Dark state
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The dark state is an interaction between matter and light, in which light can be "slowed down" or even stopped. In 2001, a team led by Lene Hau at Harvard University managed to bring a ray of light to a halt in a cold, cigar-shaped cloud of sodium atoms. This experiment has since been repeated with warm rubidium and more 'accurately' with cold caesium. Two laser beams are fired into the cloud- one a coupling beam, the other a signal beam- and if made to destructively interfere with each other will entangle the laser photons and the atoms in the gas. Dimming and eventually switching off the coupling beam will slow down the signal beam- to a complete stop if done correctly- thus storing the signal beam in this dark state. Turning the coupling beam back on again reverses the effect, although the light still interferes with the dark state atoms while at a halt.
A possible technological application for this effect is storage for a quantum computer.Template:Physics-stub