Dynamics of two qubits in a spin-bath of Quantum anisotropic Heisenberg   XY coupling type
  The dynamics of two 1/2-spin qubits under the influence of a quantum Heisenberg XY type spin-bath is studied. After the Holstein-Primakoff transformation, a novel numerical polynomial scheme is used to give the time-evolution calculation of the center qubits initially prepared in a product state or a Bell state. Then the concurrence of the two qubits, the $z$-component moment of either of the subsystem spins and the fidelity of the subsystem are shown, which exhibit sensitive dependence on the anisotropic parameter, the temperature, the coupling strength and the initial state. It is found that (i) the larger the anisotropic parameter $\gamma$, the bigger the probability of maintaining the initial state of the two qubits; (ii) with increasing temperature $T$, the bath plays a more strong destroy effect on the dynamics of the subsystem, so does the interaction $g_0$ between the subsystem and the bath; (iii) the time evolution of the subsystem is dependent on the initial state. The revival of the concurrence does not always means the restore of the state. Further, the dynamical properties of the subsystem should be judged by the combination of concurrence and fidelity. 
Jun Jing and Zhi-Guo L\"u