I then try to calculate a probability the first photon hits a particular point at the screen. If at the slits the wave function of two photons is #|AB>+|BA># where #|A># and #|B># are wave functions of a singular photon at 1st and 2nd slits respectively, Could please somebody review? For what I could see: It's been a long time since I did anything of this sort and I was never good at QM, so I might easily make a mistake. I probably got my calculations wrong, as I got that two photons cannot interfere on the two slits in my setup. If the phases of the two photons are the same (more precisely, the correlation between them is precisely controlled), and if the photons are emitted at the same time, then you can get interference. Further, the 'probability' is actually an intensity rather than a probability: the probability of observing a photon strike in a particular square nanometre in a nanosecond. At each point on the screen we need to add the fields and then square, to get a probability. * They are actually 'pre-probability' fields. Somebody that understands more about lasers would need to answer that. I expect what happens if the beams are wider than the slits would be more interesting. If we directed two laser beams at the slits, I imagine that, if the beams were narrower than the slits, they would simply pass through and make two tiny dots on the screen.

Either way, there would be nothing left to interfere. Furthermore, the act of observing them likely destroys them.

The probability fields tell us the probability of observing a 'photon strike' at each point on the screen.Īccordingly, we can't meaningfully talk about a setup with two photons crossing the slits simultaneously, because that implies that they are observed at the slit, so we no longer have waves and probability fields, just a couple of photon observations. The interference in a double slit pattern is not interference between designated photons but interference between two probability fields* - each field being associated with one of the two slits. I find it makes most sense to think of photons as observations - experimental outcomes - rather than as inputs. I believe it is not meaningful to talk of interference between two photons.

Photon vs quantum torpedoes