X. X. Yi

Robust preparation of Greenberger-Horne-Zeilinger and w states of three distant atoms

Schemes to generate Greenberger-Horne-Zeilinger(GHZ) and W states of three distant atoms are proposed in this paper. The schemes use the effects of quantum statistics of indistinguishable photons emitted by the atoms inside optical cavities. The advantages of the schemes are their robustness against detection inefficiency and asynchronous emission of the photons. Moreover, in Lamb-Dicke limit, the schemes do not require simultaneous click of the detectors, this makes the schemes more realizable in experiments.

Driving quantum systems into decoherence-free subspaces by Lyapunov control

We present a scheme to drive a finite-dimensional quantum system into the decoherence-free subspaces(DFS) by Lyapunov control. Control fields are established by Lyapunov function. This proposal works well for both closed and open quantum systems, with replacing the DFS by desired subspaces for closed systems. An example which consists of a four-level system with three degenerate states driven by three lasers is presented to gain further insight of the scheme, numerical simulations for the dynamics of the system are performed and the results are good.

Entanglement of two atoms through different couplings and thermal noise

The entanglement of two atoms is studied when the two atoms are coupled to a single-mode thermal field with different couplings. The different couplings of the two atoms is favourable to entanglement: they not only make the case that two atoms with different couplings can be entangled (whereas atoms with the same couplings are not entangled), but also enhance the entanglement when the relative difference of the two couplings is in a certain range. We also show that the relative difference of the couplings can improve the critical temperature. If the optical cavity is leaky during the time evolution, for the initial atomic state , the dissipative thermal environment benefits the production of entanglement when the cavity is initially in a vacuum state.

Geometric phases induced in auxiliary qubits by many-body systems near their critical points

The geometric phase induced in an auxiliary qubit by a many-body system is calculated and discussed. Two kinds of coupling between the auxiliary qubit and the many-body system are considered, which lead to dephasing and dissipation in the qubit, respectively. As an example, we consider the

Entanglement induced in spin-1/2 particles by a spin chain near its critical points

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Concurrence of superpositions

spin chain dephasingly coupled to a qubit. The geometric phase induced in the qubit is presented and discussed. The results show that the geometric phase might be used to signal the critical points of a many-body system, and it tends to zero as the parameters of the many-body system go away from the critical points.

Effect of intersubsystem coupling on the geometric phase in a bipartite system

A relation between entanglement and criticality of spin chains is established. The entanglement we exploit is shared between auxiliary particles, which are isolated from each other, but are coupled to the same critical spin-1/2 chain. We analytically evaluate the reduced density matrix, and numerically show the entanglement of the auxiliary particles in the proximity of the critical points of the spin chain. We find that the entanglement induced by the spin chain may reach 1, and it can signal very well the critical points of the chain. A physical understanding and experimental realization with trapped ions are presented.

Quantum correlation measure in arbitrary bipartite systems

Bounds on the concurrence of the superposition state in terms of the concurrences of the states being superposed are found in this paper. The bounds on concurrence are quite different from those on the entanglement measured by von Neumann entropy [Linden et al., Phys. Rev. Lett. 97, 100502 (2006)]. In particular, a nonzero lower bound can be provided if the states being superposed are properly constrained.

Geometric phase in dephasing systems

The influence of intersubsystem coupling on the cyclic adiabatic geometric phase in bipartite systems is investigated. We examine the geometric phase effects for two uniaxially coupled spin-(1/2) particles, both driven by a slowly rotating magnetic field. It is demonstrated that the relation between the geometric phase and the solid angle enclosed by the magnetic field is broken by the spin-spin coupling, in particular leading to a quenching effect on the geometric phase in the strong coupling limit.

Effective Hamiltonian Approach to Open Systems and Its Applications

Quantum correlation with a novel definition is presented for an arbitrary bipartite quantum state in terms of the skew information of the complete set of rank-one orthogonal projectors. This definition not only inherits the good properties of skew information including the contractivity, but also shows a powerful analytic computability for a large range of states. In addition, the measure for a general state can be easily numerically obtained by the well-developed technique of the approximate joint diagonalization. As a comparison, we give both the analytic and the numerical quantum correlation for many high-dimensional states. The relation between our measure and quantum metrology is also analyzed.