I'm trying to get to the bottom of things concerning the shaping of an optical pulse(change of state) with linear optics. Using linear optical apparatus we can shape the pulse of a photon in such methods as cavity dumping. You feed an optical cavity with near monochromatic(short bandwidth) input and allow no output. The light builds up and then you release all the light at once by opening up the cavity, and in the end you have an output with pulse length equal to the inner loop length of the cavity, even if the cavity loop is shorter than the coherence length of the input. So you can shorten the coherence length of the output which consequently widens the bandwidth. This is a change of state. In QM all changes of state can be modelled by a physically real coupling, aka the Von Neumann measurement scheme. My question is, what physically real coupling is taking place when the photon output has its change of state? You can create a mathematical model of the unitary operator which would act on the initial state description to produce the final output state, but what is the physically real meaning of the operation? Is there an answer?