Single photon sources

A single photon source is like a light bulb that only produces one particle of light (aka a photon) at a time. When this particle of light is sent out, it is guaranteed that it is the only one. This is different from regular light bulbs which produce lots and lots of particles of light all at once. Single photons are important because they can be used to send information over long distances in a secure way without the worry that someone else could be eavesdropping.