  We shall revisit the conventional adiabatic or Markov approximation, showing
its intrinsic failure in describing the proper quantum-mechanical evolution of
a generic subsystem interacting with its environment. In particular, we shall
show that -contrary to the semiclassical case- the Markov limit does not
preserve the positive-definite character of the corresponding density matrix,
thus leading to highly non-physical results. To overcome this problem, we shall
propose an alternative adiabatic procedure which (i) in the semiclassical limit
reduces to the standard Fermi's golden rule, and (ii) describes a genuine
Limblad evolution, thus providing a reliable/robust treatment of
energy-dissipation and dephasing processes.
