Exact treatment for the entanglement of the multiphoton two-qubit system with the single-mode thermal field

In this paper, we study the entanglement between two two-level atoms (qubits) when they interact simultaneously with a single-mode thermal field through exchanging k photons. We demonstrate the entanglement of the bipartite by making use of the concurrence. Contrary to previous studies, we deduce an exact form of the wave function of the system for various types of initial atomic states. We show that the asymmetric case (i.e., nonidentical atoms) can generate amounts of entanglement much greater than those of the symmetric one. These amounts are sensitive to the value of the transition parameter k and the type of initial atomic states. We present a novel phenomenon: for a particular type of Bell states, the initial entanglement can be trapped through the evolution of the system. This would be of a great interest in the framework of quantum memory.