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.     

Generation of various multiatom entangled graph states via resonant interactions

In this paper, a scheme for generating various multiatom entangled graph states via resonant interactions is proposed. We investigate the generation of various four-atom graph states first in the ideal case and then in the case in which the cavity decay and atomic spontaneous emission are taken into consideration in the process of interaction. More importantly, we improve the possible distortion of the graph states coming from cavity decay and atomic spontaneous emission by performing appropriate unitary transforms on atoms. The generation of multiatom entangled graph states is very important for constructing quantum one-way computer in a fault-tolerant manner. The resonant interaction time is very short, which is important in the sense of decoherence. Our scheme is easy and feasible within the reach of current experimental technology.     

Generation of superpositions of coherent states for an atomic sample in cavity QED

This paper proposes a scheme for generation of superpositions of coherent states of the effective bosonic mode in a collection of atoms. In the scheme an atomic sample interacts with a slightly detuned cavity mode and a resonant strong classical field. Under certain conditions the atomic system evolves from a coherent state to a superposition of coherent states.     

Creation of quantum correlations via the initial classical-mixed states

Using the pseudomode method, we theoretically analyze the creation of quantum correlations between two two-level dipole-dipole interacting atoms coupled with a common structured reservoir with different coupling strengths. Considering certain classes of initial separable-mixed states, we demonstrate that the sudden birth of atomic entanglement as well as the generation of stationary quantum correlations occur. Our results also suggest a possible way to control the occurrence time of entanglement sudden birth and the stationary value of quantum correlations by modifying the initial conditions of states, the dipole-dipole interaction, and the relative coupling strength. These results are helpful for the experimental engineering of entanglement and quantum correlations.     

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 squeezed atomic states in cavity QED

A squeezed atomic state is that state of a system of two-level atoms for which the intrinsic quantum noise in a process of measurement is less than the minimum noise obtained by using a spin coherent state. It is shown that such a state is generated in certain time intervals when a non-squeezed atomic state evolves on interaction with a single mode coherent field inside a lossless cavity. The atoms are assumed to undergo one-photon or two-photon transitions between the given two levels. The maximum atomic squeezing is found as a function of the number of atoms and the field strength. The effect of the field-dependent Stark shift is investigated in the case of the atoms undergoing two-photon transitions.     

Preparation of two and four atom entangled states

A scheme for preparing two and four atom entangled states is presented. It is based on atom cavity field interactions. Firatly, the cavity is prepared in the superposition of the number states through the atom undergoing a two photon transition, the secondly, the two or four identical two level atoms, which are all initially in their ground states, are sent through the cavity sequentially and can make resonant single photon transition in the cavity. Then atomic entangled states are created and the cav     

Transformation of bipartite non-maximally entangled states into a tripartite W state in cavity QED

We present two schemes for transforming bipartite non-maximally entangled states into a W state in cavity QED system, by using highly detuned interactions and the resonant interactions between two-level atoms and a single-mode cavity field. A tri-atom W state can be generated by adjusting the interaction times between atoms and the cavity mode. These schemes demonstrate that two bipartite non-maximally entangled states can be merged into a maximally entangled W state. So the scheme can, in some sense, be regarded as an entanglement concentration process. The experimental feasibility of the schemes is 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.     

原子与弱相干腔场相互作用系统中的量子特性

考虑腔场处于弱相干态的情况,研究了两个全同的二能级原子与耦合腔相互作用系统中原子的偶极压缩和原子间的纠缠特性。研究结果表明,随耦合腔耦合系数增大,原子的偶极压缩减弱。另一方面,随耦合腔耦合系数增大,原子间的纠缠也减弱,这一结果与耦合腔处于真空态的情况相反。     

Vacuum polarization for atomic quasienergy states

Vacuum polarization for an atomic system in the laser field is considered in the representation of quasienergy states as a radiation correction to the quasienergy. It is shown that laser effects are absent in the polarization fermion loop in case of resonant mixing of atomic levels. Perspectivity of investigation of laser effects in vacuum polarization of muonic atoms is discussed.     

Generating a superposition of spin states in an atomic ensemble

A method for generating a mesoscopic superposition state of the collective spin variable of a gas of atoms is proposed. The state consists of a superposition of the atomic spins pointing in two slightly different directions. It is obtained by using off resonant light to carry out quantum nondemolition measurements of the spins. The relevant experimental conditions, which require very dense atomic samples, can be realized with presently available techniques. Long-lived atomic superposition states may become useful as an off-line resource for quantum computing with otherwise linear operations.     

Manipulating atomic states via optical orbital angular-momentum

Optical orbital angular-momentum (OAM) has more complex mechanics than the spin degree of photons, and may have a broad range of application. Manipulating atomic states via OAM has become an interesting topic. In this paper, we first review the general theory of generating adiabatic gauge field in ultracold atomic systems by coupling atoms to external optical fields with OAM, and point out the applications of the generated adiabatic gauge field. Then, we review our work in this field, including the generation of macroscopic superposition of vortex-antivortex states and spin Hall effect (SHE) in cold atoms.      

Resonant two-photon ionization with high-order continuum states

In this paper,the resonant two-photon ionization of atoms with high-order con-tinuum state is studied.It's found that the C-C coupling among the continuum states enhancesthe two-level atomic Rabi oscillation,and the direct transition from the intermidiate excitedstate to the continuum weakens the Rabi oscillation.Therefore the photoelectron energy spec-tra and the population are changed.     

Asymptotic Entanglement Sudden Death in Two Atoms with Dipole–Dipole and Ising Interactions Coupled to a Radiation Field at Non-Zero Detuning

We investigate the time evolution and asymptotic behavior of a system of two two-level atoms (qubits) interacting off-resonance with a single mode radiation field. The two atoms are coupled to each other through dipole–dipole as well as Ising interactions. An exact analytic solution for the system dynamics that spans the entire phase space is provided. We focus on initial states that cause the system to evolve to entanglement sudden death (ESD) between the two atoms. We find that combining the Ising and dipole–dipole interactions is very powerful in controlling the entanglement dynamics and ESD compared with either one of them separately. Their effects on eliminating ESD may add up constructively or destructively depending on the type of Ising interaction (Ferromagnetic or anti-Ferromagnetic), the detuning parameter value, and the initial state of the system. The asymptotic behavior of the ESD is found to depend substantially on the initial state of the system, where ESD can be entirely eliminated by tuning the system parameters except in the case of an initial correlated Bell state. Interestingly, the entanglement, atomic population and quantum correlation between the two atoms and the field synchronize and reach asymptotically quasi-steady dynamic states. Each one of them ends up as a continuous irregular oscillation, where the collapse periods vanish, with a limited amplitude and an approximately constant mean value that depend on the initial state and the system parameters choice. This indicates an asymptotic continuous exchange of energy (and strong quantum correlation) between the atoms and the field takes place, accompanied by diminished ESD for these chosen setups of the system. This system can be realized in spin states of quantum dots or Rydberg atoms in optical cavities, and superconducting or hybrid qubits in linear resonators.     

Entangled states of atoms upon interaction with narrowband light

The process of resonant interaction of light with two-level atoms in the absence of relaxation is considered. For a special form of initial conditions, simple and exact solutions are found that describe coherent processes leading to the appearance of many-particle entangled W-class states. These processes can be used for preparation and transformation of entangled states, in particular, for problems of quantum memory and generation of entangled atomic chains.     

基于低Q腔光子Faraday旋转的远程态制备

基于低Q腔中单光子的输入与输出关系,提出了利用偏振光Faraday旋转分别遥远制备单原子态和两原子纠缠态的可行方案.研究结果表明,当初始原子态的系数为实数时,通过选择合适的偏振光、腔场与原子相互作用系统的参数,单原子态与两原子纠缠态的远程制备均可确定性地得以实现.与以前的原子态远程制备方案相比,本文方案采用光子作为飞行比特来传递量子信息,故原则上可实现原子态的真正长距离制备.由于原子态的信息编码在耗散单边腔囚禁的Λ型三能级原子的两个基态能级,且原子仅虚激发,因此本文方案对腔衰减和原子自发辐射不敏感.此外,本文所提出的两种方案不需要两体或多体正交测量,仅涉及单体直积态测量,而且两种方案都工作在低Q腔,不需要原子与光腔的强耦合,从而有效降低了实验难度.     

Nonadiabatic effects on population transfer of two Bose－Einstein condensates induced by atomic interaction

We investigate the stimulated Raman adiabatic passage for Bose－Einstein condensate (BEC) states which are trapped in different potential wells or two ground states of BEC in the same trap. We consider that lasers are nearly resonant with the atomic transitions. The difference of population transfer processes between BEC atoms and usual atoms is that the atomic interaction of the BEC atoms can cause some nonadiabatic effects, which may degrade the process. But with suitable detunings of laser pulses, the effects can be remedied to some extent according to different atomic interactions.     

Decoherence-immune generation of highly entangled states for two atoms

This paper proposes a decoherence-immune scheme for generating highly entangled states for two atoms trapped in a cavity. The scheme is based on two resonant atom-cavity interactions. Conditional upon the detection of no photon, the two atoms may exchange an excitation via the first resonant interaction, which leads to entanglement. Due to the loss of the excitation, the two atoms are in a mixed entangled state. With the help of an auxiliary ground state not coupled to the cavity mode, the state related to the excitation loss is eliminated by the detection of a photon resulting from the second resonant interaction. Thus, the fidelity of entanglement is almost not affected by the decoherence.     

Entangling Two-Atom Through Cooperative Interaction Under Stimulated Emission

We discuss the generation of two two-level atoms entanglement inside a resonant microcavity under stimulated emission (STE) interaction. The amount of entanglement is studied based on different atomic initial states. For each kind of initial state, we obtain the entanglement period and the entanglement critical point, which are found to deeply depend on driving field density. If both atoms are initially in excited state, the entanglement can be induced due to STE. While when one of them initially lies in its ground state, we find there is a competition between driving field induced entanglement and STE induced entanglement. PACS number: 03.75. Gg 03.75. Lm.