Solutions to the quantum Rabi model with two equivalent qubits
  Using extended coherent states, an analytically exact study has been carried out for the quantum Rabi model with two equivalent qubits. Compact transcendental functions of one variable have been derived leading to exact solutions. The energy spectrum is clearly identified and analyzed. Also obtained analytically is the necessary and sufficient conditions for the occurrence of isolated exceptional solutions, which are not doubly degenerate as in the one-qubit quantum Rabi model. 
Hui Wang, Shu He, Liwei Duan, Yang Zhao, and Qing-Hu Chen
quant-ph