Decoherence of two qubits in a non-Markovian reservoir without   rotating-wave approximation
  The decoherence of two initially entangled qubits in a non-Markovian reservoir has been investigated exactly without Born Markovian approximation and rotating-wave approximation(RWA). The non-perturbative quantum master equation is derived and its exact solution is obtained. The decoherence behaviors of two qubits, initially entangled in Bell states, has been investigated in three different cases of parameters. The results show that the counter-rotating wave terms have great influence on the decoherence behavior, and there are differences between the exact solution of the Hamiltonian with RWA and that of the exact Hamiltonian without RWA. It might be improper to take RWA when the atom is in a non-markovian reservoir. 
Fa-Qiang Wang, Zhi-Ming Zhang and Rui-Sheng Liang