From reading the article it seems likes they're using the fact that photons are Poisson distributed. Just like raindrops or particles emitted by radioactive dec

From reading the article it seems likes they're using the fact that photons are Poisson distributed. Just like raindrops or particles emitted by radioactive decay, you can determine an average rate of flux and the probability distribution of where particles will hit your detector, but for any given moment, you don't know if a particle will hit at that instant, and you don't know where. With a super fast camera, they're recording many instantaneous snapshots to see if and where a photon has been detected.