Two Qubits Tavis-Cummings Model Beyond the Rotating Wave Approximation:   Degenerate Regime
  We study the dynamics of two qubits interacting with a single mode of a harmonic oscillator beyond the rotating wave approximation in the ideally degenerate regime. Exact analytic expressions are obtained for state properties of interest, including qubit entanglement for a certain class of initial states of the oscillator and the qubits. Qualitative differences and similarities in the evolution of the qubits in the degenerate regime when the oscillator is treated quantum mechanically and classically are discussed. 
S. Agarwal, S.M. Hashemi Rafsanjani and J.H. Eberly
quant-ph