NOON states in entangled cavities

We show how NOON states may be generated entangling two cavities by passing atoms through them. The atoms interact with each cavity via two-photon resonant transitions. We take advantage of the fact that depending on the state the atom enter (excite or ground), it leaves or takes two photons per interaction and leaves the cavities in a pure state.     

Transferring an N-atom state between two distant cavities via an optical fiber

This paper proposes a scheme for transferring an N-atom state between two distant cavities via an optical fiber.The scheme is based on adiabatic passage along a dark state.In the scheme,all the atoms are always in ground state,the field mode of the fiber remains in vacuum state,and the field mode of the cavities being excited can be negligible under certain conditions.Therefore,the scheme is very robust against decoherence.The successful probability of implementing the quantum state transfer increases with increasing number of atoms.Furthermore,the interaction time does not need to be accurately adjusted as long as the adiabaticity condition is fulfilled.     

Generation of maximally entangled state from a single-photon state in fiber-connected cavities

Starting from the physical model consisting of two remotely located atoms trapped in high finesse fiber-connected cavities, generation of maximally entangled state is firstly realized theoretically from a initial single-photon state. Result shows that under the regime of strong atom-cavity coupling strength, the averaged fidelity can remarkably exceed 0.9980. The distant creation of maximally entangled state may provides certain opportunities and be helpful in the accomplishment of scalable quantum networks.     

Preparation of the W state via cavity QED

A scheme for preparation of the tripartite W state via cavity quantum electrodynamics is presented in this paper. And the scheme can be generalized to prepare the n-atom W states. The second part of this paper shows how to prepare n-cavity W states. All cavities involved are initially in the vacuum states, thus the requirement on the quality factor of the cavities is greatly loosened.     

Entanglement dynamics of two distant atoms in two detuning cavities

We investigate the entanglement dynamics via the concurrence of two distant atoms interacting off-resonantly with two cavity fields in Fock states,respectively.We find that the evolution of entanglement has sudden death and sudden birth phenomena,that with the increase of photon numbers in the two cavities,the alternate frequency of sudden death and sudden birth turns fast,and that the amplitude of concurrence oscillates regularly with oscillation frequency becoming slow when the cavity fields have the same photon numbers.While,the maximum of concurrence declines and the amplitude of concurrence oscillates irregularly when the two cavity fields have different photon numbers.In addition,we find the length of death time is dependent on the initial entanglement.     

相对论速调管振荡器中两个调制腔耦合系数的计算

 用场方法研究了相对论速调管振荡器中两个调制腔的耦合，解析地得到了两腔耦合系数与腔和波导的尺寸、间隙及两腔之间的距离等参数之间的关系。耦合系数的数值与由耦合与否腔的频率测量确定的实验结果在数量级上符合得较好。     

W类态三纠缠原子与相干态光场相互作用过程中光场的量子特性

研究了处于W类态的三纠缠原子与相干态光场相互作用过程中光场的量子特性;运用数值方法,讨论了三纠缠原子初始状态和相干态光场的强弱对系统光场压缩和二阶相干特性的影响。     

Alternative scheme to generate a supersinglet state of three-level atoms

In this Letter we propose an alternative scheme to generate a supersinglet state of three three-level atoms via a single-mode of a cavity QED based on the two-photon transitions described by the ‘full microscopical Hamiltonian approach’. In it, three three-level atoms prepared in suitable initial states are sequentially sent through the cavity originally prepared in its vacuum state. After an appropriate choice of the atom-cavity interaction times plus a field detection the state that describes the whole atom-field system is projected in the desired supersinglet state. The fidelity and success probability of the state as well as the practical feasibility of the scheme are discussed.     

原子在弱相干场光纤耦合腔系统中的纠缠特性

研究由两个相同的二能级原子分别处于用单模光纤耦合的两弱相干光场系统的共生纠缠特性, 通过数值计算研究了光纤模-腔模与原子-腔模的耦合强度比、弱相干光场的强度和两光场相对相位差等因素对系统纠缠演化的影响. 结果表明: 两腔中的两原子之间、两光场之间和每个腔中的原子与光场之间的纠缠随时间呈现周期或准周期性演化, 两腔场之间的纠缠与腔中的两原子的纠缠可以相互转换, 与两原子之间和两光场之间的纠缠相比, 每个腔中光场与原子之间的纠缠随时间变化的周期缩短. 光纤模-腔模与原子-腔模的耦合强度比与两腔中光场相位差对系统纠缠的影响很大, 较小的光纤模-腔模与原子-腔模的耦合强度之比可以获得较大的系统纠缠度. 关键词： 弱相干场 光纤耦合腔 耦合强度 量子纠缠     

Highly efficient scheme for the preparation of four-photon polarization entangled state via quantum non-demolition measurement with classical feed-forward

A simple and efficient scheme is proposed to prepare a four-photon entangled state, which can be used as a resource for realizing a controlled-NOT (CNOT) gate pointed out by Gottesman and Chuang [Nature 402 (1999) 390], with the help of the cross-Kerr nonlinearity. The distinct feature of the present scheme is its preparation with near unity success probability and near perfect fidelity. It is not necessary in the present scheme that the cross-Kerr nonlinearity is very large because the weak nonlinearity can be compensated by using a coherent light laser probe beam with very large amplitude. This makes us more confident in the feasibility of the proposed scheme.     

Quantum effect in a generalized N-atom Dicke model

We have studied the geometric phase and the sub-Poissonian photon distribution of a generalized N two-level atoms Dicke model in the thermo-dynamical limit and the off-resonant coupling case. It is found that the geometric phase in the ground state is relative to the atom number, the coupling strength between the atom and the light field, the frequency of the electromagnetic wave and the energy difference between two levels of the atom. The photons may exhibit the sub-Poissonian distribution in the ground state.     

Resonant scheme for realizing quantum phase gates for two separate atoms via coupled cavities

We propose a scheme for realizing conditional quantum phase gates for two atoms that are distributed in two coupled cavities. Due to the resonant interaction in temporal evolution of the entire system, the gate operation time is greatly reduced as compared with that of the nonresonant schemes. We study the influence of imperfections in the interaction and the effect of decoherence and find the gate to be robust. We discuss the issue related to the practical implementation and show that the gate is accessible within the current cavity QED technology.     

Preparation of a stable and maximally entangled state of two distant qutrits trapped in separate cavities

We have proposed a simple scheme to entangle two distant qutrits trapped in separate optical cavities. The quantum information of each qutrit is skillfully encoded on the degenerate ground states of a pair of atoms, hence the entanglement between them is relatively stable against spontaneous emission. In Lamb-Dicke limits, it is not necessary to require coincidence detections, which will relax the conditions for the experimental realization. The scheme is robust against the inefficient detections.     

Scheme for generating the singlet state of three atoms trapped in distant cavities coupled by optical fibers

An effective scheme is proposed to generate the singlet state with three four-level atoms trapped in three distant cavities connected with each other by three optical fibers, respectively. After a series of appropriate atom–cavity interactions, which can be arbitrarily controlled via the selective pairing of Raman transitions and corresponding optical switches, a three-atom singlet state can be successfully generated. The influence of atomic spontaneous decay, photon leakage of cavities and optical fibers on the fidelity of the state is numerically simulated showing that the three-atom singlet state can be generated with high fidelity by choosing the experimental parameters appropriately.     

A system with two photon states and two polarization-entangled photon modes as configuration of minimal coupling to a classical pump field

The configuration of minimum interaction energy between two polarization-entangled photon modes and a classical pump field is determined by treating the corresponding interaction Hamiltonian operator. In fact, the above configuration is found to be of two photon states.     

Tunable two-axis spin model and spin squeezing in two cavities

Multi-mode cavities have now attracted much attention both experimentally and theoretically. In this paper, inspired by recent experiments of cavity-assisted Raman transitions, we realize a two-axis spin Hamiltonian H = q(J_x~2+ χJ_y~2) + ω_0J_z in two cavities. This realized Hamiltonian has a distinct property that all parameters can be tuned independently. For proper parameters, the well-studied one- and two-axis twisting Hamiltonians are recovered, and the scaling of N~(-1) of the maximal squeezing factor can occur naturally. On the other hand, in the two-axis twisting Hamiltonian, spin squeezing is usually reduced when increasing the atomic resonant frequency ω_0. Surprisingly, we find that by combining with the dimensionless parameter χ(-1), this atomic resonant frequency ω_0 can enhance spin squeezing greatly. These results are beneficial for achieving the required spin squeezing in experiments.     

Atomic NOON state generation in distant cavities by virtual excitations

A general scheme of generating NOON states of virtually-excited 2N atoms is proposed. The two cavities are fibre-connected with N atoms in each cavity. Although we focus on the case of N = 2, the system can be extended to a few atoms with N 〉2. It is found that all 2N atoms can be entangled in the form of NOON states if the atoms in the first cavity are initially in the excited states and atoms in the second cavity are all in the ground states. The feasibility of the scheme is carefully discussed, it shows that the NOON state with a few atoms can be generated with good fidelity and the scheme is feasible in experiment.     

Effective Approach to Constructing n-Mode Wigner Operator in the Entangled State Representation

By extending Fan-Klauder entangled state representation to multipartite case. We construct n-mode Wigner operator in the common eigenvector of the multipartite centre-of-mass coordinate and two mass-weighted relative momenta, and its canonical conjugate state, they are both more complicated entangled state of continuum variables. the technique of integration within an ordered product (IWOP) of operators is essential in our derivation.     

Efficient scheme for realizing quantum dense coding with GHZ state in separated low-Q cavities

We propose an efficient scheme for realizing quantum dense coding with three-particle GHZ state in separated low-Q cavities. In this paper, the GHZ state is first prepared with three atoms trapped, respectively, in three spatial separated cavities. Meanwhile, with the assistance of a coherent optical pulse and X-quadrature homodyne measurement, we can implement quantum dense coding with three-particle GHZ state with a higher probability. Our scheme can also be generalized to realize N-particle quantum dense coding.     

Atomic N00N state generation in distant cavities by virtual excitations

A general scheme of generating N00N states of virtually-excited 2N atoms is proposed. The two cavities are fibre-connected with N atoms in each cavity. Although we focus on the case of N=2, the system can be extended to a few atoms with N>2. It is found that all 2N atoms can be entangled in the form of N00N states if the atoms in the first cavity are initially in the excited states and atoms in the second cavity are all in the ground states. The feasibility of the scheme is carefully discussed, it shows that the N00N state with a few atoms can be generated with good fidelity and the scheme is feasible in experiment.