Decay times of quantum states in one- and two-photon absorption processes

We introduce two definitions of the transition time to the ground state from an arbitrary initial (pure or mixed) quantum state: ''conventional'' and ''mean'' times. The dependences of these quantitative characteristics of the relaxation process on the initial conditions show the qualitative difference between the one- and two-photon absorption processes: in the one-photon case, the transition times go to infinity with increase of the initial mean energy, whereas these times tend to some constant values in the multi-photon processes (for both definitions). The analysis of the evolution of different initial states (Fock's, coherent, even/odd, squeezed, thermal) shows the important role of the quantum fluctuations, because the decay times of the thermal and vacuum squeezed states turn out to be smaller than the decay times of the Fock and coherent states with the same initial mean photon number, both in one- and two-photon processes.