The Entanglement Entropy of Some Few-qubit XY Model in a Transverse Field
Zhang chao, Ma Chuan-tao, Gong Yan-xiang

Abstract
The entanglement entropy of three-qubit and four-qubit for anisotropic XY model in a transverse field are discussed in this paper. We find that the entanglement entropy scales with the volume of the subsystem with the increment of temperature, and the RS (the rate of the entropies of the subsystems) changes from some constant to another one with temperature getting high, and has the similar behavior with the transverse field varies. The ?S (the entropy of the system minus the sum of the entropies of the subsystems) from zero to some negative constant with the field strengthen, and the derivative of which can be used to study the quantum phase transition.

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