Zhi-Xi Wang

Deterministic secure direct communication using GHZ states and swapping quantum entanglement

We present a deterministic secure direct communication scheme via entanglement swapping, where a set of ordered maximally entangled three-particle states (GHZ states), initially shared by three spatially separated parties, Alice, Bob and Charlie, functions as a quantum information channel. After ensuring the safety of the quantum channel, Alice and Bob apply a series of local operations on their respective particles according to the tripartite stipulation and the secret message they both want to send to Charlie. By three of Alice, Bob and Charlies Bell measurement results, Charlie is able to infer the secret messages directly. The secret messages are faithfully transmitted from Alice and Bob to Charlie via initially shared pairs of GHZ states without revealing any information to a potential eavesdropper. Since there is no transmission of the qubits carrying the secret message between any two of them in the public channel, it is completely secure for direct secret communication if a perfect quantum channel is used.

Assisted state discrimination without entanglement

It is shown that the dissonance, a quantum correlation which is equal to quantum discord for separable state, is required for assisted optimal state discrimination. We find that only one side discord is required in the optimal process of assisted state discrimination, while another side discord and entanglement is not necessary. We confirm that the quantum discord, which is asymmetric depending on local measurements, is a resource for assisted state discrimination. With the absence of entanglement, we give the necessary and sufficient condition for vanishing one side discord in assisted state discrimination for a class of d nonorthogonal states. As a by-product, we find that the positive-partial-transposition condition is the necessary and sufficient condition for the separability of a class of 2 x d states.

A lower bound of concurrence for multipartite quantum states

We study the concurrence of arbitrary multipartite mixed quantum states. An explicit lower bound of the concurrence is derived, which detects quantum entanglement of some states better than some separability criteria, and gives sufficient conditions for distilling GHZ states from tripartite states. An interesting relation between the lower bound of the concurrence for bipartite states and for tripartite states has been presented.

Super-quantum correlation and geometry for Bell-diagonal states with weak measurements

We propose “weak one-way deficit” by weak measurements as the generalization of one-way deficit defined for standard projective measurements. The weak one-way deficit for Werner state is obtained analytically. We find that weak one-way deficit is smaller than the standard one-way deficit, which contrasts with a straightforward expectation based on the known fact that super-quantum discord by weak measurement is always larger than the quantum discord defined by projective measurement. On the other hand, by tuning the weak measurement continuously to the projective measurement, both weak one-way deficit and super-quantum discord converge to the same value, which is either the one-way deficit or the quantum discord both quantifying quantum correlation. In this sense, weak measurement does not necessarily capture more quantumness of correlations. We also give the geometry of super-quantum discord of the Bell-diagonal states with explicit geometrical figures. As an application, the dynamic behavior of super-quantum correlation including super-quantum discord and weak one-way deficit under decoherence is investigated. We find that the order relation of the super-quantum correlation and the quantum correlation remained unchanged under the phase flipping channel for the Bell-diagonal states and the Werner states.

A note on entanglement of formation and generalized concurrence

We discuss a kind of generalized concurrence for a class of high-dimensional quantum pure states such that the entanglement of formation is a monotonically increasing convex function of the generalized concurrence. An analytical expression of the entanglement of formation for a class of high-dimensional quantum mixed states is obtained.

One-Way Information Deficit and Geometry for a Class of Two-Qubit States

The work deficit, as introduced by Jonathan Oppenheim et al. [Phys. Rev. Lett. 89 (2002) 180402] is a good measure of the quantum correlations in a state and provides a new standpoint for understanding quantum non-locality. In this paper, we analytically evaluate the one-way information deficit (OWID) for the Bell-diagonal states and a class of two-qubit states and further give the geometry picture for OWID. The dynamic behavior of the OWID under decoherence channel is investigated and it is shown that the OWID of some classes of X states is more robust against the decoherence than the entanglement.

Separability of rank two quantum states on multiple quantum spaces

Explicit sufficient and necessary conditions for separability of N-dimensional rank two multiparty quantum mixed states are presented. A nonseparability inequality is also given, for the case where one of the eigenvectors corresponding to nonzero eigenvalues of the density matrix is maximally entangled.

Experimental Determination of Entanglement for Arbitrary Pure States

We present a way of experimentally determining the concurrence in terms of the expectation values of local observables for arbitrary multipartite pure states. Instead of the joint measurements on two copies of a state in the experiment for two-qubit systems [S. P. Walborn et al., Nature (London) 440, 20 (2006)], we only need one copy of the state in every measurement for any arbitrary dimensional multipartite systems, avoiding the preparation of twin states or the imperfect copy of the state.

A note on fully entangled fraction

We investigate the general characteristics of the fully entangled fraction for quantum states. The fully entangled fractions of isotropic states and Werner states are analytically computed.

One-way deficit of two-qubit

Quantum deficit originates in questions regarding work extraction from quantum systems coupled to a heat bath (Oppenheim et al. in Phys Rev Lett 89:180402, 2002). It links quantum correlations with quantum thermodynamics and provides a new standpoint for understanding quantum non-locality. In this paper, we propose a new method to evaluate the one-way deficit for a class of two-qubit states. The dynamic behavior of the one-way deficit under decoherence channel is investigated, and it is shown that the one-way deficit of the