  A perturbative treatment of reduced density operators of quantum subsystems
is implemented in the same spirit as Fermi Golden Rule for scattering. Analytic
expressions for linear entropy (a measure of purity loss, and in some cases of
coherence loss) and for subsystem's energy variations (dissipation) are given.
Application to electromagnetic field superposition states in a dissipative
cavity is performed. Evaluation of typical field and reservoir time scales
($\tau_{F}$ and $\tau_{R}$) show that even for small temperatures they are very
different. We also indicate the condition on the cutoff temperature above which
the decoupling assumption is quantitatively justified.
