Double split experiment - location of single photons

Question

The double split experiment is setup so you shine a laser beam over the double slit i.e the beam completely covers the slit.

In the case of sending a single photon how can this be an equivalent setup? Surely a single photon would be blocked by the slit or just pass through the slit with plenty of room.

I understand the wave like motion that explains this phenomenon but am struggling to apply this to a single photon. It is almost like the entire cross-sectional area of the beam is being called a photon. Otherwise what is determining the how wide an area the single photon can interact with? If the slit was made huge obviously the single photon would pass right through just as a normal beam would, so it's like the cross-sectional size of the beam is preserved.

In experiments a single photon is isolated by using filtering. So one would assume that these filters affect the entire cross-sectional area of the beam equally. Apparently a photon has a particular and very small size e.g much smaller than the surface area of the beam. So how is it that only one photon across the entire beam makes it through, especially as this is coherent light?

If it is the case that the material of the filter is so complex that it randomly allows through one photon with enough time before the next one (to allow a measurement) that is fair enough but then I go back to the original part of the question and assume that this photon has originated from a particular point on the cross-section of the beam and therefore could be thought of a just a tiny laser that could be sent through the slit like any other beam. It's not like that photon could have originated from any part of the beam cross-section, it should have a distinct origin. Or does it?

In other words when filtering a beam to a single photon how is this any different than just setting up a very tiny laser beam. As far as I understand there is no connection between the individual photons usually being emitted by the laser so the fact the single photon emerged from a larger light source should be irrelevant.

That said of course if the laser was tiny it would just shine straight through the slit if positioned correctly. So what is the connection between filtering down to a single photon and that photon originating from a larger source of light?

Answer 1

I think you are just grappling with the amazingness of the double slit experiment. What makes the experiment so crazy, is that even though each individual photon can indeed be thought of as a very small particle, in fact each individual photon is going through both of the two slits and interfering.

It sounds like you are saying "When I have a wide beam, I understand that the part of the beam that goes through the left slit interferes with the part of the beam that goes through the right slit, but how is it possible for a single photon to interfere because it needs to go through one slit?". In fact what is occurring, is that in your wide beam, every single photon goes through both slits, there is not left side of the beam/right side of the beam. This same process still applies when we shoot one photon at a time, the wave of light passes through both slits at once, interferes with itself and given a wave interference probability distribution appears on the screen at the far end.

Answer 2

That said of course if the laser was tiny it would just shine straight through the slit if positioned correctly. So what is the connection between filtering down to a single photon and that photon originating from a larger source of light?

You are making the mistake of identifying photons with the classical electromagnetic radiation, ignoring the quantum nature of photons. There is no straight through for photons, which obey quantum mechanics.

It can be shown that photons quantum mechanically in large numbers make up the classical electromagnetic radiation, but the four vectors (direction, energy and momentum) of photons cannot be predicted by the classical ray optics. Only the accumulation of photons can show the classical interference effect of two slits.

Here is the single photon at a time through a double slit accumulation:

Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.

It is the accumulation of the random positions of single photons on the left that ends up to show the classical light interference pattern. Quantum mechanics can predict probability distributions, not tracks of single particles.

The experiment's boundary conditions of " photon scattering off two slits given distance apart given width" come out with a quantum mechanical wavefunction $Ψ$ for the setup, which will give the probability distribution $Ψ^*Ψ$ for the path of the photons.