Search

By adopting the concept of fidelity, we investigated efficiency of quantum state transfer with the XX chain as the quantum channel. Different from the previous works, we concentrated on effects of spin and magnetic impurity on fidelity of quantum state transfer. Our results revealed that the spin impurity cannot prevent one from implementing perfect transfer of an arbitrary one-qubit pure state across the spin channel, however, the presence of magnetic impurity or both spin and magnetic impurities may destroy the otherwise perfect spin channels., Comment: 5 pags

Entanglement dynamics of the N-qubit XY model in thermal and dephasing environments are investigated by solving the Lindblad form of the master equation. Analytical solutions for the two-qubit case and numerical solutions for the multi-qubit case are obtained. For the two-qubit case, our results revealed two main features for entanglement evolution from different initial states. First, the thermal reservoir always induces degradation of the entanglement, and the entanglement may undergo sudden death during certain intervals of the evolution time. Second, the dephasing environment induces damped oscillation of the entanglement for initially separable states and mixed states with relative large values of Δ or J; however, it always induces exponentially decay of the entanglement for the initial Bell states. For the multi-qubit case, our results show that the entanglement decreases monotonically as the time evolves for the initial W state, and behaves as damped oscillation for the initial “one-particle” state. Particularly, for system with large number of qubits, the curves of the concurrence C12 with different N are almost overlapped in dephasing environment.

By analyzing parameter-dependence of the linear entropy and the negativity of a block of L/2 odd-number spins, we study quantum phase transitions (QPTs) in the spin-1/2 Heisenberg XXZ model, the dimerized Heisenberg model, the dimerized-frustrated Heisenberg model and the spin-1 quadratic–biquadratic Heisenberg model. The numerical results reveal that most of the QPTs can be detected exactly by discontinuities, singularities or extreme points of these two quantities with finite-size samples. This fact implies that the linear entropy and the negativity can serve as efficient indicators of QPTs occurring in a variety of condensed-matter systems.

By analytically solving the master equation, we examine entanglement dynamics of the two-qubit spin system coupled via the XY interaction and in the presence of population relaxation. We found that the entanglement dynamical behaviors are very sensitive to different initialized states. Particularly, for initial states ϱ=(|01〉〈01|+|10〉〈10|)/2 and ϱ=(|00〉〈00|+|11〉〈11|)/2, a combination of the XY coupling with the population relaxation can even be used to enhance the entanglement. Moreover, we show that among different initially separable states, ϱ=|01〉〈01| and ϱ=|10〉〈10| are more robust in generating pairwise entanglement.