Hi,
Through most of our schooling we are trained to think in terms of energy
eigenstates. Atom's and photons do not need to be in energy eigenstates. There
are experiments that show atoms oscillating between two energy eigenstates well
after the excitation process. With light, the measurement of its coherence
length is a measure of its linewidth. This can be done with a Michelson
interferometer. Basically the coherence length is related to how different the
path lengths can be and still seeing interference. It is my understanding that
even at very low light levels, the coherence length stays the same.