The raw material for many quantum computers and quantum key distribution systems are single photons. For a quantum computer, the current ideal implementation would allow users to perform computations using the quantum states of atoms or ions, and communicate between the atoms and ions using single photons. Likewise, most quantum key distribution is performed by encoding a quantum state in a single photon. The upshot of this reliance on photons is that network and computational flexibility requires some method to route photons to different locations.More at; http://arstechnica.com/science/news/2009/03/routing-light-with-a-single-atom.ars
