Yu Changshui

Separability criterion of tripartite qubit systems

In this paper, we present a method to construct full separability criterion for tripartite systems of qubits. The spirit of our approach is that a tripartite pure state can be regarded as a three-order tensor that provides an intuitionistic mathematical formulation for the full separability of pure states. We extend the definition to mixed states and give out the corresponding full separability criterion. As applications, we discuss the separability of several bound entangled states, which shows that our criterion is feasible.

Global entanglement for multipartite quantum states

Based on the residual entanglement [Phys. Rev. A 71, 042331 (2005)], we present the global entanglement for a multipartite quantum state. The measure is shown to be also obtained by the bipartite partitions of the multipartite state. The distinct characteristic of the global entanglement is that it consists of the sum of different entanglement contributions. The measure can provide sufficient and necessary condition of fully separability for pure states and be conveniently extended to mixed states by minimizing the convex hull. To test the sufficiency of the measure for mixed states, we evaluate the global entanglement of bound entangled states. The properties of the measure discussed finally show the global entanglement is an entanglement monotone.

Faithful Controlled Teleportation of an Arbitrary Unknown Two-Atom State via Special W-States and QED Cavity

A scheme is proposed for the controlled teleportation of an arbitrary two-atom state via special W-type entangled states and QED cavity. The scheme does not involve the direct joint Bell-state-measurement (BSM). We show that the quantum information is split into two parts, thus the original atomic state cannot be perfectly restored by the receiver without the other agents collaboration and classical communication. In addition, the physical realization of this scheme is not difficult.

Teleportation of an Arbitrary Multipartite GHZ-Class State by One EPR Pair

We present a scheme for perfectly teleporting an arbitrary and unknown N-particle GHZ-class state from a sender to a receiver. We just need one quantum channel composed of two or three particles in the maximally entangled state. The sender performs one Bell-state measurement on two of her particles and N−1 Hadamard operations and N−1 von Neumann measurements on the rest N−1 particles. The receiver adopts one corresponding unitary transformation on his particles shared with the sender. After that, the receiver can obtain the original N-particle GHZ-class state by introducing N−1 ancillary particles and carrying out N−1 controlled-NOT operations. We also generalize the scheme to the case of controlled teleportation.

Existence criterion of genuine tripartite entanglement

In this paper, an intuitive mathematical formulation is provided to generalize the residual entanglement for tripartite systems of qubits [Phys. Rev. A 61, 052306 (2000)] to the tripartite systems in higher dimension. The spirit lies in the tensor treatment of tripartite pure states [Phys. Rev. A 72, 022333 (2005)]. A distinct characteristic of the present generalization is that the formulation for higher dimensional systems is invariant under permutation of the subsystems, hence is employed as a criterion to test the existence of genuine tripartite entanglement. Furthermore, the formulation for pure states can be conveniently extended to the case of mixed states by utilizing the Kronecker product approximate technique. As applications, we give the analytic approximation of the criterion for weakly mixed tripartite quantum states and consider the existence of genuine tripartite entanglement of some weakly mixed states.

Teleportations of Mixed States and Multipartite Quantum States

In this paper, we propose a protocol to deterministically teleport an unknown mixed state of qubit by utilizing a maximally bipartite entangled state of qubits as quantum channel. If a non-maximally entangled bipartite pure state is employed as quantum channel, the unknown mixed quantum state of qubit can be teleported with 1−(1−C2)1/2 probability, where C is the concurrence of the quantum channel. The protocol can also be generalized to teleport a mixed state of qudit or a multipartite mixed state. More important purpose is that, on the basis of the protocol, the teleportation of an arbitrary multipartite (pure or mixed) quantum state can be decomposed into the teleportation of each subsystem by employing separate entangled states as quantum channels. In the case of deterministic teleportation, Bob only needs to perform unitary transformations on his single particles in order to recover the initial teleported multipartite quantum state.

Quantum nondemolition measurement of the Werner state

We propose a theoretical scheme of quantum nondemolition measurement of two-qubit Werner state. We discuss our scheme with the two qubits restricted in a local place and then extend the scheme to the case in which two qubits are separated. We also consider the experimental realization of our scheme based on cavity quantum electrodynamics. It is very interesting that our scheme is robust against the dissipative effects introduced by the probe process. We also give a brief interpretation of our scheme finally.

Genuine tripartite entanglement and quantum phase transition

A new simplified formula is presented to characterize genuine tripartite entanglement of (2 ⊗ 2 ⊗ n)-dimensional quantum pure states. The formula turns out equivalent to that given in (Quant. Inf. Comp. 7(7) 584 (2007)), hence it also shows that the genuine tripartite entanglement can be described only on the basis of the local (2 ⊗ 2)-dimensional reduced density matrix. In particular, the two exactly solvable models of spin system studied by Yang (Phys. Rev. A 71 030302(R) (2005)) are reconsidered by employing the formula. The results show that a discontinuity in the first derivative of the formula or in the formula itself of the ground state just corresponds to the existence of quantum phase transition, which is obviously different from the concurrence.

Probing Bell Diagonal State without Disturbing Its Correlations

In this paper, we propose a very simple scheme to probe the quantum and classical correlation including quantum entanglement of Bell diagonal state. In the probing process, the correlation of Bell diagonal state, even the state itself, is not disturbed, which means a non-destructive probing. In addition, our scheme can be performed even though the two qubits of the Bell diagonal state are separate in space.

Elementary Quantum Gates Based on Intrinsic Interaction Hamiltonian

A kind of new operators, the generalized pseudo-spin operators are introduced and a universal intrinsic Hamiltonian of two-qubit interaction is studied in terms of the generalized pseudo-spin operators. A fundamental quantum gate U(θ) is constructed based on the universal Hamiltonian and shown that the roles of the new quantum gate U(θ) is equivalent, functionally, to the joint operation of Hadamard and C-Not gates.