The speed of light in a vacuum is more than 670 million miles per hour. The speed of light in glass is about two-thirds of that. But the speed of light in a cloud of sodium cooled to a few billionths of a degree above absolute zero is only 38 mph[1], a remarkable discovery reported by Harvard physicist Lene Vestergaard Hau last February.

Hau starts with a magnetic trap inside a vacuum chamber, which she uses to create a strange kind of matter called a Bose-Einstein condensate-a cloud of atoms (sodium in this case) reduced to the same quantum state of near-zero energy so that they become profoundly cold and still. Next she and her fellow researchers send a pulse of light from a "probe" laser into one end of this tenth-of-a-millimeter-long, cigar-shaped cloud. "Then we just wait and measure the pulse when it comes out," she says.

If that were all there was to her setup[2], the pulse would never come out. The sodium cloud would simply absorb all the photons. Hau's solution to this problem is to illuminate the cloud from the side with a second laser-the "coupling" laser. Its slightly different photon energy primes[3] the sodium atoms for a quantum leap[4] different from the one suggested by the probe laser-and that confuses the atoms. The upshot is that the whole pulse is slowed dramatically.

Hau sees applications for her scheme in optical communications and computing. "It's like a transistor-it's really an optical switch," she says. Meanwhile, Hau continues to improve on her record. In October she slowed a pulse to half a meter per second, a shade[5] over one mph. Her goal is a centimeter per second: light as slow as a turtle.