Entanglement and coherence of a three-level atom in Λ configuration interacting with two fields

We investigate the entanglement of a three-level atom in λ configuration interacting with two quantized field modes by using logarithmic negativity. Then, we study the relationship of the atomic coherence and the entanglement between two fields which are initially prepared in vacuum or thermal states. We find that if the two fields are prepared in thermal states, the atomic coherence can induce the entanglement between two thermal fields. However, there is no coherence-induced entanglement between two vacuum fields.     

Continuous-Variable Entanglement Generation from a Four-State Atom under Raman Excitation

We investigate the generation and the evolution of continuous-variable (CV) entanglement from a laser-driven four-state atom inside a doubly resonant cavity under Raman excitation. Two transitions in the four-state atom independently interact with the two cavity modes, while two other transitions are driven by coupling laser fields. By including the atomic relaxation as well as cavity losses, we show that the CV entanglement with large mean number of
photons can be generated in our scheme. We also show that the intensity of the coupling laser fields can influence effectively the entanglement period of the cavity field. Different from the conventional resonant excitation scheme where zero one-photon detuning are required, it is found that the intensity and period of entanglement between the two cavity modes as well as the total mean photon number of the cavity field can be adjusted by properly
modulating the frequency detuning.     

Tripartite Entanglement via Microwave Driven Atomic Coherence

We propose a new scheme to achieve the tripartite entanglement based on the standard criteria [Phys. Rev. A 67（2003） 052315] in a inverse-tripod atomic system. In our scheme, the atomic coherence is introduced by two microwave fields which drive the upper three levels of atom. By numerically simulating the dynamics of system, we investigate the generation and evolution of entanglement in the presence of atom and cavity decay. As a result, the present research provides an efficient approach to achieve fully tripartite entanglement with different frequencies and initial states for each entangled mode, which may have impact on the progress of multicolored multi-notes quantum information networks.     

The Entanglement Properties in the Coupled-Cavity System with the Detuning between an Atom and Cavity Field

We consider the situation that two atomic ensembles are separately trapped in coupled single-mode cavities, and each atom non-resonantly interacts with cavity field via a one-photon hopping. By employing the negativity measure of entanglement, we investigate the temporal evolution of entanglements between the cavities as well as between the cavity and atomic ensemble. By means of the numerical calculations, we discuss the influences of the number of atoms in an atomic ensemble and the detuning on the entanglement. The results show that as the number of atoms increases, the entanglement between the cavities is strengthened, contrary the entanglement between atomic ensemble and cavity is weakened. On the other hand, as the detuning increases, the entanglement between the cavities is weakened, contrary the entanglement between atomic ensemble and cavity is strengthened.

     

Entanglement evolution and transfer in a double Tavis--Cumming model in cavity QED

We have studied entanglement evolution and transfer in a double Tavis--Cumming model where two pairs of entangled two-level atoms AB and CD interact with two single-mode cavity fields a and b. We show that the Bell-like initial state of atoms AB can exhibit entanglement sudden death which should be independent of the initial entanglement of atoms CD. Also, we show that the initial entanglement of one atomic pair can be transferred into another pair, as well as the possible subsystems, that become entangled during evolution.     

Nonlinear Effect and Atomic Entanglement in a Coupled-Cavity Array

By introducing the nonlinear effects that arise from Kerr medium, we theoretically study the nonlinear effect and the entanglement between two atoms in two coupled cavities. We give out the process of dynamic stability and solve the eigen problem of the system under high-intensive fields. The dynamics of the two coupled cavity with high-intensity fields inside is also studied numerically, the effects of atom-field coupling on the self-trapping as well as on the entanglement are also analyzed and discussed. In vacuum and high-intensity fields we calculate the concurrence of the two atoms in both theoretical and realistic situation, and discuss the nonlinear effect on the atomic entanglement. The result shows that the nonlinear interaction can play a controlling role in entangling two atoms.     

von Neumann Mutual Information for Anisotropic Coupled Oscillators Interacting with a Single Two-Level Atom

We consider the interaction between a two-level atom and two electromagnetic fields injected simultaneously within a cavity, with the interaction between the fields in parametric frequency-converter form. The wave function in Schr ödinger picture is obtained under certain conditions and consequently the density matrix. By employing a generalization of the von Neumann mutual information (in the context of Tsallis' nonextensive statistics) we measure the degree of entanglement for the present system. An important change is observed in the generalized mutual information depending on the entropic index. We also measure the minimum degree of entanglement during the transition from collapse to revival and vice-versa. Successive revival peaks show a lowering of the local maximum point indicating a dissipative irreversible change in the atomic state.     

Quantum optics phenomena in atomically doped carbon nanotubes

Quantum optics phenomena, including light absorbtion and atomic states entanglement, are discussed for carbon nanotubes doped with atoms (ions). It has been shown that, similar to semiconductor microcavities and photonic band-gap materials, carbon nanotubes may qualitatively change the character of the atom-electromagnetic-field interactions, yielding strong atom-field coupling regime with the formation of quasi-one-dimensional atomic polaritons. These may be observed experimentally via the effect of the absorption line splitting (Rabi splitting) in the frequency range close to the atomic transition frequency. A scheme for entangling atomic polaritons is investigated using the photon Green function formalism for quantizing electromagnetic fields in the presence of quasi-one-dimensional absorbing and dispersing media. Small-diameter metallic nanotubes are shown to result in sizable amounts of the two-qubit atomic entanglement with no damping for sufficiently long times, thus challenging novel applications of atomically doped carbon nanotubes in quantum information science. The text was submitted by the author in English.     

Entanglement dynamics of two atoms successively passing a cavity

By means of concurrence, we investigate the dynamics of entanglement between two initially separate atoms in succession passing through a cavity and their interaction with a Fock state field. We then analyze the effects of the atomic coherence, photon number, and atomic motion on the time evolution of atom-atom entanglement. The results show that there can be entanglement between two separate atoms, and that the threshold time for the creation of the entanglement is controllable by the photon number, atomic motion, and field-mode structure.     

Extended entanglement to quantum networks

Connecting individual quantum systems through quantum channels leads to develop quantum networks crucial to perform multipartite communication or quantum cryptography. We present two techniques to generate entanglement among different parties at larger scale. In the first approach cavity QED technique is used to produce extended entanglement in atomic internal and external degrees of freedom. In this scheme we entangle two tagged atoms in their momentum state with cavity fields. Later, interaction of two auxiliary atoms with the two cavity fields in non-dispersive and dispersive fashion transforms the atoms–fields entanglement to atoms–atoms entanglement. Quantum measurement on auxiliary atoms generates extended entangled state in atomic degrees of freedom. In the second approach we take three cavities in which the two cavities have separate entangled state with third cavity in two modes which are distinguishable. Applying quantum measurement process on third cavity, we develop extended entangled state among the three cavities. We provide experimental parameters to realize the work in laboratory experiment.     

纠缠相干态光场驱动下∧-型三能级原子的辐射谱

采用时间演化算符方法,研究∧-型=三能级原子与纠缠相干态光场共振相互作用的辐射谱.给出了辐射谱一般公式,并讨论在纠缠相干态光场驱动下的辐射频谱结构.结果表明,无论下能级简并与否纠缠相干态光场平均光子数很小时均出现拉比分裂,且强度随双模光场纠缠程度的增加而增加.当两下能级简并时,若两模场的平均光子数较小,辐射谱呈现对称多峰结构,若两模场的平均光子数较大,辐射谱呈现对称五峰结构.当两下能级非简并时,若两模场的平均光子数较小,辐射谱呈现对称多峰结构.若两模场的平均光子数较大,辐射谱呈现对称十峰结构.纠缠相干光与非纠缠相干光辐射谱的本质差别有两点:一是双模光场强量子关联导致纠缠度越强拉比峰强度越高;二是存在纠缠时由于两模场相干性导致辐射谱呈现对称多峰结构.     

Coherence-Enhanced Entanglement Induced by a Two-Mode Thermal Field

Considering two identical two-level atoms interacting with two mode thermalfield through a nondegerate two-photon process, we study the entanglement dynamics between two atoms when the atomic coherence exists. It shows that the entanglement is dependent on the initial atomic states, and is greatlyenhanced due to atomic coherence as compared with the case when the atomic coherence is ignored. The results also show that the entanglement can be controlled by changing the relative phases and the amplitudes of the polarized atoms.     

Cloaks for suppression or enhancement of scattering of diffuse photon density waves

In this paper we investigate the entanglement dynamics between two two-level atoms interacting with two coherent fields in two spatially separated cavities which are filled with a Kerr-like medium. We examine the effect of nonlinear medium on the dynamical properties of entanglement and atomic occupation probabilities in the case of even and odd deformed coherent states. The results show that the deformed fields play important roles in the evolution of entanglement. Also, the results demonstrate that entanglement sudden death, sudden birth and long-distance can be controlled by the deformation and nonlinear parameters.     

Generation of two-mode entangled states in a four-level atomic system via the Raman process

In this paper, we have theoretically investigated the generation of two-mode entangled states from a four-level atomic system via the Raman process. We show that the degree of entanglement between the two cavity modes could be strongly adjusted by both the Rabi frequencies and the detunings of the pumping fields. Our numerical results reveal that entanglement between the steady state of the two cavity modes depends on the difference of the two detunings of the atomic levels with the classical laser fields or the difference of the two Rabi frequencies. Finally, our result also shows that when such atomic system is operated above the threshold, it is possible to obtain the macroscopic entangled states.     

Generation of fully three-mode continuous-variable entanglement in a single-atom laser

We proposed a scheme for generating fully three-mode continuous-variable (CV) entanglement between three nondegenerate cavity modes in a single-atom laser. In our scheme, the single-atom laser consists of a four-level atom inside a triply resonant cavity, and the atomic coherence is induced by two classical laser fields driving the corresponding atomic transitions. To demonstrate the generation of entanglement, we numerically simulated the dynamics of this system, and the numerical simulation shows that the single-atom laser considered here can be seen as a three-mode CV entanglement amplifier even in the presence of cavity losses. Moreover, we also show that the generation of entanglement doesn’t depend intensively on the initial condition of cavity field, and the fully three-mode CV entanglement can be realized no matter the three entangled (nondegenerate) modes are initially in the same state or different states based on our scheme.     

微波驱动双模四能级单原子中连续变量纠缠的制备

通过两个经典微波场驱动相应的原子精细跃迁诱导产生原子相干, 研究在双模单原子激光器中连续变量量子纠缠的制备和演化. 研究结果表明: 微波场强度可以有效地控制腔场纠缠特性; 通过调节相应的频率失谐, 能够同步增加腔场总的平均光子数、腔模间的纠缠时间和强度. 关键词： 四能级单原子 原子相干 连续变量纠缠     

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.     

Entanglement generation via full-resonant four-wave mixing

We present to use a full-resonant four-level atomic system to generate the steady-state entanglement. This scheme is based on the nondegenerate four-wave mixing process with the four fields being resonant with the corresponding bare atomic transitions, unlike the previous mixing schemes, where the entanglement is achieved under the condition of off-resonance. Another feature of such a mixing is that four fields are coupled to the different atomic transitions, respectively. By choosing the atoms appropriately, entanglement occurs between the fields with the large frequency difference.     

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.     

Preparation of Cluster States of Atomic Qubits in Cavity QED

We propose an optical scheme to generate cluster states of atomic qubits, with each trapped in separate optical cavity, via atom-cavity-laser interaction. The quantum information of each qubit is encoded on the degenerate ground states of the atom, hence the entanglement between them is relatively stable against spontaneous emission. A single-photon source and two classical fields are employed in the present scheme. By controlling the sequence and time of atom-cavity-laser interaction, we show that the atomic cluster states can be produced deterministically.