California Institute of Technology researchers have fashioned a single atom into a light source that generates single photons on demand.

The light source could eventually be used in quantum cryptography systems to guarantee perfectly secure cryptographic keys. It could also be used in quantum computers, which have the potential to solve certain types of very large problems many orders of magnitude faster than today's classical computers, including those that would render today's security codes obsolete.

A single atom can make a single photon -- the trick is isolating and controlling the atom. The researchers' system traps a cesium in a cavity formed by a pair of highly reflective mirrors.

The researchers have showed that it is possible to generate quantum entanglement between atoms and photons using the system, and then transfer that entanglement so that atoms in separate cavities can be entangled. The properties of entangled particles remain in lockstep regardless of the distance between them. Entanglement promises to make it possible to build quantum communications networks.

There's a lot to be done before the system can be used practically. Today the system takes up an entire room. In 10 to 20 years the technology may exist to integrate large numbers of atoms, each with its own minicavity onto a chip of some sort, according to the researchers. And the lasers and electronics would also have to be miniaturized.

It will be least two decades before the system could be used practically, according to the researchers. The work appeared in the February 26, 2004 issue of Sciencexpress.