We study the quasienergy spectrum and the nonadiabatic time dynamics of the open quantum system, which incorporates the two-level atom dipole coupled to both the quantized mode and the polychromatic classical field. Both the numeric calculations and the original perturbation technique are applied to track the quasilevel positions against the strength of the classical field. The first-order perturbation theory predicts the singular behavior of the system including the infinitely large squeezing of the quantized mode quadratures at certain values of the field strength. The singularity is removed here by allowing for the nonadiabatic transitions among the quasienergy states and the higher perturbations as well.

• Received 14 May 2001

DOI:https://doi.org/10.1103/PhysRevA.64.053801