How to enforce only 1 photon at a time in double slit Excuse my noobishness here, but I'm trying to understand the double slit experiment exactly. The way I understand it is the weirdness lies in the fact that when you send single photons through a double slit, one at a time, that it over time results in the same diffraction pattern you would expect when sending multiple photons through. Classically we wouldn't think of light as photons but as waves.
But anyway, it seems that the "strangeness" of light acting both as a wave and a particle depends largely on whether or not we actually ARE sending a single photon through the double slit apparatus at a time. Otherwise, if multiple photons are being sent through the double slit, they will interfere with each other and we have a completely rational explanation for the interference pattern.
So, given that what i've said above is true, which I may not be and need to be corrected on, from the apparatuses and experiments I have seen, I'm not completely convinced that they are indeed only sending one photon "at a time". Heres one such resource
https://www.animations.physics.unsw.edu.au/jw/light/youngs-experiment-single-photons.html
it seems that they are trying to basically run the power very low and also filter it through a filter and a single slit, and assume that that will result in one photon being emitted at a time. But that does not seem like a guarantee in this situation. Maybe 2 could be released from the lamp at a time? How do we know exactly?
Perhaps there's some semantic issues in the way this problem is traditionally explained to laymen and its leaving out some details that would clarify this. Either way, would love some input on this. Thanks!
 A: One simply just adds filters and uses a sensitive photodetector such as PMT (photo multiplier tube). With that you can rates of any where from 1 count per second or even one count per year if you want.  Classical explanations of the DSE have major problems, for example if 2 photons cancel then where does the energy go .... its a violation of conservation of energy.
Every photon has particle properties like momentum and wave properties like color (i.e wavelength).  It only matters in what you are interested to measure. The DSE experiment is historically most important for its illustration of the wave nature ... it was a great revelation.  The single photons version shows both particle and wave ... and it has the important consequence of showing each photon acts on its own ... historically stated "as each photon interferes with itself" by Dirac.
When single photons are created the exact timing is random ... so it possible to get 2 photons closely in time on occasion.
A: I don’t think those experiments actually are about shooting single photons through the double slit. Rather they shall highlight that QM is above our perception while experimental results are indisputable.
In the experiment the light source is reduce to such low output (see answer by @PhysicsDave) that, when detected, it will produce single photon events.
Because of this „single event”-detection, people are inclined to use sentences like as if a single photon is send through the double slit and interferes with itself. But this is nonsense. A single particle does not and cannot interfere with itself.
The quite simple meaning of those experiments is: Their results are inconceivable to the human mind. Any effort of interpretation is useless (at least for humans!)
