One can experimentally measure the length of a single photon.
Isn't this a measurement of the length of a coherent wavetrain?
I thought that this coherence-length was caused by collisions in the gas
molecules which emit the light... as opposed to photons themselves having
a length.
If a molecule is being knocked about, then obviously can only emit an EM
wavetrain having a certain number of cycles (a certain length) before it's
output is suddenly Doppler-shifted during a collision.