Yong-De Zhang

Maximal violation of Bell's inequalities for continuous variable systems.

We generalize Bells inequalities to biparty systems with continuous quantum variables. This is achieved by introducing the Bell operator in perfect analogy to the usual spin- 1/2 systems. It is then demonstrated that two-mode squeezed vacuum states display quantum nonlocality by using the generalized Bell operator. In particular, the original Einstein-Podolsky-Rosen states, which are the limiting case of the two-mode squeezed vacuum states, can maximally violate Bells inequality due to Clauser, Horne, Shimony, and Holt. The experimental aspect of our scheme is briefly considered.

Teleportation scheme ofS-level quantum pure states by two-level Einstein-Podolsky-Rosen states

Unknown quantum pure states of arbitrary but definite S-level of a particle can be transferred onto a group of remote two-level particles through two-level EPRs as many as the number of those particles in this group. We construct such a kind of teleportation, the realization of which need a nonlocal unitary transformation to the quantum system that is made up of the s-level particle and all the two-level particles at one end of the EPRs, and measurements to all the single particles in this system. The unitary transformation to more than two particles is also written into the product form of two-body unitary transformations.

Comprehensive test of entanglement for two-level systems via the indeterminacy relationship.

A 3-setting Bell-type inequality enforced by the indeterminacy relation of complementary local observables is proposed as an experimental test of the 2-qubit entanglement. The proposed inequality has an advantage of being a sufficient and necessary criterion of the separability. Therefore any entangled 2-qubit state cannot escape the detection by this kind of tests. It turns out that the orientation of the local testing observables plays a crucial role in our perfect detection of the entanglement.

Multipartite pure-state entanglement and the generalized Greenberger-Horne-Zeilinger states

We show that not all four-party pure states are Greenberger-Horne-Zeilinger (GHZ) reducible (i.e., can be generated reversibly from a combination of two-, three-, and four-party maximally entangled states by local quantum operations and classical communication asymptotically). We also present some properties of the relative entropy of entanglement for those three-party pure states that are GHZ reducible, and then we relate these properties to the additivity of the relative entropy of entanglement.

Classifying N-qubit entanglement via Bell's inequalities.

All the states of N qubits can be classified into N-1 entanglement classes from 2-entangled to N-entangled (fully entangled) states. Each class of entangled states is characterized by an entanglement index that depends on the partition of N. The larger the entanglement index of a state, the more entangled or the less separable is the state in the sense that a larger maximal violation of Bells inequality is attainable for this class of state.

A necessary and sufficient criterion for multipartite separable states

Abstract We present a necessary and sufficient condition for the separability of multipartite quantum states, this criterion also tells us how to write a multipartite separable state as a convex sum of separable pure states. To work out this criterion, we need to solve a set of equations, actually it is easy to solve these equations analytically if the density matrix of the given quantum state has few nonzero eigenvalues.

Classical capacity of a quantum multiple-access channel

We consider the transmission of classical information over a quantum channel by two senders. The channel capacity region is shown to be a convex hull bound by the Von Neumann entropy and the conditional Von Neumann entropy. We discuss some possible applications of our result. We also show that our scheme allows a reasonable distribution of channel capacity over two senders.

Quantum transformation theory in fermion Fock space

In this article a general linear quantum transformation U for the following fermion system with n modes, (b’+,b’)=U(b+,b)U−1=(b+,b)(B, A, CD), is studied. All above transformations in Fock space are proved to form a ray representation of a group which is isomorphic with O(2n,C). The explicit expressions of operator U in terms of the normal ordered product and non‐normal ordered product are obtained. A series of auxiliary identities are given. It has been mentioned that the results are applicable to the case of fermion fields with interaction.

Adiabatic Condition and Quantum Geometric Potential

In this paper, we present a U(1)-invariant expansion theory of the adiabatic process. As its application, we propose and discuss new sufficient adiabatic approximation conditions. In the new conditions, we find a new invariant quantity referred as quantum geometric potential (QGP) contained in all time-dependent processes. Furthermore, we also give detailed discussion and analysis on the properties and effects of QGP.

COMMENT ON: ON THE QUANTUM ZENO EFFECT BY NAKAZATO ET AL.

Abstract Recently, using a specific example involving neutron spin, Nakazato et al. [Phys. Lett A 199 (1995) 27] argued that the uncertainty principle would impose a remarkable limitation on the mathematical limit involved in the quantum Zeno effect. In this paper we show that it is impossible for Nakazato et al. to obtain this limitation by the uncertainty relation, because an unsuitable assumption has been used in their deduction process.