Light is the fleetest of phenomena. Indeed, "the speed of light" is synonymous with the universe's ultimate speed limit. Yet even light slows down when it has to slog its way through matter--glass or optical fiber, for example, cuts light back to about 70 percent of its top speed, which is still fast enough to circumnavigate the earth five times in a second. Two and a half years ago physicists demonstrated how a specially prepared gas could slow light by a factor of 20 million, to the pace of a speeding bicycle. Now two groups have used such a system to bring light, in effect, to a complete halt and then controllably release it back on its way. The process could have applications ranging from extremely precise measurements of properties of atoms to quantum computing.

Lene V. Hau's group at the Rowland Institute for Science in Cambridge, Mass., and at Harvard University carries out these tricks in a tiny cloud of sodium atoms chilled to less than a microkelvin above absolute zero. The other group, led by Ronald L. Walsworth and Mikhail D. Lukin of the Harvard-Smithsonian Center for Astrophysics, also in Cambridge, achieves much the same results in a four-centimeter-long cell of rubidium vapor almost as hot as boiling water. Both use the same two-step process to freeze the light.

This article was originally published with the title Ultimate Stop Motion.

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