One the other hand, I know about the stimulated emission which out of a photon produce exactly the same (wavelength, polarisation, etc...). Maybe the fact is that the excited atom do a measure. (Let's say it can stimulate emission with only one direction of polarisation.)
But now if I have a statiscal number of atoms with random "polarisation" direction, I should be able to copy any incident photon. I've made a cloning machine.
This cannot be true because of the no-cloning theorem. But I can't figure why.