Hi Everyone This may sound like a question with an obvious answer but with my current knowledge, it is not obvious to me. Can one photon interfere with another photon or can each photon only interfere with itself? Most people on this forum will know that Young's Slits produces interference patterns even with "one-at-a-time" photons. This means that it is not necessary for one photon to interfere with another photon in order for Young's Slits to produce an interference pattern. One way of looking at this question is as follows: Imagine that you have two seemingly identical beams of parallel light which are as powerful as each other and have exactly the same pure wavelength. Let's say the two beams are coming from two unmarked emitters so that you can't tell exactly how each emitter is producing its beam but you have been told that one of the emitters is a laser device which produces completely phase-aligned light and the other emitter contains just a normal but powerful LED which produces light which is not phase-aligned. Can Young's Slits tell the difference between the two beams? Can the normal but powerful LED produce visible interference patterns? If it is not possible to tell the difference between the two beams using Young's Slits then I would say that this would mean that one photon cannot interfere with another photon. What do the experts on this forum think? Thank you very much.