Quantum theory of dissipation of a harmonic oscillator coupled to a non-equilibrium bath; Wigner-Weisskopf decay and physical spectra

We extend the quantum theory of dissipation in the context of a system-reservoir model, where the reservoir in question is kept in a non-equilibrium condition. Based on a systematic separation of time scales involved in the dynamics, appropriate generalizations of the fluctuation-dissipation and Einstein's relations have been pointed out. We show that the Wigner-Weisskopf decay of the system mode results in a rate constant which, depending on the relaxation of a non-equilibrium bath, is dynamically modified. We also calculate the time-dependent spectra of a cavity mode with a suitable gain when the cavity is kept in contact with a non-equilibrium bath.