Entanglement Dynamics of Two Atoms in the Squeezed Vacuum and the Coherent Fields

We investigate the entanglement dynamics of two atoms in a double damping Jaynes-Cummings model. The two atoms are initially in the Bell states and each is in a squeezed vacuum cavity field or coherent cavity field. Compared with the case in coherent field, the atomic entanglement in the squeezed vacuum field is stronger under the same conditions. The results show that we can adopt appropriate parameters such as mean photon number, detuning, the atomic spontaneous decay and the cavity decay, to realize better control of atomic entanglement in quantum information processing. What’s worth mentioning is that proper choosing of the last two parameters enables us to decrease disentanglement period and postpone the moment when the entanglement disappears. Finally, the atomic entanglement in double damping and non-identical Jaynes-Cummings model is obtained

     

Entangling Two Multi-atomic Clusters Through a Cavity Field

Two atomic clusters, which have NA and Ns two-level atoms, respectively, are placed in a cavity but separated spatially. There is no direct interaction between the atoms. All the atoms interact with a single-mode of the cavity field. Quantum entanglement between the two atomic clusters is investigated for various initial states of the two atomic clusters and the field. When the cavity field is initially in a Fock state, we find that the time evolution of entanglement quasi-periodically oscillates regardless of the initial states of atoms. The oscillation period increases as the initial photon number increases. When all the atoms in both of the atomic clusters are initially in the excited state, we show that there is no entanglement between the atomic clusters with NA = NB = 1 regardless the initial state of the cavity field. However, when either NA or NB is larger than one, we find that the entanglement always exists even for a strong thermal field. In cases with different initial states of the atomic clusters, we notice that the entanglement becomes stronger as number of the atoms increases. When all the atoms in both of the clusters in the ground state, we also find that the entanglement can be enhanced even by a thermal field. We also notice that a single qubit can be entangled with multi-atoms which are initially in the ground state by the cavity field initially being in vacuum, thermal, coherent, and squeezed states.     

Creation of pure multi-mode entangled states in a ring cavity

Practical schemes for creation of multi-mode squeezed (entangled) states of atomic ensembles located inside a high-Q ring cavity are discussed. It is assumed that the cavity is composed of two degenerate mutually counter-propagating modes that can simultaneously couple to the atomic ensembles with the same coupling strengths. The ensembles are composed of ultra-cold atoms which are modeled as four-level systems driven by two laser fields, both co-propagating with one of the cavity directions. We illustrate a procedure that constructs multi-mode squeezed states from the vacuum by a unitary transformation associated with the collective dynamics of the atomic ensembles subjected to driving lasers of a suitably adjusted amplitudes and phases. The lasers pulses together with the cavity dissipation prepare the collective modes in a desired stationary squeezed state.     

Concentration of Non-maximally Entangled States via Entanglement Swapping in Cavity QED

Using the highly detuned interaction between Λ-type three-level atoms and coherent optical fields in cavity QED, we can create a maximally entangled state between an atom and a cavity mode from a non-maximally entangled atomic state and a non-maximally entangled coherent state via entanglement swapping. The averaged output entanglement of the scheme is the product of the initial two non-maximal entanglements rather than the sum of them. The impact made by the spontaneous emission from the atomic excited levels has been canceled here, which makes the current protocol more feasible.     

Entangling distant atoms by interference of polarized photons

We propose a scheme to generate the entangled state of two Lambda-type three-level atoms trapped in distant cavities by using interference of polarized photons. Two possible spontaneous emission channels of each excited atom result in a coherent superposition of the states of two atoms. The subsequent detection of the different polarized photons reveals that both atoms are in different ground states, but an interference effect prevents us from distinguishing which atom is in which ground state; the atoms are thus entangled. In comparison with the original proposal of interference-induced entanglement [C. Cabrillo, J. Cirac, P. Garcia-Fernandez, and P. Zoller, Phys. Rev. A 59, 1025 (1999)]], in our scheme the weakly driven condition is not required, and the influence of atomic excitement and atomic recoil on the entanglement fidelity can be eliminated.     

Spontaneously created entanglement between two atoms

Spontaneous emission as a potential tool for creation of entanglement between two atoms is investigated. We assume that the atoms are coupled to the same environment and study entanglement engineering between the atoms and its transfer between different states. The role of the atomic coherence induced by spontaneous emission will be explored which, in contrast to what is generally believed, can create entanglement between initially unentangled atoms. We quantify entanglement by the concurrence and find that it exhibits threshold properties that can lead to interesting noncontinuous phenomena of sudden birth and sudden death of entanglement. In addition, we consider the mechanism involved in creation of entanglement between distant atoms coupled to a single-mode cavity field. We include a possible variation of the coupling constants between the atoms and the cavity mode with location of the atoms in a standing-wave cavity mode. Effectively, we engineer two coupled atoms whose the dynamics are analogous to that of interacting and collectively damped two nonidentical atoms. We illustrate the interesting result that spatial variations of the coupling constants can lead to a stationary entanglement between the atoms. We explain this effect in terms of the trapping phenomenon of atomic population in a non-decaying entangled state.     

两个V型三能级原子系统的纠缠突然死亡与复苏

研究了在真空辐射场作用下,两个V型三能级原子系统的纠缠随时间的演化特性.发现当两原子间距较远,自发辐射会导致纠缠退化,甚至导致纠缠突然死亡,而原子激发态衰变的速率会影响纠缠死亡的时间;当两原子间距非常小,由于原子间的合作效应,死亡后的纠缠会在一段时间后复苏,初始的纠缠和复苏的纠缠由不同的原因引起.     

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.     

被驱动的V型三能级原子在压缩真空中的稳态行为

研究了相干驱动的Ｖ型三能级在原子在压缩真空入射的劣质腔中的稳态行为,分析了原子的稳态粒子布居数与库场平均光予数、压缩关联强度和相位及相干激光场的强度的关系。对相干捕获现象的发生作了讨论。     

Control of entanglement between two atoms by the Stark shift

Considering a quantum model consisting of two effective two-level atoms and a single-mode cavity,this paper investigates the entanglement dynamics between the two atoms,and studies the effect of the Stark shift on the entanglement.The results show that,on the one hand the atom-atom entanglement evolves periodically with time and the periods are affected by the Stark shift;on the other hand,the two atoms are not disentangled at any time when the Stark shift is considered,and for large values of the Stark shift parameter,the two atoms can remain in a stationary entangled state.In addition,for the initially partially entangled atomic state,the atom-atom entanglement can be greatly enhanced due to the presence of Stark shift.These properties show that the Stark shift can be used to control entanglement between two atoms.     

Atomic Bell states generation in an open driven cavity QED system

We show that two uncorrelated two-level atoms can become maximally entangled if they are both off-resonantly coupled to a dissipative cavity mode, initially in the vacuum state, and strongly driven by a resonant coherent field. For moderate atom-field detuning we find that the quantum correlations in the tripartite system can alternatively concentrate either in the atom-atom subsystem or in the two atom-field subsystems. In the first case Bell states as well as their superpositions are generated for low enough cavity decay rates. In a dispersive coupling regime the atomic entanglement grows up monotonically to the maximum value where it remains nearly stationary without being affected by cavity dissipation.     

Generation of Quantum States for Atomic Ensemble via Cavity QED

A scheme is proposed for generating quantum states of atomic ensemble. In this scheme, a beam of three-level atoms in the Λ configuration is trapped in a cavity, then squeezed vacuum state and squeezed coherent state of the atomic ensemble are prepared by choosing different initial states of the system. The scheme is based on the off-resonant interaction between the atom and cavities, so the high-level of the atom is eliminated adiabatically.     

Effect of mode mode-competition on atom-atom entanglement

A system consisting of two atoms interacting with a two-mode vacuum is considered, where each atom is resonant with the two cavity modes through two different competing transitions. The effect of mode--mode competition on the atom--atom entanglement is investigated. We find that the entanglement between the two atoms can be induced by the mode--mode competition. For the initial atomic state |\varPsi(0)\rangle, whether the atoms are initially separated or entangled, a large or even maximal entanglement between them can be obtained periodically by introducing the mode--mode competition. For the initial atomic state |\varPhi(0)\rangle, the strong mode--mode competition can prevent the two atoms entangled initially from suffering entanglement sudden death; besides, it makes them in a more stable and longer-lived entanglement than in the non-competition case.     

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.     

Coherent effects under suppressed spontaneous emission

An ensemble of resonance atoms is considered, which are doped into a medium with well developed polariton effect, when in the spectrum of polariton states there is a band gap. If an atom with a resonance frequency inside the polariton gap is placed into the medium, the atomic spontaneous emission is suppressed. However, a system of resonance atoms inside the polariton gap can radiate when their coherent interaction is sufficiently strong. Thus the suppression of spontaneous emission for a single atom can be overcome by a collective of atoms radiating coherently. Conditions when such collective effects can appear and their dynamics are analysed. Received 7 June 2000     

Continuous-variable Entanglement in a Correlated Spontaneous Emission Laser

We discuss the generation and evolution of entangled light in a correlated spontaneous emission laser in the linear regime. The master equation for the two-mode cavity field is derived and solved analytically in phase space. The time-dependent characteristic function in the Wigner representation for the two-mode field is obtained. It shows that the two-mode field in the cavity evolves in a two-mode Gaussian state. The entanglement degree of the two- mode field in the cavity increases initially, then decreases, and finally vanishes as the field evolves from an initial vacuum. The period of the entanglement is extended as the intensity of the driving field is increased. It is found that the entanglement still exists even when the two-mode squeezing disappears. During the entanglement period, the intensity of the field is amplified. The entanglement for the initial field being a two-mode squeezed vacuum and the entanglement of the output field are also discussed.     

Deterministic generation of entangled coherent states for two atomic samples

This paper proposes an efficient scheme for deterministic generation of entangled coherent states for two atomic samples. In the scheme two collections of atoms are trapped in an optical cavity and driven by a classical field. Under certain conditions the two atomic samples evolve from an coherent state to an entangled coherent state. During the interaction the cavity mode is always in the vacuum state and the atoms have no probability of being populated in the excited state. Thus, the scheme is insensitive to both the cavity decay and atomic spontaneous emission.     

量子关联与原子间相互作用距离关系的研究

本文将两个二能级原子注入一个腔中,用共生纠缠的方法来度量两原子之间的纠缠并推出它们之间的量子失协,还给出了原子与环境之间的共生纠缠与量子失协的计算公式,讨论了原子自发衰变率的变化对量子纠缠及失协的影响,及不同的初始状态情况下,纠缠及失协随原子距离的演化情况。结果表明：原子自发衰变率减小,原子与环境之间的量子纠缠及失协增加;选择不同的初态,可以控制原子间出现纠缠死亡的现象或量子失协为零的状态。     

量子关联与原子间相互作用距离关系的研究

本文将两个二能级原子注入一个腔中,用共生纠缠的方法来度量两原子之间的纠缠并推出它们之间的量子失协,还给出了原子与环境之间的共生纠缠与量子失协的计算公式,讨论了原子自发衰变率的变化对量子纠缠及失协的影响,及不同的初始状态情况下,纠缠及失协随原子距离的演化情况.结果表明:原子自发衰变率减小,原子与环境之间的量子纠缠及失协增加;选择不同的初态,可以控制原子间出现纠缠死亡的现象或量子失协为零的状态.     

耗散腔中双原子与光场的纠缠演化特性

采用全量子理论方法,研究了处于耗散腔中的双原子与单模相干光场相互作用系统,分析了双原子与光场之间以及两原子之间的纠缠演化特性,讨论了腔场的衰减以及原子与光场间的失谐量对双原子与光场之间以及两原子之间纠缠演化特性的影响.结果表明,当腔场存在损耗时,原子与光场之间可出现纠缠,但在长时极限下,纠缠逐渐消灭|而失谐量对原子与光场间的纠缠存在着显著影响|初始处于最大纠缠的两原子之间的纠缠,由于光场的衰变而逐渐减弱,但原子-光场之间的失谐可抑制这一衰减.