自旋链中的热纠缠、混合度和量子通讯

研究了热平衡态下具有三个自旋的自旋链中的两体热纠缠和混合度，讨论了温度和外界磁场对量子纠缠和混合度的影响。发现系统处于基态时，通过调节外界磁场可以得到一种重要的量子混合态－最大纠缠混合态，这为制备最大纠缠混合态和调控纠缠提供了另一种途径。在此基础上，利用自旋链量子态作为量子信道进行量子通讯。讨论了温度和外界磁场对保真度的影响，发现温度和外界磁场对通讯保真度有重要影响；给出了保真度和两体热纠缠的关系，发现两体热纠缠越大不一定意味着保真度越高。

Thermal entanglement, mixture and quantum communication in a spin chain

The two-body thermal entanglement and mixture in a three-spin spin chain in thermal equilibrium state are investigated, and the effects of the temperature and magnetic field on the entanglement and mixture are discussed. It is found that, in the ground state, an important class of states, i.e., maximally entangled mixed states, can be generated by controlling magnetic field, which provide a new way to generate maximally entangled mixed states and control entanglement. Further, the spin chain in the thermal equilibrium is used as a quantum channel for quantum communication. The effects of temperature and magnetic field on fidelity are discussed. It is shown that the temperature and magnetic field have vast effects on the fidelity. The relation between fidelity and two-body thermal entanglement is given, which shows that more amount of entanglement does not means higher fidelity.