Spin bath decoherence of quantum entanglement

We study an analytically solvable model for decoherence of a two spin system embedded in a large spin environment. As a measure of entanglement, we evaluate the concurrence for the Bell states (Einstein-Podolsky-Rosen pairs). We find that while for two separate spin baths all four Bell states lose their coherence with the same time dependence, for a common spin bath, two of the states decay faster than the others. We explain this difference by the relative orientation of the individual spins in the pair. We also examine how the Bell inequality is violated in the coherent regime. Both for one bath and two bath cases, we find that while two of the Bell states always obey the inequality, the other two violate the inequality at early times.     

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.     

Sudden death time of two-qubit entanglement in a noisy environment

We present a simple equation to predict the sudden death time of two-qubit entanglement in a noisy environment. This result is valid for all two-qubit systems, no matter what kind of noise is considered.     

Local basis-dependent noise-induced Bell-nonlocality sudden death in tripartite systems

We demonstrate that multipartite Bell-inequality violations can be fully destroyed in finite time in three-qubit systems subject only to the mechanism of local external asymptotic dephasing noise. This broadens the study of local-noise-induced sudden death of nonlocal behavior, extending it beyond the realm of bipartite systems, to which it had previously been restricted.     

Controlling entanglement sudden death and birth in cavity QED

We present a scheme to control the entanglement sudden birth and death in a cavity quantum electrodynamics system, which consists of two noninteracting atoms each locally interacting with its own vacuum field, by applying and adjusting classical driving fields.     

Entanglement in eight-qubit graph states

Any 8-qubit graph state belongs to one of the 101 equivalence classes under local unitary operations within the Clifford group. For each of these classes we obtain a representative which requires the minimum number of controlled-Z gates for its preparation, and calculate the Schmidt measure for the 8-partite split, and the Schmidt ranks for all bipartite splits. This results into an extension to 8 qubits of the classification of graph states proposed by Hein, Eisert, and Briegel [M. Hein, J. Eisert, H.J. Briegel, Phys. Rev. A 69 (2004) 062311].     

Generic tripartite Bell nonlocality sudden death under local phase noise

We definitively show, using an explicit and broadly applicable model, that local phase noise that is capable of eliminating state coherence only in the infinite-time limit is capable of eliminating nonlocality in finite time in three two-level systems prepared in the Bell-nonlocal tripartite states of the generic entanglement class.     

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.     

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.     

Robust entanglement

It is common belief among physicists that entangled states of quantum systems lose their coherence rather quickly. The reason is that any interaction with the environment which distinguishes between the entangled sub-systems collapses the quantum state. Here we investigate entangled states of two trapped Ca⁺ ions and observe robust entanglement lasting for more than 20 s.     

Control of the entanglement of a two-level atom in a dissipative cavity via a classical field

We investigate the entanglement dynamics and purity of a two-level atom, which is additionally driven by a classical field, interacting with a coherent field in a dissipative environment. It is shown that the amount of entanglement and the purity of the system can be improved by controlling the classical field.     

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.     

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.     

Entanglement of Bell diagonal mixed states

A sufficient and necessary condition for separability of Bell diagonal mixed states for bipartite systems in higher dimensions is presented. Moreover, we present a necessary condition for genuine entanglement of Bell diagonal mixed states in higher dimensions for multipartite systems.     

Sudden death and birth of entanglement beyond the Markovian approximation

We investigate the entanglement dynamics of two initially entangled qubits interacting independently with two uncorrelated reservoirs beyond the Markovian approximation. Quite different from the Markovian reservoirs [C.E. López, et al., Phys. Rev. Lett. 101 (2008) 080503], we find that entanglement sudden birth (ESB) of the two reservoirs occurs without certain symmetry with respect to the entanglement sudden death (ESD) of the two qubits. A phenomenological interpretation of entanglement revival is also given.     

Bounds on bipartitely shared entanglement reduced from superposed tripartite quantum states

For a tripartite pure state superposed by two individual states, the bipartitely shared entanglement can always be achieved by local measurements of the third party. Consider the different aims of the third party, i.e. maximizing or minimizing the bipartitely shared entanglement, we find bounds on both the possible bipartitely shared entanglement of the superposition state in terms of the corresponding entanglement of the two states being superposed. In particular, by choosing the concurrence as bipartite entanglement measure, we obtain calculable bounds for tripartite (2 ⊗ 2 ⊗ n)-dimensional cases.     

Three methods to distill multipartite entanglement over bipartite noisy channels

We first assume that there are only bipartite noisy qubit channels in a given multipartite system, and present three methods to distill the general Greenberger-Horne-Zeilinger state. By investigating the methods, we show that multipartite entanglement distillation by bipartite entanglement distillation has higher yield than ones in the previous multipartite entanglement distillations.     

Strengthened Bell inequalities for orthogonal spin directions

We provide bounds on correlations of locally orthogonal observables in two-qubit separable states. These bounds strengthen the Bell inequality and improve upon some alternative entanglement criteria. They provide necessary and sufficient criteria for separability of pure states and test the correlations allowed by local hidden variable models against those allowed by separable quantum states.     

A controlled quantum dialogue protocol in the network using entanglement swapping

A quantum dialogue network is constructed, which uses GHZ states as quantum channel. As its application, a quantum dialogue protocol using entanglement swapping is presented. If the network is safe, any two users in the network can communicate under the control of the other users.     

Measurement-induced nonlocality and geometric quantum discord in two SC-charge qubits

By using geometric quantum discord and measurement-induced nonlocality, quantum correlations are investigated for two superconducting (SC) charge qubits that share a large Josephson junction where the field is assumed to be prepared initially in a coherent state. It is found that the difference between measure measurement-induced nonlocality and geometric quantum discord, of the final state of the two SC-charge qubits system which is especial case of X-states, is equal to a constant value. It is found that the quantum correlations and entanglement of the qubits are very sensitive to the mean number of the coherent photons. The entanglement exists in small intervals of death quantum discord and measurement-induced nonlocality. This is further evidence in support of the fact that quantum correlation and entanglement are not synonymous.