With respect to Thomas Young's double-slit experiment, I have heard that it can be done with a single photon. But how does the single photon go through both slits? And how far apart are the slits?
The electromagnetic field is interacting with the slits and detector placed around it (say the screen) everywhere. You can make the field so weak that by Fermi's golden rule the probability of the field interacting with any point on the screen is so small that you see only one "hit" at a time. This does not mean however that we dealt with a photon travelling through any of the slits arriving at the screen. A photon is not a localized particle. It is a quanta that a can be annihilated from field when interacting with the fields in the screen with certain probability.