Combined effects of photons occupying the same state Will multiple photons occupying the same quantum state have a detectable combined effect? I mean, is there any difference in their combined property? I am assuming yes for this. How can  we determine whether there is just one or multiple photons?
 A: An easy way to know there are multiple photons is if you see light. In this one photon at a time experiment one sees how an interference pattern typical of light is generated by the accumulation of single photons. These photons do not overlap in time as you would want, but the quantum mechanical probability functions make the wave paternt appear.
A demonstration of the overlap of the zillion of photons that make up a laser beam can be seen in this MIT video. It shows an experiment with a laser beam split in two beams, and how the interference pattern comes up on the screen, even though the individual photons are not interacting. Lasing is a quantum mechanical phenomenon, where individual photons can be guided optically through the beam they make up. What is interesting is total interference of the two beams , showing a black screen, and the explanation of where the energy of the beam goes ( back tot the lasing source) . It proves to me how complicated a classical system is, when viewed quantum mechanically.
