  We discuss the statistics of emission of photons by a single atom or ion
illuminated by a laser beam at the frequency of quasi-resonance between two
energy levels, a situation that corresponds to real experiments. We extend this
to the case of two laser beams resonant with the energy differences between two
excited levels and the ground state (three level atom in V-configuration). We
use a novel approach of this type of problem by considering Kolmogorov equation
for the probability distribution of the atomic state which takes into account
first the deterministic evolution of this state under the effect of the
incoming laser beam and the random emission of photons during the spontaneous
decay of the excited state(s) to the ground state. This approach yields
solvable equations in the two level atom case. For the three level atom case we
set the problem and define clearly its frame. The results obtained are valid
both in the opposite limits of rare and of frequent spontaneous decay, compared
to the period of the optical Rabi oscillations due to the interaction between
the resonant excitation and the atomic levels. Our theory gives access to
various statistical properties of the fluorescence light, including one showing
that its fluctuations in time are not invariants under time reversal. This
result puts in evidence the fundamentally irreversible character of quantum
measurements, represented here by the emission of photons of fluorescence.
