When light encounters a medium in which the index of refraction changes dramatically with wavelength, the group velocity of light -- the speed at which the wave pulse propagates -- can be considerably lowered, even to zero. The energy and information in the original light beam can be stored, without any heating, in the form of a wave of excitations in the spins of the atoms in the medium.

Earlier this year, two different experiments at Harvard stopped and stored light in a vapor sample. Now the feat has been carried out in a solid at MIT and at the Air Force Research Laboratory in Hanscom, Massachusetts. This is a promising advance, since most information processing is carried out in solid-state integrated devices.

The medium used, a yttrium-silicate crystal doped with atoms of the rare earth praseodymium, is already commonly used as a medium for high-density optical data storage. The researchers foresee many applications for slow or stopped light in a solid in areas such as quantum computing, ultra-sensitive magnetometry and acousto-optics (if light is slowed to subsonic speeds, strong coupling between light and sound waves becomes possible).