LIGHT & SYMPATHETIC VIBRATIONS
Text: Deep-frozen atoms stop beam of light in its tracks (Filed: 17/02/2002) http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2002/02/17/waa117.xml&sSheet=/portal/2002/02/17/por_right.html AMERICAN scientists have succeeded in a feat that will boggle the minds of those who believe that nothing travels as fast as light: they have made light stop. A team at the Rowland Institute in Cambridge, Massachusetts, led by Prof Lene Hau, told the American Association for the Advancement of Science yesterday that it had succeeded in making a pulse of laser light slow down to a complete halt. Then, after a relatively long interval in physicists' terms - about a thousandth of a second - they made it start up again as if nothing had happened. The experiment is deeply troubling for people who can remember being told in their physics lessons that the speed of light is supposed to be the most constant, unvarying quantity in the universe. The breakthrough, which will have implications for designing the supercomputers of the future, has been achieved by using a tiny, dense cloud of sodium atoms that have been chilled to one-millionth of a degree above absolute zero (-273.15C). By using a tuned laser beam to get these atoms into just the right state, scientists can get a light beam passing through that cloud to move slower and slower. A handful of other scientists around the world, including some at Stanford University and the University of Colorado, have worked on similar techniques, and a group at Texas A & M University hopes to go one better by stopping light and then reversing its direction. The work relies on creating an ultracold state of matter - something called a Bose-Einstein Condensate, in which a cloud of atoms is chilled to a point so close to absolute zero that they become, for practical purposes, a single superatom. Because of the Uncertainty Principle - one of the fundamental rules of quantum mechanics - the more precisely the momentum (or velocity) of a particle is known, the less precisely is it possible to measure its position, and vice versa. At absolute zero degrees (which could never be reached), a particle would have zero momentum, a precise value. The particle's position would then be highly uncertain; it might be found anywhere within a large volume of possible places. The volume occupied by each ultra-cold sodium atom expands enormously, so much so that atoms are forced to overlap and merge into the Bose-Einstein condensate, in which the atoms' quantum wave functions are combined. Once the condensate is created, a "coupling" laser tuned to resonate with the trapped mass of atoms is beamed into the trap chamber so that the atoms and photons of light become "entangled", behaving as if they were a single entity. A pulsed laser probe is shot into the "laser dressed" condensate from a different direction. By passing it through an illuminated atomic cloud, they can cut the speed of a pulse of yellow laser light from 186,000 miles per second to a crawl.
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