What is time? This question intrigued Einstein. In 2005 as the World Year of Physics recalls Einstein's miraculous year of 1905, which changed forever our understanding of Nature, we continue to be excited by time and its measurement. Atomic clocks are the most accurate timepieces ever made, and are essential for modern life. For example, the Global Positioning System (GPS), which guides aircraft, cars, and hikers to their destinations, depends on atomic clocks and on Einstein's theories. The limitations of atomic clocks come from the thermal motion of the atoms: hot atoms move rapidly and suffer from time shifts, which are also predicted by Einstein's Theory of Relativity.
Contrary to intuition, we can cool things by shining laser light on them. With laser cooling, applying ideas that originated with Einstein, we cool gases to less than one millionth of a degree above Absolute Zero. The slowly moving atoms in such a gas allow us to make even more accurate clocks, already so good that they would gain or lose only a second in 40 million years. Laser cooling has also made possible the observation of a long-standing prediction of Einstein: Bose-Einstein condensation, hailed as one of the most important recent scientific developments, and the coolest thing yet!
William D. Phillips shares the 1997 Nobel Prize in Physics with Stephen Chu (Stanford University) and Claude Cohen-Tannoudji (Collège de France and École Normale Supérieure, Paris) for development of methods to cool and trap atoms with laser light. After earning his Ph.D. in physics and completing post-doctoral research at the Massachusetts Institute of Technology, Phillips came to the National Institute of Standards and Technology (NIST) (then the National Bureau of Standards) in 1978 to work in the Electricity Division. While at MIT, Phillips had completed two thesis experiments, one in the well-established area of magnetic resonance and the other with newly available tunable laboratory lasers. His official duties were related to his first thesis experiment, involving precision electrical measurements. However, he explains, he was allowed to use stolen moments to dabble in laser-cooling with lab equipment he brought from MIT. With encouragement from management, he continued experiments and demonstrated that a beam of neutral atoms could be slowed and cooled with radiation pressure from a laser.
NIST's accomplished and internationally recognized laser cooling and trapped atom research program grew out of these early experiments. In the mid-1980s, Phillips' team found serious discrepancies between its own measurements and the generally accepted Doppler cooling limit. They demonstrated that it was actually possible to chill atoms well below the accepted limits down to a few microKelvins, or just millionths of a degree above absolute zero. This discovery paved the way for the 1995 creation (by a NIST/University of Colorado group in Boulder, Colorado) of the first Bose-Einstein condensation, an exotic new form of matter in which atoms all fall into their lowest energy levels and merge into a single quantum state.
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