During Neil deGrasse Tyson’s talk on Tuesday, he talked about firing photons one at a time to produce a result.  During almost any talk about physics experiments you will eventually hear about physicists shooting individual photons.  Despite its near ubiquity and my hundreds of hours of physics classes I’ve never really heard an explanation of how physicists manage to produce a single photon.  None of my friends had either.  So I decided to hit the interwebs and find an answer.  It is so simple, I am almost ashamed that I didn’t come up with an explanation on my own.  More proof that my brain is slowly turning to mush from lack of exercise.

Think about a flashlight.  Turn it on and light goes beaming out.  That light is composed of untold schmillions of photons.  Electricity is running through the filament causing the (usually) tungsten filament to heat up and produce light.  The electrons passing through the tungsten will collide with an individual atom temporarily causing the individual atom’s outer shell electron to jump to a higher energy level.  This is an unstable condition for the atom so the electron will eventually fall back down and photons will be produced.

“But that doesn’t help us, we still have multiple photons being produced!”, you might exclaim.  And you’d be right.  Here’s where a little ingenuity is involved.  We need a source which produces a known number of photons at known wavelengths.  Enter the spectrometer!  Thanks to that lovely invention, we know the wavelengths of the photons produced for just about everything.  Prior to that, I assume it was mostly trial and error.

Now it’s just a matter of choosing a substance, isolating it, and running just enough electricity through it to produce a reaction on a single atom.  But what shall we choose?  It turns out there are plenty, but the easiest example I could find is sodium.  A single sodium atom produces two distinct wavelengths of light at 588.9950 nm and 589.5924 nm.  Oops, but that’s still two photons, what do we do now?  Filters!  We just place a filter across the path of the photons that blocks one of the wavelengths and voila, a single photon source!

Obviously, creating a device capable of firing a controlled amount of electricity across a controlled number of atoms is non-trivial, but that’s the basic concept.  You just take your basic light source and winnow it down to its atomic level.