Multiphoton interaction of a qutrit with single-mode quantized field in   the ultrastrong and deep strong coupling regimes
  We consider multiphoton dynamics of a quantum system composed of a three-state atom (a qutrit) and a single-mode photonic field in the ultrastrong and deep strong coupling regimes, when the coupling strength is comparable to or larger than the oscillator energy scale. We assume a qutrit to be in a polar-$\Lambda $ configuration in which two lower levels have mean dipole moments. Direct multiphoton resonant transitions revealing generalized Rabi oscillations, collapse, and revivals in atomic excitation probabilities for the ultrastrong couplings are studied. In the deep strong coupling regime particular emphasis is placed on the ground state of considering system which exhibits strictly nonclassical properties. 
H.K. Avetissian, A.K. Avetissian, G.F. Mkrtchian, O.V. Kibis
quant-ph