DECAY TIMES OF QUANTUM STATES IN ONE-PHOTON AND 2-PHOTON ABSORPTION PROCESSES

We introduce two definitions of the transition time to the ground stat e from an arbitrary initial (pure or mixed) quantum state: ''conventio nal'' and ''mean'' times. The dependences of these quantitative charac teristics of the relaxation process on the initial conditions show the qualitative difference between the one- and two-photon absorption pro cesses: in the one-photon case, the transition times go to infinity wi th increase of the initial mean energy, whereas these times tend to so me 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 vacu um squeezed states turn out to be smaller than the decay times of the Fock and coherent states with the same initial mean photon number, bot h in one- and two-photon processes.