[General] How to know it's 1 photon at a time

[Hodge John]

Chandra
"A single photon in the experiment at a time" means 1 photon between the mask and screen at a time. If there are a million photons impinging on the mask and only 1 goes through the slit, it is 1 at a time.
So, how an experimenter can tell if it is 1 at a time is get a photon counter and hook it to a speaker to hear the clicks. 1 click every few nano seconds or longer is 1 photon in the experiment at a time. I referenced a video in which the experimenter did just that. He also showed the screen accumulates the dots (photon encounters) 1 at a time.  
My paper used the 1 at a time in the simulation. The actual experiment used a laser that was certainly millions at a time. The point is the simulation and laser experiment agreed with the sinc() input (Hodge) experiment.
Hodge