On 16 Mag, 16:03, Mitchell Jones <mjo...@21cenlogic.com> wrote: > ***{Fine. How do you know you did that, other than by counting the > clicks on the photon detector? Maybe you actually released a million > photons, and only got one click. --MJ}***
It's a well known fact (since Plank and Einstain) that photons have quantized energy: The energy of a single photon is equal to: hc/l, where h is the Plank's constant, c is the speed of light in the vacuum and l is the wavelength of the radiation.
When you desing a light source you can control the total emitted power (total energy/time) and the wavelength (the 'color' of the light). Since the total energy emitted in a given time interval is just the sum of the energy of all photons emitted in that interval, you can simply calculate the average number of photons per interval: n = El/ hc, where E is the total energy.
Therefore, by keeping a low total power and/or a low wavelength you can archive a small number of photons per time interval. In particular, you can desing the source so that it emits only one photon in an interval much larger than the time required for the photon to traverse your experimental setup. Then you can be sure (with high probability) that there is at most one photon at time traveling in your experimental system.
