  There are many ways of calculating photon statistics in quantum optics in
general and single molecule spectroscopy in particular such as the generating
function method, the quantum jump approach or time ordering methods. In this
paper starting with the optical Bloch equation, within the paths interpretation
of Zoller, Marte and Walls we obtain the photon statistics from a sequence of
laser pulses expressed by means of quantum trajectories. We find general
expressions for Pn(t) - the probability of emitting n photons up to time t,
discuss several consequences and show that the interpretation of the quantum
trajectories (i) emphasizes contribution to the photon statistics of the
coherence paths accumulated in the delay interval between the pulses and (ii)
allows simple classification of the terms negligible under certain physical
constraints . Applying this method to the concrete example of two square laser
pulses we find the probabilities of emitting 0,1 and 2 photons, examine several
limiting cases and investigate the upper and lower bounds of P0(t), P1(t) and
P2(t) for a sequence of two strong pulses in the limit of long measurement
times. Implication to single molecule non-linear spectroscopy and theory of
pairs of photons on demand are discussed briefly.
