A study on entanglement dynamics for a four-qubit model

In this paper, we consider the entanglement dynamics of a four-qubit model 2006 {\em Phys. Rev.} A \textbf{74} 042328] where two entangled qubits a and b locally interact with separate qubits A and B via the spin-exchange-like Hamiltonian. We study the effect of purity of initial entangled state of qubits a, b on the entanglement evolution and its relation with energy transfer. Also, we find that the total bipartite entanglement of qubits a, b plus A, B is not a constant any longer when the initial entangled state of a, b is not pure, which is a complement to the result in the paper 2007 {\em J. Phys.} B \textbf{40} S45] for the pure case.