Quanutm Eraser

[Phymath]

when performing a double slit experiment using photons it is said that if you fire individual photons that over time as more photons are fired the interference pattern starts to appear. My question is how do they define a single photon? what is the method that they can fire one photon at a time controllably so? how do they define a photon?

[olgranpappy]

One "tick" in their detector (a photomultiplier tube or something) is a single photon. That's what particle *are*. They are things that come into your detector and go "tick!"

[Phymath]

so just to clarify there is no known way to fire a single photon from a source?

[Integral]

Yes, they can fire single photons. That is how they do the experiments you are talking about.

[mgb_phys]

Easiest way is to get a dim light source and then put neutral density filters (welding mask glass is good) in front of it until you are only getting a few photons / sec. You can confirm this by pointing the detector direcly at the source without any sllits.
You can of course add more filters to reduce the rate until you are convinced there is only one photon in the system at once.

[ZapperZ]

so just to clarify there is no known way to fire a single photon from a source?

There are already many schemes that produce single photon on demand. See, for example

http://physics.nist.gov/Divisions/Div844/facilities/cprad/PhotonSource.htm

Zz.