Is the double slit experiment performed measuring single photons? As a hobby physicist I don't understand how the double slit experiment is performed in terms of single photons. Is the experiment really conducted by sending and measuring single photons? Is there not always interference of some kind (except for some device like the LHC)?
If yes: what would happen if the experiment is conducted over a prolonged period of time, for instance: 1 photon every n hours. Would they theoretically interfere with "themselves"? (don't know if i'm still making sense here).
If no: why is the interference considered to be originating from the sent photons and not from an external influence of some sort?
Thanks!
 A: Yes, there are single photon through a double slit experiments , as also single electron ones.
Here is one single photon at a time:


. 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.

You can see the single photons hitting on the left frame, the hits seem random. By the time the accumulation has reached 1000 frames(about 50.000 photons counting the ones in the single frame)  the interference pattern starts appearing, and it is the classical interference of light  in the far right (~25.000.000 photons).
The external influence is the boundary condition: two slits d1 width each, d2 distance apart. Photon hits the boundary conditions. One sees the probability distribution for a single photon to scatter off these specific double slits. Each photon follows that probability distribution, except individually it looks random. In the accumulation the wave nature of the photon wavefunction is demonstrated.
The timing would have no influence on the pattern if the experimental geometry remains fixed.
