In 1999 Lene Hau and her colleagues at Harvard University slowed light traveling at 186,282 miles a second to bicycle speed (38 miles an hour). A few years later the team stopped a beam of light completely. This year Hau's team added another quantum trick: turning light into matter and then back into light again. The matter was a pair of ultracold atomic gases called Bose-Einstein condensates (BECs). In Hau's experiment a laser pulse struck one BEC and imparted its energy to the condensate. Thus imprinted, the BEC's atoms formed a matter wave that traveled 160 microns before hitting the second BEC, which absorbed the atoms. Those atoms radiated a light pulse that was an exact duplicate of the original. Hau speculates that the technique, described February 8 in Nature, may someday be used in optical communications or ultraprecise navigation systems.
This article was originally published with the title "Stop-and-Go Light" in Scientific American 296, 4, 30 (April 2007)