Production of three-dimensional entanglement for two atoms with a single resonant interaction

A scheme is proposed for generating three-dimensional maximally entangled states for two atoms. In the scheme the atoms are trapped in a two-mode cavity. The scheme only requires a single resonant interaction of the atoms with the cavity modes. Therefore, the scheme is very simple and required interaction time is very short, which is important in view of decoherence.     

Complete entanglement transfer between light and qubits

Using the two-mode two-photon Jaynes-Cummings model, entanglement transfer between atoms and field is studied. It is found that when the field is in state constructed from the two-mode photon number states |00〉,|11〉 or the two-mode squeezed vacuum states, full entanglement exchange can be attained no matter the atoms are initially in pure or mixed states. These investigations show that CV entangled states can act as perfectly as the entangled number states in entangling initially separable atoms. The two-mode two-photon atom-field interaction also provides a simple way for the quantum teleportation of atomic or field states.     

The entanglement properties of non-Gaussian states prepared by photon subtraction from two-mode squeezed thermal states

We analyze the entanglement condition of a new kind of non-Gaussian quantum state, which is prepared by photon number subtraction from the two mode squeezed thermal state. Fock space criterion and Shchukin-Vogel criterion are applied, and they give the same condition which is also that of the input state.     

Atomic squeezed states in a driven Dicke model

We consider the dissipative N two-level atom Dicke system driven by a c.w. laser field. In the steady state, the cooperativity among the atoms via the radiation field produces spin squeezing. Even in the absence of dipole-dipole interaction, the system shows spin squeezing which is attributed to strong nonlinear interaction with the driving field.     

Enhanced squeezing and entanglement in a non-degenerate three-level cascade laser with injected squeezed light

We consider a non-degenerate three-level cascade laser coupled to a two-mode squeezed vacuum reservoir via the lossy single-port mirror. Applying the pertinent master equation, we analyze the effects of the injected squeezed light on the quadrature squeezing, entanglement and normalized intensity difference fluctuations. We show that the injected squeezed light considerably enhances the degree of squeezing and entanglement in the two-mode light for certain initial conditions. Moreover, the injected squeezed light increases the mean photon number where the squeezing and entanglement is significant. We also show that the presence of the injected squeezed light greatly reduces the noise in the intensity difference.     

Scheme for Generating Cluster States with Charge Qubits in a Cavity

Based on superconducting charge qubits （SCCQs） coupled to a single-mode microwave cavity, we propose a scheme for generating charge cluster states. For all SCCQs, the controlled gate voltages are all in their degeneracy points, the quantum information is encoded in two logic states of charge basis. The generation of the multi-qubit cluster state can be achieved step by step on a pair of nearest-neighbor qubits. Considering effective long-rang coupling, we provide an efficient way to one-step generating of a highly entangled cluster state, in which the qubit-qubit coupling is mediated by the cavity mode. Our quantum operations are insensitive to the initial state of the cavity mode by removing the influence of the cavity mode via the periodical evolution of the system. Thus, our operation may be against the decoherence from the cavity.     

Generation of Greenberger-Horne-Zeilinger states for three atoms trapped in a cavity beyond the strong-coupling regime

A scheme is proposed for generating maximally entangled states for three atoms trapped in a two-mode cavity. The scheme is based on resonant atom-cavity interaction and linear optics elements. The fidelity of the entangled state is not affected by both the decoherence and detection inefficiencies. The scheme works beyond the strong-coupling regime, which is important for high-fidelity entanglement engineering under realistic conditions.     

Inseparability of Phase-Damped Bipartite Gaussian States

For bipartite non-Gaussian states which are prepared by applying the phase damping to Gaussian states, we use Fock subspace inseparable criterion and the Shchukin-Vogel inseparable criterion to analyze their inseparability. The lowest order of the two criteria is obtained. Fock subspace criterion is more efficient in detecting the entanglement of damped Gaussian state by numerical results. Two photon Fock subspace criterion does not improve the entanglement detecting condition.     

Optical Generation of Single- or Two-Mode Excited Entangled Coherent States

With nonlinear Mach-Zehnder interferometer （NLMZI） and a typed beta-barium borate （BBO） crystal, we optically generate single-mode excited entangled coherent states. This scheme can be easily generalized to generate two-mode excited entangled coherent states. We simply analyse different influences of single- and two-mode photon excitations on entangled coherent states.     

Entangled atomic state generation via adiabatic evolution of dark eigenstates in cavity QED

We propose a scheme for generating a two-atom entangled state and an N-atom W state using adiabatic evolution of dark eigenstates in cavity QED. The time required to complete the process does not need precise control. Since the cavity modes are never excited during the operations by engineering adiabatic evolution and controlling the atom–cavity couplings, the decoherence of the cavity decay can be suppressed.     

A Theoretical Scheme for Entanglement Transfer under Intensity-Dependent Couplings

We consider a theoretical scheme for entanglement transfer between a two-mode squeezed vacuum field and two initially separable atoms through intensity-dependent couplings. We find that the entanglement transfer between the field and the atoms has an exact period for any given squeezing. We also find that the maximum achievable entanglement of the atomic subsystem is a simple increasing function of r.For sufficiently large squeezing parameter r, it is possible for the atoms to be entangled into a Bell state at half the periodic time points.     

The influence of non-Kolmogorov turbulence on the entanglement of spatial two-qubit states in a slant channel

The effects of atmospheric turbulence on the entanglement of spatial two-qubit states that are prepared using the signal and idler photons produced by parametric down-conversion are studied. Utilizing the non-Kolmogorov model for atmospheric turbulence and Rytov approximation method, we quantify the effects of atmospheric turbulence on the entanglement of the two-qubit state in terms of Wootters's concurrence. Our results show that the effects of the zenith angle of communication channel and the outer scale of turbulence on the concurrence of a spatial two-qubit state can be ignored and the smaller inner scale of turbulence, the smaller refractive-index power α, the shorter wavelength of beams and the longer propagation distance will lead to the larger fluctuations of the concurrence of a spatial two-qubit state.     

Robust Implementation of a Nonlocal N-Qubit Phase Gate by Interference of Polarized Photons

We propose a protocol to directly implement the nonlocal phase gate of N A-type three-level atoms trapped in distant cavities by using interference of polarized photons. The protocol uses the effects of quantum statistics of indistinguishable photons emitted by the atoms inside optical cavities.     

Experimental Non-Local Generation of Entanglement from Independent Sources

We experimentally demonstrate a non-local generation of entanglement from two independent photonic sources in an ancilla-free process. Two bosons （photons） are entangled in polarization space by steering into a novel interferometer setup, in which they have never met each other. The entangled photons are delivered to polarization analyzers in different sites, respectively, and a non-local interaction is observed. Entanglement is further verified by the way of the measured violation of a CHSH type Bell＇s inequality with S-values of 2.54 and 27 standard deviations. Our results will shine a new light onto the understanding of how quantum mechanics works, have possible philosophic consequences on the one hand and provide an essential element for quantum information processing on the other hand. Potential applications of our results are briefly discussed.     

Concentration of Unknown Atomic Entangled States via Entanglement Swapping through Raman Interaction

We show that entanglement concentration of unknown atomic entangled states is achieved via the implementation of entanglement swapping based on Raman interaction in cavity QED. A maximally entangled state is obtained from a pair of partially entangled states probabilistically. Due to Raman interaction of two atoms with a cavity mode and an external driving field, the influence of atomic spontaneous emission has been eliminated. Because of the virtual excitation of the cavity mode, the decoherence of cavity decay and thermal field is neglected.     

Dynamical creation of entanglement versus disentanglement in a system of three-level atoms with vacuum-induced coherences

The dynamics of entanglement between three-level atoms coupled to the common vacuum is investigated. We show that the collective effects such as collective damping, dipole-dipole interaction and the cross coupling between orthogonal dipoles, play a crucial role in the process of creation of entanglement. In particular, the additional cross coupling enhances the production of entanglement. For the specific initial states we find that the effect of delayed sudden birth of entanglement, recently invented by Ficek and Tana? [Ficek, R. Tana?, Phys. Rev. A 77 (2008) 054301] in the case of two-level atoms, can also be observed in the system. When the initial state is entangled, the process of spontaneous emission causes destruction of correlations and its disentanglement. We show that the robustness of initial entanglement against the noise can be changed by local operations performed on the state.     

Linear entropy in the Jaynes-Cummings model with a Kerr nonlinearity

The dynamical properties of quantum entanglement in the integrable Jaynes-Cummings model with a Kerr nonlinearity are studied in terms of the reduced-density linear entropy with various Kerr coupling parameters and initial states, where the initial states are prepared by the coherent states placed in the corresponding phase space described in terms of canonical variables. The mean entanglement averaged over time is employed to investigate the behavior of entanglement of those coherent states. It is shown that the mean entanglement of the coherent states put near the centers of periodic orbits, both with a strong Kerr coupling and without a Kerr coupling, tends to be the minimal, and that the mean entanglement of the coherent states centered near the boundary with a strong Kerr coupling is the minimal while that without Kerr coupling is the maximal.     

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.     

Generation of Cluster-Type Entangled Coherent States via Cross-Kerr Nonlinearity

We propose an optical scheme for the generation of the cluster-type entangled coherent states in free travelling optical fields via cross-Kerr nonlinearity. The required resources for the generation are coherent state source, beam splitters, photodetectors, and Kerr media. We also discuss the implementation of the Hadamard gate operation for coherent states and the homodyne detection.     

Effect of dissipation and reservoir temperature on squeezing exchange and emergence of entanglement between two coupled bosonic modes

We study the effect of a thermal reservoir on the squeezing transfer and entanglement between two identical harmonic oscillators, caused by a bilinear coupling of the RWA type. We analyze the evolution of the invariant squeezing coefficients of each mode for arbitrary initially factorized mixed states, as well as the separability coefficient based on Simon's criterion for Gaussian states. We show the importance of initial squeezing for the emergence of entanglement and the existence of critical temperatures above which no squeezing transfer or entanglement are possible.