I got to ask "How long is a photon" and was a tad gob smacked by the answer. There was me thinking "point source" and I get the answer "As long as a piece of string ...".
Basically, to be a photon, you must have at least one complete wave, so a Long Wave radio photon is at least 1,500m long! Gamma ray photons are typically very short.
But photons have a bell shaped envelope for something he called c?, to do with time, so there is a probability of finding it anywhere. The more waves a photon has, the better you know its frequency, and the less probability you have of finding it away from the concentration of the waves. In private discussion afterwards he made some comment about [a well known public scientific figure's] understanding of photons!
I came across this
https://www.physicsforums.com/threads/size-of-photon-particle.32102/page-2#post-292379
He described how he creates single photons but I did not really follow it. It's all based on probabilities because it is quantum.
You cannot just shine a laser through "sunglasses" and attenuate the number of photons until you get them coming out as singles because the laser is sending 10^15 photons/second (his laser pointer does), they travel in bunches where the average rate is 1 per 10^-15 sec, but the actual number of photons in a small interval has a Poisson distribution. (I think that was correct, but it may be that the attenuation is probabilistic? or both are probabilistic? Whatever, your photons after the sunglasses are probabilistically distributed in time.) So, if you attenuate to an average of 3 per time interval, you actually get, say, from 0 to 10 per interval on a Poisson curve which goes off to an infinite number at a tiny probability.
Make the glasses darker so you get an average 1 per time interval but now most intervals have 0, very few have 1, very very fewer have 2 etc. It's Sod's Law - the darker the glasses, the fewer "single photons per interval" you get.
The key is
getting rid of the intervals with 0 photons because the largest set left is
intervals with 1 photon.
To do this he shines a very powerful laser pulse which is very short (10^-15 sec), so limiting the number of photons (still 10^lots!) into a glass (in his case fibre) which produces a highly non-linear electric field response. As a consequence (a bit like a diode detector for an AM radio signal) you generate pairs of photons, one with higher and one with lower frequency. (His tiddly $5 green laser pointer is actually an IR laser which is frequency doubled to get green.)
There is a lot of statistics and basically you produce photon pairs for about 1 in 10^2 incoming photons, and 2 x photon pairs for 1 in 10^4 and 3x photon pairs for 1 in 10^6. You now detect one of the pair, which absorbs it, but this means you know you have also generated a "nice, untouched, virgin photon" as its pair (and 2 virgins in 1 in 10^4 cases and 3 in 1 in 10^6 cases).
Critically, you can discard all those instances when you do
not create a photon pair because you don't detect any photons, so you have got rid of the "0 photons bunch". The parameters are set to have a very low probability (1%) of producing a single pair, because that simultaneously means you produce few double and few few triple pairs. But until you have a detector which discriminates between single and multiple photons - he doesn't - you just have to live with the odd double or triple or quadruple photon - which screws up your experiments.
How long do you need to wait for getting a some photons out? It's a log scale - one comes out in 10^-10 seconds, 3 takes about 1 second ... but 12 takes 10^20 secs, or longer than the age of the universe. He runs 4 x guns in parallel and combines the outputs to get 12 photons per second.
When a single photon hits the Mona Lisa it is absorbed. The excited electron drops back and emits a new photon in any random direction. So, you only see the Mona Lisa because so many photons hit it, that enough are randomly generated in your eye direction for you to see it.
