基于二比特自旋压缩模型中的热纠缠研究
Thermal entanglement in a Two-qubit spin squeezing model

利用Concurrence判据，讨论了基态和有限温度下二比特自旋压缩模型中的量子热纠缠性质. 探究了自旋压缩参量 、外磁场 以及温度T对系统纠缠性质的影响. 结果发现，首先，Concurrence随着耦合参量 （或 ）的增大先增大到最大值，然后逐渐衰减到零，Concurrence的最大值随参数 （或 ）的增大而增大；Concurrence的衰减率会随参数 （或 ）的增大而减小，从而可以在较大的范围内得到体系热纠缠. 其次，较大的耦合参量取值可以提升Concurrence存在的温度范围. 最后，在有限温度条件下，当参数 （或 ）取值较小时，体系的纠缠保持为零，即存在纠缠“死亡间隔”现象；然而随着参量 （或 ）的取值增大到一定值时，Concurrence会突然产生，即存在纠缠瞬间产生（“Sudden birth”）现象，值得注意的是：纠缠的“死亡间隔”会随着参数 （或 ）的增大而减小.
Using the concurrence criterion, the thermal entanglement properties in a two-qubit spin squeezing model are investigated in detail in the cases of ground state and the finite temperature. We explore the influences of spin squeezing parameter 、magnetic field and temperature T on the quantum entanglement properties. Firstly, the results show that the concurrence initially increases and then decreases to zero with the improving coupling parameter （or ）, the maximum value of the concurrence increases with the improving parameter （or ）. The rate of entanglement decays is decreased with the improvement of the two parameters, and it can broaden the region of the entanglement existences. Secondly, larger values of the coupling parameters can broaden the temperature region of the nonzero concurrence. Finally, in the finite temperature case, when the value of （or ）becomes smaller the entanglement persists to be zero, that is to say, there is a “entanglement sudden death” phenomenon. Whereas as the parameter （or ）is further increased to one fixed constant value, the concurrence suddenly appears, that is to say, the entanglement quantified by concurrence exhibits a “sudden birth” phenomenon. One interesting point is that the interval of entanglement sudden death is decreased with the improvement of （or ）.