cavities can be prepared, including also the macroscopic ＂Schrodinger cats＂ of the high- order squeezed vacuum states, the entangled states of the macroscopic ＂Schrodinger cats＂, and so on. Possible extension and application of our scheme are discussed. Our scheme is reachable within the current techniques in the cavity QED.     

Complete entanglement transfer between light and qubits

Using the two-mode two-photon Jaynes-Cummings model, entanglement transfer between atoms and field is studied. It is found that when the field is in state constructed from the two-mode photon number states |00〉,|11〉 or the two-mode squeezed vacuum states, full entanglement exchange can be attained no matter the atoms are initially in pure or mixed states. These investigations show that CV entangled states can act as perfectly as the entangled number states in entangling initially separable atoms. The two-mode two-photon atom-field interaction also provides a simple way for the quantum teleportation of atomic or field states.     

Entanglement dynamics of non-interacting two-qubit system under a squeezed vacuum environment

Entanglement dynamics of two non-interacting atoms in a squeezed vacuum reservoir is studied. Several examples with different initial entangled states are investigated, and it is found that entangled atoms become disentangled faster in squeezed vacuum than in ordinary vacuum, and larger squeezing results in faster entanglement decay. The time evolution of the concurrence and the separability "distance" A can be used to explain this novel entanglement sudden death phenomenon.     

Investigations on quantum correlation of coupled qubits in squeezed vacuum reservoir

In this paper, we investigate the quantum correlation of coupled qubits which are initially in maximally entangled mixed states in squeezed vacuum reservoir. We compare and analyse the effects of squeezed parameters on quantum discord and quantum concurrence. The results show that in squeezed vacuum reservoir, the quantum discord and quantum concurrence perform completely opposite behaviors to the change of squeezed parameters. Quantum discord survives longer with the increase of squeezed amplitude parameter, but entanglement death turns faster on the contrary. The results also indicate that the classical correlation of the system is smaller than quantum discord in vacuum reservoir, while it is bigger than quantum discord in squeezed vacuum reservoir. The quantum discord and classical correlation are more robust than quantum concurrence in the two reservoir environments, which indicates that the entanglement actually is easily affected by decoherence and quantum discord has stronger ability to avoid decoherence in squeezed vacuum reservoir.     

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.     

Entanglement and disentanglement from cascade beam-splitter for squeezed vacuum state inputs

Properties of the entanglement at the outputs of a cascade beam-splitter are investigated for two single-mode squeezed vacuum state inputs. It is shown that the entanglement depends on the squeezing amplitudes of the input states and the reflection coefficients of the cascade beam-splitter, and the composite phase shift Δ between the cascade beam-splitter and the input fields has a great effect on the entanglement. In particular, the properties of the entanglement of a cascade beam-splitter differ from those of only one beam-splitter. A further method for manipulating entanglement by adjusting the parameters of the cascade beam-splitter and the input fields is proposed.