A light beam can be "seen" as an electromagnetic wave or as a flow of particles (photons). The number of photons in a light beam depends on its energy and of course the length of time we are considering. If we take a laser pulse, that is the one used in telecommunications, a single pulse is in the range of a few ns (from a hundredth of a nanosecond to a hundred of nanosecond) and at the energy level used such a pulse will consist of something like 10 million billion photons (plus or minus some hundred billion photons...).
That's quite a lot!
Now suppose you want to extract just one photons out of those million billions... It looks pretty challenging. Moreover, when you detect a photon you are basically annihilating it (it gets absorbed by an electron changing its energy level).
Well, at the Weizmann Institute of Science, in Israel, a team of researchers has found a way to pick up a single photon out of a laser beam redirecting it. The have reported the method in an article on Nature.
They have used a physical effect called SPRINT, single photon Raman Interaction, where a single atoms interacts with a light beam resulting in the diversion of a single photon. This is basically extracting a photon from the laser pulse changing its direction without causing its adsorption by an electron. Hence that photon can be used in anyway desired.
The researchers point out that this mechanism provides a a tool for further studying particular states of light and for eavesdropping on imperfect quantum-cryptography systems relying on single photons, eventually increasing security in quantum cryptography.
I guess that, as in many other leading edge discovery, we will see practical use, if ever, over the next decades, possibly in ways that we are not foreseen today.