Controlling entanglement sudden death and birth in cavity QED

We present a scheme to control the entanglement sudden birth and death in a cavity quantum electrodynamics system, which consists of two noninteracting atoms each locally interacting with its own vacuum field, by applying and adjusting classical driving fields.     

Controlling Collapse and Revival of Multipartite Entanglement Under Decoherence via Classical Driving Fields

We investigate the effects of classical driving fields on the dynamics of purity, spin squeezing, and genuine multipartite entanglement (based on the Peres-Horodecki criterion ) of three two-level atoms within three separated cavities prepared in coherent states in the presence of decoherence. The three qubits are initially entangled and driven by classical fields. We obtain an analytical solution of the present system using the superoperator method. We find that the genuine multipartite entanglement measured by an entanglement monotone based on the Peres-Horodecki criterion can stay zero for a finite time and revive partially later. This phenomenon is similar to the sudden death of entanglement of two qubits and can be controlled efficiently by the classical driving fields. The amount of purity, spin squeezing, and genuine multipartite entanglement decrease with the increase of mean photon number of cavity fields. Particularly, the purity and genuine multipartite entanglement could be simultaneously improved by the classical driving fields. In addition, there is steady state genuine multipartite entanglement which can also be adjusted by the classical driving fields.     

驱动两能级原子和真空场相互作用系统中纠缠突然死亡的操控

利用并发度和线性熵作为纠缠度量研究了两个驱动两能级原子和真空场相互作用系统中的纠缠动力学特性,分析了经典驱动场频率、原子和经典场的耦合系数以及参数α对并发度和线性熵的影响。结果发现通过调控经典驱动场能够提高两原子之间和两原子与场之间的纠缠,实现两原子之间纠缠突然死亡现象的操控,理论上提供了一种调控纠缠的方式。     

Induced entanglement revival and entanglement protection in a two qubit system

A system of two initially entangled qubits interacting with a bosonic environment is considered. The interaction induces a loss of the initial entanglement of the two qubits, and for specific initial states it causes entanglement sudden death. An investigation of the modifications on the entanglement dynamics by a single pulse control field, performed in the two qubit system, shows that the control field can not only protect entangled states against sudden death but also induce a revival of entanglement in the two qubit system.     

Control of entanglement sudden death by Kerr medium

The entanglement dynamics of system, where atoms A and B interact with single mode cavity fields a and b respectively, is studied. The interaction between atom A and cavity a may be described by using the typical Jaynes Cummings model, while that between the atom B and cavity b filled with a Kerr medium is of a two-photon process. For a certain initial atom entanglement state, there is an entanglement sudden death effect between the two atoms. The Kerr medium in the cavity b can effectively prevent the undesirable entanglement sudden death from occurring. Also, from the viewpoint of the population dynamics, we discuss why the Kerr medium can do so.     

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.     

Erratum to “Effect of the time-dependent coupling on a superconducting qubit-field system under decoherence: Entanglement and Wehrl entropy” [Ann. Physics 361 (2015) 247–258]

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.     

Tunable entanglement sudden death and three-partite entanglement in Tavis-Cummings model with an added nonlinear kerr-like medium

We investigate the entanglement dynamics and the phenomenon of entanglement sudden death in the two-photon Tavis-Cummings model with an added nonlinear kerr-like medium. It is shown that the phenomenon of entanglement sudden death can be controlled by the nonlinear kerr-like medium. Furthermore, the influence of dipole-dipole interaction on entanglement between atoms is also discussed. It is found that the phenomenon of entanglement sudden death can be weakened by the dipole-dipole interaction. Finally, we investigate the influence of kerr medium and dipole-dipole interaction on the three-partite entanglement of the system by making use of the state preparation fidelity.     

控制Tavis-Cummings模型中两原子X态的纠缠突然死亡与突然产生

研究了初态为X态时Tavis-Cummings模型中具有偶极相互作用两原子的纠缠演化特性,在演化过程中,同时号码出现了两原子的纠缠突然死亡(ESD)与突然产生(ESB)两种有趣的现象.详细分析了两原子初始态的纯度、偶极相互作用、光场粒子数对这两种现象出现时间的影响,进一步给出了初始为混态时ESB与ESD的转换条件.计算结果表明,上述系统参量对两原子的纠缠演化、ESB与ESD有重要的影响,偶极相互作用会改变纠缠度的振荡周期,使出现ESD的时间间隔减少;随着初始两原子纠缠纯度的增大,纠缠突然产生以及纠缠突然死亡存在的时间缩短,并且可以提高两原子之间的纠缠;对于特殊的初态,产生了纠缠不变性以及固定的两原子纠缠,该定值受两原子初始状态的纯度控制。     

Classical correlation, quantum discord and entanglement for two-qubit system subject to heat bath

Classical correlation (CC), quantum discord (QD) and entanglement (QE) of two qubits in one-side and two-side decoherence models are investigated. The sudden change of quantum discord (DSC) as well as classical correlation and sudden death of entanglement (ESD) are found. It is proved that QE (QD) presents no sudden change (sudden death). We prove that, for nonzero occupation number of the reservoir, QE must suffer sudden death; For zero occupation number and X-form initial states, we obtain the states which are robust and the states which experience sudden death. It is verified that if DSC and ESD occur under one-side decoherence, then it must appear in the two-side decoherence, while the reverse does not hold. We obtain the boundaries of CC-QE plane and QD-QE plane, and give the state possessing maximal amount of CC (QD) for a given amount of QE.     

Quantum entanglement and classical bifurcations in a coupled two-component Bose-Einstein condensate

We investigate the relation between quantum states and classical fixed-point bifurcations in a coupled two-component Bose-Einstein condensate (BEC). It is shown that the classical bifurcations are closely related to a topological change of the corresponding quantum levels, and such a structure change can be manifested in the entanglement properties of the corresponding quantum states. That is, the entanglement of the quantum states displays some peaks near the classical bifurcation points.     

Control and Transfer of Entanglement between Two Atoms Driven by Classical Fields under Dressed-State Representation

We have studied the dynamics and transfer of the entanglement of the two identical atoms simultaneously interacting with vacuum field by employing the dressed-state representation. The two atoms are driven by classical fields. The influence of the initial entanglement degree of two atoms, the coupling strength between the atom and the classical field and the detuning between the atomic transition frequency and the frequency of classical field on the entanglement and atomic linear entropy is discussed. The initial entanglement of the two atoms can be transferred into the entanglement between the atom and cavity field when the dissipation is neglected. The maximally entangled state between the atoms and cavity field can be obtained under some certain conditions. The time of disentanglement of two atoms can be controlled and manipulated by adjusting the detuning and classical driving fields. Moreover, the larger the cavity decay rate is, the more quickly the entanglement of the two atoms decays.     

Generation of maximally entangled atom pairs in driven dissipative cavity QED systems

We investigate the entanglement of an open tripartite system where a cavity field mode in thermal equilibrium is off-resonantly coupled with two atoms that are simultaneously driven by a resonant coherent field. For moderately detuned atom-field coupling and strong atomic driving we show the generation, at given interaction times and for low enough cavity decay rates, of atomic Bell states and of Bell state superpositions relevant for quantum gates implementation. The system can oscillate between bi-separable and fully separable states. Also we describe the distribution of quantum correlations between the atom-atom and the two atom-field subsystems. In the dispersive coupling regime with strongly driven atoms we show the generation of nearly stationary Bell states which remain protected from cavity dissipation.     

Entanglement dynamics for the double Tavis-Cummings model

A double Tavis-Cummings model (DTCM) is developed to simulate the entanglement dynamics of realistic quantum information processing where two entangled atom-pairs AB and CD are distributed in such a way that atoms AC are embedded in a cavity a while BD are located in another remote cavity b. The evolutions of different types of initially shared entanglement of atoms are studied under various initial states of cavity fields. The results obtained in the DTCM are compared with that obtained in the double Jaynes-Cummings model (DJCM) [M. Yönaç, T. Yu, J.H. Eberly, J. Phys. B 40, S45 (2007)] and an interaction strength theory is proposed to explain the parameter domain in which the so-called entanglement sudden death occurs for both the DTCM and DJCM.     

The transfer of entanglement and Bell-nonlocality from system to environment

The entanglement among three cavities is transferred to three reservoirs. So does the Bell-nonlocality. Sudden death of entanglement among the three cavities could occur before, simultaneously with, or after sudden birth of entanglement among the three reservoirs. However, sudden death of Bell-nonlocality among the three cavities always happens before sudden birth of Bell-nonlocality among the three reservoirs. The survival time of entanglement is much longer than that of the Bell inequality violation.     

Enhancing entanglement generation of two atoms in a cavity with white noise using classical driving fields

We investigate the entanglement dynamics of a quantum system consisting of two two-level atoms in a cavity with classical driving fields in the presence of white noise.The cavity is initially prepared in the vacuum state.Generally,the entanglement of two atoms decreases with the intensity of the thermal fields and the coupling strength of the two-level atoms to the thermal fields.However,we find that the entanglement of the quantum system can be enhanced by adjusting the frequency and the strength of the classical driving fields in the presence of white noise.     

Thermal entanglement of two interacting qubits in a static magnetic field

We study systematically the entanglement of a two-qubit Heisenberg XY model in thermal equilibrium in the presence of an external arbitrarily-directed static magnetic field, thereby generalizing our prior work [G. Lagmago Kamta, A.F. Starace, Phys. Rev. Lett. 88, 107901 (2002)]. We show that a magnetic field having a component in the xy-plane containing the spin-spin interaction components produces different entanglement for ferromagnetic (FM) and antiferromagnetic (AFM) couplings. In particular, quantum phase transitions induced by the magnetic field-driven level crossings always occur for the AFM-coupled qubits, but only occur in FM-coupled qubits when the coupling is of Ising type or when the magnetic field has a component perpendicular to the xy-plane. When the magnetic field has a component in the xy-plane, the cut-off temperature above which the entanglement of both the FM- and AFM-coupled qubits vanishes can always be controlled using the magnetic field for any value of the XY coupling anisotropy parameter. Thus, by adjusting the magnetic field, an entangled state of two spins can be produced at any finite temperature. Finally, we find that a higher level of entanglement is achieved when the in-plane component of the magnetic field is parallel to the direction in which the XY exchange coupling is smaller.     

Entanglement death and teleportation pre-emption in a dissipative system of two coupled qubits

We discuss the entanglement of two coupled qubits each of which is interacting with a multi-mode squeezed vacuum field reservoir and the possibility to use them in a teleportation protocol. The entanglement sudden death (ESD) and teleportation pre-emption (TPE) are discovered in this system. We consider the interplay between the coupling strength and the squeezing parameter, and find that they play competing roles in entanglement as well as in teleportation. We show in detail how the entanglement decay affects the teleportation. It is interesting to show that TPE precedes ESD in general, and for given measurement result and squeeze parameter all output states of teleportation for a long time arrive at a fixed asymptotic point in the Bloch sphere.     

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.     

Synthesis of maximally entangled mixed states and disentanglement in coupled Josephson charge qubits

We analyze a controllable generation of maximally entangled mixed states of a circuit containing two-coupled superconducting charge qubits. Each qubit is based on a Cooper pair box connected to a reservoir electrode through a Josephson junction. Illustrative variational calculations were performed to demonstrate the effect on the two-qubits entanglement. At sufficiently deviation between the Josephson energies of the qubits and/or strong coupling regime, maximally entangled mixed states at certain instances of time is synthesized. We show that entanglement has an interesting subsequent time evolution, including the sudden death effect. This enables us to completely characterize the phenomenon of entanglement sharing in the coupling of two superconducting charge qubits, a system of both theoretical and experimental interest.