Spin squeezing and entanglement via hole-burning in atomic coherent states

We study the generation of spin squeezing via the hole burning of selected Dicke states out of an atomic coherent state prepared for a collection of N two-level atoms or ions. The atoms or ions of the atomic coherent state are not entangled, but the removal of one or more Dicke states generates entanglement, and spin squeezing occurs for some ranges of the relevant parameters. Spin squeezing in a collection of two-level atoms or ions is of importance for precision spectroscopy.     

Nonsymmetric entanglement of atomic ensembles

The entanglement of multiatom quantum states is considered. In order to cancel noise due to inhomogeneous light-atom coupling, the concept of matched multiatom observables is proposed. As a means to eliminate an important form of decoherence this idea should be of broad relevance for quantum information processing with atomic ensembles. The general approach is illustrated on the example of rotation angle measurement, and it is shown that the multiatom states that were thought to be only weakly entangled can exhibit near-maximum entanglement.     

Multiparticle entanglement via resonant interaction between light and atomic ensembles

Multiparticle entangled states generated via interaction between narrowband light and an ensemble of identical two-level atoms are considered. Depending on the initial photon statistics, correlation between atoms and photons can give rise to entangled states of these systems. It is found that the state of any pair of atoms interacting with weak single-mode squeezed light is inseparable and robust against decay. Optical schemes for preparing entangled states of atomic ensembles by projective measurement are described.     

Unconditional two-mode squeezing of separated atomic ensembles

We propose schemes for the unconditional preparation of a two-mode squeezed state of effective bosonic modes realized in a pair of atomic ensembles interacting collectively with optical cavity and laser fields. The scheme uses Raman transitions between stable atomic ground states and under ideal conditions produces pure entangled states in the steady state. The scheme works both for ensembles confined within a single cavity and for ensembles confined in separate, cascaded cavities.     

Spin squeezing inequalities for arbitrary spin

We determine the complete set of generalized spin squeezing inequalities, given in terms of the collective angular momentum components, for particles with an arbitrary spin. They can be used for the experimental detection of entanglement in an ensemble in which the particles cannot be individually addressed. We also present a large set of criteria involving collective observables different from the angular momentum coordinates. We show that some of the inequalities can be used to detect k-particle entanglement and bound entanglement.     

Optimal spin squeezing inequalities detect bound entanglement in spin models

We determine the complete set of generalized spin squeezing inequalities. These are entanglement criteria that can be used for the experimental detection of entanglement in a system of spin-1/2 particles in which the spins cannot be individually addressed. They can also be used to show the presence of bound entanglement in the thermal states of several spin models.     

Spin squeezing inequalities and entanglement of N qubit states

We derive spin squeezing inequalities that generalize the concept of the spin squeezing parameter and provide necessary and sufficient conditions for genuine 2-, or 3-qubit entanglement for symmetric states, and sufficient condition for N-qubit states. Our inequalities have a clear physical interpretation as entanglement witnesses, can be easily measured, and are given by complex but elementary expressions.     

基于二比特自旋压缩模型中的热纠缠研究

利用Concurrence判据,讨论了基态和有限温度下二比特自旋压缩模型中的量子热纠缠性质.探究了自旋压缩参量μ、外磁场Ω以及温度T对系统纠缠性质的影响.结果发现,首先,Concurrence随着耦合参量μ(或Ω)的增大先增大到最大值,然后逐渐衰减到零,Concurrence的最大值随参数μ(或Ω)的增大而增大;Concurrence的衰减率会随参数μ(或Ω)的增大而减小,从而可以在较大的范围内得到体系热纠缠.其次,较大的耦合参量取值可以提升Concurrence存在的温度范围.最后,在有限温度条件下,当参数μ(或Ω)取值较小时,体系的纠缠保持为零,即存在纠缠"死亡间隔"现象;然而随着参量μ(或Ω)的取值增大到一定值时,Concurrence会突然产生,即存在纠缠瞬间产生("Sudden birth")现象,值得注意的是:纠缠的"死亡间隔"会随着参数μ(或Ω)的增大而减小.     

Collective atomic spin squeezing and control

Using a quantum theory for an ensemble of two- or three-level atoms driven by electromagnetic fields in an optical cavity, we show that the various spins associated with the atomic ensemble can be squeezed. Two kinds of squeezing are obtained: on the one hand self-spin squeezing when the input fields are coherent ones and the atomic ensemble exhibits a large non-linearity; on the other hand squeezing transfer when one of the incoming fields is squeezed. Received 14 August 2001 and Received in final form 7 November 2001     

Spin squeezing and light entanglement in coherent population trapping

We show that strong squeezing and entanglement can be generated at the output of a cavity containing atoms interacting with two fields in a coherent population trapping situation, on account of a nonlinear Faraday effect experienced by the fields close to a dark-state resonance in a cavity. Moreover, the cavity provides a feedback mechanism allowing to reduce the quantum fluctuations of the ground state spin, resulting in strong steady state spin squeezing.     

Scheme for preparation of multi-partite entanglement of atomic ensembles

We show a scheme of preparing multipartite W type of maximally entangled states among many atomic ensembles with the generation time increasing with the party number only polynomially. The scheme is based on laser manipulation of atomic ensembles and single-photon detection, and fits well the status of the current experimental technology. We also show one of the applications of this kind of W state, demonstrating Bell theorem without inequalities.     

Coherent manipulation of spin squeezing in atomic Bose-Einstein condensate via electromagnetically induced transparency

We propose a scheme to coherently control spin squeezing of atomic Bose-Einstein condensate (BEC) via the technique of electromagnetically induced transparency (EIT). We study quantum dynamics of the mean spin vector and spin squeezing. It is shown that the mean spin vector and spin squeezing of the BEC can be controlled and manipulated by adjusting the external coupling fields or/and internal nonlinear interactions of the BEC. It is indicated that the spin squeezing can be generated rapidly in the dynamical process and maintained in a long time interval. It is found that a larger effective Rabi coupling between atoms and lasers can produce a stronger spin squeezing, and the squeezing can maintain a longer time interval.     

An efficient scheme for preparation of multipartite entanglement of atomic ensembles

We propose an efficient scheme to prepare multipartite entanglement of atomic ensembles trapped in separate cavities. Our scheme has high fidelity even with realistic noise based on the repeat-until-success strategy. By employing the quantum memory of the atomic internal state, the scaling efficiency decreases only with the number of atomic ensembles by a slow polynomial law. Moreover, the atomic ensembles also can function as quantum repeaters, which enable our system to compatible with the current experimental technique for quantum communication using atomic ensembles.     

Strongly enhanced spin squeezing via quantum control

We describe a new approach to spin squeezing based on a double-pass Faraday interaction between an optical probe and an optically dense atomic sample. A quantum eraser is used to remove residual spin-probe entanglement, thereby realizing a single-axis twisting unitary map on the collective spin. This interaction can be phase matched, resulting in exponential enhancement of squeezing as a function of optical density for times short compared to the decoherence time. In practice the scaling and peak squeezing depends on decoherence, technical loss, and noise. Including these imperfections, our model indicates that ～10 dB of squeezing should be achievable with laboratory parameters.     

Spin squeezing via one-axis twisting with coherent light

We propose a new method of spin squeezing of atomic spin, based on the interactions between atoms and off-resonant light which are known as paramagnetic Faraday rotation and the fictitious magnetic field of light. Since the projection process, squeezed light, or special interactions among the atoms are not required in this method, it can be widely applied to many systems. The attainable range of the squeezing parameter is zeta greater, similarS(-2/5), where S is the total spin, which is limited by additional fluctuations imposed by coherent light and the spherical nature of the spin distribution.     

Multipartite entanglement inequalities via spin vector geometry

We introduce inequalities for multipartite entanglement, derived from the geometry of spin vectors. The criteria are constructed iteratively from cross and dot products between the spins of individual subsystems, each of which may have arbitrary dimension. For qubit ensembles the maximum violation for our inequalities is larger than that for the Mermin-Klyshko Bell inequalities, and the maximally violating states are different from Greenberger-Horne-Zeilinger states. Our inequalities are violated by certain bound entangled states for which no Bell-type violation has yet been found.     

Efficient multiqubit entanglement via a spin bus

We propose an experimentally feasible architecture with controllable long-range couplings built up from local exchange interactions. The scheme consists of a spin bus, with strong, always-on interactions, coupled dynamically to external qubits of the Loss and DiVincenzo type. Long-range correlations are enabled by a spectral gap occurring in a finite-size chain. The bus can also form a hub for multiqubit entangling operations. We show how multiqubit gates may be used to efficiently generate W states (an important entanglement resource). The spin bus therefore provides a route for scalable solid-state quantum computation, using currently available experimental resources.     

Single-atom entropy squeezing and quantum entanglement in Tavis--Cummings model with atomic motion

We examine the single-atom entropy squeezing and the atom-field entanglement in a system of two moving twolevel atoms interacting with a single-mode coherent field in a lossless resonant cavity.Our numerical calculations indicate that the squeezing period,the squeezing time and the maximal squeezing can be controlled by appropriately choosing the atomic motion and the field-mode structure.The atomic motion leads to a periodical time evolution of entanglement between the two-atom and the field.Moreover,there exists corresponding relation between the time evolution properties of the atomic entropy squeezing and that of the entanglement between the two atoms and the field.     

Spin waves in a Bose-Einstein--condensed atomic spin chain

The spin dynamics of atomic Bose-Einstein condensates confined in a one-dimensional optical lattice is studied. The condensates at each lattice site behave like spin magnets that can interact with each other through both the light-induced dipole-dipole interaction and the static magnetic dipole-dipole interaction. We show how these site-to-site dipolar interactions can distort the ground-state spin orientations and lead to the excitation of spin waves. The dispersion relation of the spin waves is studied and possible detection schemes are proposed.     

Heralded entanglement between atomic ensembles: preparation, decoherence, and scaling

Heralded entanglement between collective excitations in two atomic ensembles is probabilistically generated, stored, and converted to single-photon fields. By way of the concurrence, quantitative characterizations are reported for the scaling behavior of entanglement with excitation probability and for the temporal dynamics of various correlations resulting in the decay of entanglement. A lower bound of the concurrence for the collective atomic state of 0.9+/-0.3 is inferred. The decay of entanglement as a function of storage time is also observed, and related to the local dynamics.