  This paper studies composite quantum systems, like atom-cavity systems and
coupled optical resonators, in the absence of external driving by resorting to
methods from quantum field theory. Going beyond the rotating wave
approximation, it is shown that the usually neglected counter-rotating part of
the Hamiltonian relates to the entropy operator and generates an irreversible
time evolution. The vacuum state of the system is shown to evolve into a
generalized coherent state exhibiting entanglement of the modes in which the
counter-rotating terms are expressed. Possible consequences at observational
level in quantum optics experiments are currently under study.
