We consider a two-photon quantum model of radiation-matter interaction
between a single two-level atom and a degenerate bimodal high-Q cavity field.
Within this tripartite system the explicit construction of two collective
radiation modes, one of which freely evolving and the other one quadratically
coupled to the matter subsystem, is reported. Meaning and advantages of such a
decoupling treatment are carefully discussed.Comment: accepted for publication in Physica Script
