Generation of remote W-type entangled state via tripartite entanglement swapping of continuous variables

We propose a scheme for generating remote W-type entangled state via tripartite entanglement swapping of continuous variables, where two EPR pairs and a local W-type entangled state are required. Because of the co-existence of both bipartite and tripartite entanglement in a W-type entangled state, the three involved remote regions, without direct interaction, will become entangled after the prescribed entanglement swapping.     

Thermal entanglement of a two-qutrit XX spin chain with Dzialoshinski-Moriya interaction

The effect of Dzialoshinski-Moriya (DM) interaction on thermal entanglement of a two-qutrit XX spin chain in a homogenous magnetic field is investigated. Our results imply that DM interaction and the magnetic field play competing roles in enhancing thermal entanglement.     

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.     

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.     

Experimental realization of deterministic entanglement transformations of bipartite pure states

We experimentally realize a kind of entanglement transformations of bipartite pure states with 100% efficiency. The protocol employs two-outcome positive operator-valued measures (POVMs) and can transform two-photon maximally entangled state to any two-photon entangled pure state with 100% efficiency. The average fidelity of all output states is 96%. Moreover, the scheme to implement arbitrary POVM on single-photon polarization state is also discussed. In principle, our setup can be applied to any kind of entanglement transformations of two-qubit entangled states to achieve optimal successful probability.     

A generalized microscopic model of quantum jump superoperators

In this paper, we discuss a microscopic model for the detector-field system in the theory of the quantum counting process. The detector-field system is described by a generalized Jaynes-Cummings model, from which we derive an analytic expression of the general quantum jump superoperators. To illustrate the universality and convenience of our method, we consider two specific examples with the help of the analytic expression.     

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.     

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.     

Analysis of a teleportation scheme involving cavity field states in a linear superposition of Fock states

We analyse a teleportation scheme of cavity field states. The experimental sketch discussed makes use of cavity quantum electrodynamics involving the interaction of Rydberg atoms with superconducting (micromaser) cavities as well as with classical microwave (Ramsey) cavities. In our scheme the Ramsey cavities and the atoms play the role of auxiliary systems used to teleport a field state, which is formed by a linear superposition of vacuum |∅〉 and the one-photon state |1〉, from a micromaser cavity to another.     

Stability investigation on quantum correlations of coupled qubits in non-Markovian process

Utilizing the concurrence and the quantum discord as the measure method, in this paper we compare and investigate the dynamic evolution features of quantum correlations of coupled qubits in non-Markovian process. We focus attention on decoherence effect influences the stability of quantum correlations. The investigation results show that because of the decoherence influence between the system and environment, the concurrence always evolves with time in oscillation form in the way of deaths and survivals, however, the quantum discord time evolution does not appear the deaths and survivals. The quantum discord survives obviously longer than concurrence, which indicates that quantum discord has a stronger ability to resist decoherence than entanglement. Through regulating and controlling the purity and entanglement of the initial quantum state, we can effectively suppress the decay of the quantum correlations, which is advantaged to complete the quantum information processing.     

Comment on “Controlled DSQC using five-qubit entangled states and two-step security test”

A controlled deterministic secure quantum communication protocol [X.M. Xiu, L. Dong, Y.J. Cao, F. Chi, Opt. Commun. 282 (2009) 333] with five-qubit entangled states was proposed recently. The aim of Xiu et al. was that the successful realization of communication between Alice and Bob needed the cooperation of a controller, Charlie. However, we show that the controller Charlie’s role could be excluded unknowingly. Moreover, an eavesdropper can entangle an ancilla without introducing any error in the first security test and then she can distill a quarter of the secret messages from her entangled ancilla.     

Critical properties of entanglement in the Dicke model with the dipole-dipole interactions

In this paper, we investigate two aspects of entanglement properties of the ground state for the Dicke model with the dipole-dipole interaction between the atoms in the thermodynamic limit, and observe how they are affected by the quantum phase transition. The appearance of dipole-dipole interactions between the atoms does not change the maximum atom-field entanglement at the critical point, while it changes the maximum atom-atom entanglement at the critical point, and has an important influence on the atom-atom entanglement behavior.     

Efficient implementation of a nonlocal gate with nonmaximal entanglement

We present two schemes for efficient implementation of a nonlocal gate with nonmaximal entanglement. The main strategy of the schemes is local conversion of pure states, which consists of a generalized measurement described by a positive operator-valued measure (POVM), one-way classical communication, and corresponding unitary operations. First, we discuss the way to generate determinately the nonlocal gate via any pure shared entangled state combined with entanglement-assistance. Then we propose the other way to generate probabilistically the nonlocal gate via any pure entangled state with the aid of ancillary particles.     

Relative entropy of entanglement of rotationally invariant states

We calculate the relative entropy of entanglement for rotationally invariant states of spin- and arbitrary spin-j particles or of spin-1 particle and spin-j particle with integer j. A lower bound of relative entropy of entanglement and an upper bound of distillable entanglement are presented for rotationally invariant states of spin-1 particle and spin-j particle with half-integer j.     

Scheme for implementing perfect quantum dense coding with three-atom W-class state in cavity QED

We propose an experimentally feasible scheme of implementing perfect quantum dense coding with three-atom W-class state in cavity QED. In this scheme atoms interact simultaneously with a highly detuned cavity field and the cavity is only virtually excited, thus the scheme is insensitive to the cavity decay, which is very important in view of experiment. Moreover, we also propose a scheme of transmitting three bits of classical information by sending one qubit and one classical bit with 3-qubit W-class and GHZ states.     

Robustness investigations on quantum entanglement and quantum discord of coupled qubits in squeezed vacuum reservoir

We investigate the quantum discord of coupled qubits in squeezed vacuum reservoir and compare it with the quantum entanglement of system. We find that the quantum discord and entanglement perform completely oppositely with the change of squeezed parameters. Quantum discord survives longer with the increase of squeezed amplitude parameter and entanglement death faster on the contrary. Under high squeezed amplitude parameter, the quantum discord can keep nonzero which indicate that the quantum discord is more robust than entanglement. We also find that the purity reduction of the initial quantum state will lead to the decay of concurrence or quantum discord. However, the quantum discord damps remarkably more slowly and survives longer than concurrence.     

Purity and entanglement of two-photon polarization states generated by spontaneous parametric down-conversion

We elucidate the dependence of purity and entanglement of two-photon states generated by spontaneous parametric down-conversion on the parameters of the source, such as crystal length, pump beam divergence, frequency bandwidth, and detectors angular aperture. The effect of crystal anisotropy is taken into account. Numerical simulations are presented for two types of commonly used source configurations.     

Quantum information and entanglement transfer for qutrits

We propose a scheme for the transfer of quantum information among distant qutrits. We apply this scheme to the distribution of entanglement of qutrits states among distant nodes and to the generation of multipartite antisymmetric states. We also discuss applications to quantum secret sharing.     

Atom-field entanglement in two-atom Jaynes-Cummings model with nondegenerate two-photon transitions

An exact solution of the problem of two two-level atoms with nondegenerate two-photon transitions and nondegenerate Raman transitions interacting with two-mode radiation field is presented. Asymptotic solutions for system state vectors are obtained in the approximation of large initial coherent fields. The atom-field entanglement is investigated on the basis of the reduced atomic entropy dynamics. The possibility of the system being initially in a pure disentangled state to revive into this state during the evolution process for both models is shown. Conditions and times of disentanglement are derived.