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 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.     

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.     

Possible Realization of Cluster States and Quantum Information Transfer in Cavity QED via Raman Transition

We present a scheme to generate cluster states with many scheme, no transfer of quantum information between the atoms in cavity QED via Raman transition. In this atoms and cavities is required, the cavity fields are only virtually excited and thus the cavity decay is suppressed during the generation of cluster states. The atoms are always populated in the two ground states. Therefore, the scheme is insensitive to the atomic spontaneous emission and cavity decay. We also show how to transfer quantum information from one atom to another.     

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.     

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.     

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.     

Preparation of Cluster States with Trapped Ions in Thermal Motion

A potential scheme is proposed for generating cluster states of many trapped ions in thermal motion, in which the effective Hamiltonian does not involve the external degree of freedom and thus the scheme is insensitive to the external state, allowing it to be thermal state. The required experimental techniques of the schemes are within the scope that can be obtained in the ion-trap setup.     

Effective Scheme for Generating Cluster States in Cavity QED

We propose a scheme to prepare many two-mode cavities into one-dimensional cluster states in the context of cavity QED. The left-circularly polarized state and right-circularly polarized state of the cavity are encoded as the logic zero and one of the qubits. In the scheme, the atomic spontaneous emission is suppressed, and the fidelity is unaffected by the cavity decay on the assumption that the detection efficiencies of all the photondetectors are 1.     

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.     

A Scheme for Generating Entangled Squeezed States Based on Cavity QED

We propose a scheme for generating entangled squeezed vacuum states of electromagnetical fields. The scheme is based on cavity QED. In this scheme, an atom interacts, successively, with a classical field, two quantum cavity fields, and another classical field. By detecting the final states of the atom, the two quantum cavity fields will be projected to an entangled state.     

Generation of Multi-Photon Cluster States through the Cavity Input--Output Process

We propose a scheme to generate the multi-photon cluster states via the cavity input-output process and the single-bit rotations. The method can be generalized to construct a series of multi-photon graph states, and the successful probability is close to unity in the ideal condition.     

Generation of Two-Atom Cluster State via Cavity QED

We propose a physical scheme for generating a two-atom cluster state through the simultaneous interaction of two two-level atoms with a single-mode cavity field prepared initially in an odd-coherent state under a large-detuned limit. The influence of the dissipation constant, the intensity of the field and the imperfect manipulation on the preparation scheme are investigated. It is shown that when the intensity of the cavity is large enough, the influence of the cavity decay is ettlciently suppressed. The possible error in the implementation of the cluster state is negligible when the time difference between two atoms crossing the cavity axis is small. It is suggested that the scheme can be realized by current technologies.     

A Scheme for Atomic Entangled States and Quantum Gate Operations in Cavity QED

We propose a scheme for controllably entangling the ground states of five-state W-type atoms confined in a cavity and realizing swap gate and phase gate operations. In this scheme the cavity is only virtually excited and the atomic excited states are almost not occupied, so the produced entangled states and quantum logic operations are very robust against the cavity decay and atomic spontaneous emission.     

A Simple Scheme for the Preparation of Four-Photon GHZ States Based on Cavity QED

A simple scheme is presented for generating four-photon Greenberger-Horne-Zeilinger states with a large detuned interaction between a three-level atom and two bimodal cavities. In the proposed protocol, the quantum information is encoded on Fock states of the cavity-fields, and the atomic spontaneous emission can be effectively suppressed due to the fact that the excited state of the atom is adiabatically eliminated under the large detuning condition. The detection of the atom can collapse the cavity to the desired state. The experimental feasibility of our proposal is also discussed.     

Qualitative aspects of the entanglement in the three-level model with photonic crystals

This communication is an enquiry into the circumstances under which concurrence and phase entropy methods can give an answer to the question of quantum entanglement in the composite state when the photonic band gap is exhibited by the presence of photonic crystals in a three-level system. An analytic approach is proposed for any three-level system in the presence of photonic band gap. Using this analytic solution, we conclusively calculate the concurrence and phase entropy, focusing particularly on the entanglement phenomena. Specifically, we use concurrence as a measure of entanglement for dipole emitters situated in the thin slab region between two semi-infinite one-dimensionally periodic photonic crystals, a situation reminiscent of planar cavity laser structures. One feature of the regime considered here is that closed-form evaluation of the time evolution may be carried out in the presence of the detuning and the photonic band gap, which provides insight into the difference in the nature of the concurrence function for atom-field coupling, mode frequency and different cavity parameters. We demonstrate how fluctuations in the phase and number entropies affected by the presence of the photonic-band-gap. The outcomes are illustrated with numerical simulations applied to GaAs. Finally, we relate the obtained results to instances of any three-level system for which the entanglement cost can be calculated. Potential experimental observations in solid-state systems are discussed and found to be promising.     

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.     

Realization of Arbitrary Positive-Operator-Value Measurement of Single Atomic Qubit via Cavity QED

Positive-operator-value measurement （POVM） is the most general class of quantum measurement. We propose a scheme to deterministically implement arbitrary POVMs of single atomic qubit via cavity QED catalysed by only one ancilla atomic qubit. By appropriately entangling two atomic qubits and sequentially measuring the ancilla qubit, any POVM can be implemented step by step. As an application of our scheme, the realization of a specific POVM for optimal unambiguous discrimination （OUD） between two nonorthogonal states is given.     

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.     

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.