The "wave train" of a laser is often very long, for example,
100 meters or more. Obviously, it can not be identified
with a single photon from one atomic transition. It is a
sequence of photons matching each other perfectly.
Suppose that the train is made from 1000 photons and
that one of them, from somewhere in the middle, happens
to ionize an atom. What happens to the train? Is it divided
into two uncorrelated trains of shorter length?
Ludwik Kowalski