Bringing Light To A Stop

Albert Einstein showed nearly a century ago that light always travels at one speed--670 million m.p.h.--but his Special Theory of Relativity had an often overlooked qualifier: this blistering pace applies only in a vacuum. When it passes through air, say, or glass or water, light goes a bit slower.

Even Einstein would probably be taken aback, though, by experiments announced in Cambridge, Mass., last week: two teams of scientists have slowed light to a dead stop. The achievements, to be reported in this week's issue of Nature and next week's Physical Review Letters, could one day lead to a new generation of compact, superfast computers that operate by light, not electricity.

Both groups, one at Harvard's physics department and the private Rowland Institute for Science and the other at the Harvard-Smithsonian Center for Astrophysics, took advantage of the fact that a beam of light can transform matter--a cloud of rubidium atoms in one case, supercooled sodium in the other--from ordinary opaqueness into temporary transparency. Having done that, they shone a second light on the clouds, and then turned the first one off.

Result: the second beam was compressed more than 100 millionfold in length and "stored" in the form of altered energy states and spin patterns in the atoms. When the triggering beam was turned back on, the light was reconstituted and sped on its way.

Eventually, such optical sleights of hand could be used in switches and memory-storage devices at the heart of so-called quantum computers, which use subatomic effects for processing information. Such applications are a decade away at least; for now, physicists are happy to probe the strange interplay of light and matter. "It's great fun," says Lene Hau, leader of the Harvard-Rowland team. "And it's great science."