Three APS Fellows Share 1997 Nobel Prize in Physics

William Phillips is congratulated by APS Treasurer Thomas McIlrath.

In October, the Swedish Academy of Sciences selected three APS Fellows as recipients of the 1997 Nobel Prize in Physics: Steven Chu of Stanford University, William Phillips of National Institute of Standards and Technology (NIST) and Claude Cohen-Tannoudji of the Ecole Normale Suprieure in France. The scientists were recognized for their development of methods to cool and trap atoms with laser light. The award will be officially presented in Stockholm, Sweden, on December 10, 1997. The three newest Nobelists are expected to give invited talks at the APS Spring Meeting in April.

Laser cooling techniques, in conjunction with magnetic trapping and cooling techniques, have allowed physicists to cool atoms closer to absolute zero than ever before. Cooling atoms to millionths or even billionths of a degree above absolute zero allows the creation and study of such new states of matter as Bose-Einstein condensates and promises even further improvements in already ultraprecise atomic clocks.

Published articles on the Nobel Prize-winning work of these three scientists have appeared in Physical Review Letters over the years. In a 1985 paper, Chu reported his discovery that "the motion of atoms in the intersection region is similar to the movement in a hypothetical viscous medium," and coined the term "optical molasses." In an early 1988 paper, Phillips reported that the atoms had a temperature of about 40 Microkelvins, much below the predicted Doppler limit of 240 Microkelvins. They also found that the lowest temperatures were reached under conditions that contradicted those of the theoretical Doppler limit. Finally, Cohen-Tannoudji demonstrated how the Doppler effect can be used to ensure that only the coldest atoms end up in the dark state. His so-called velocity selective coherent population trapping (VSCPT) method was first applied, and reported on in PRL: in 1988 in one dimension, in 1994 in two dimensions and in 1995 in three dimensions.

In these experiments, an array of laser beams converges on a gas of atoms. In the simplest type of laser cooling, the wavelength of the light is tuned so that just the fastest atoms moving in a particular direction will absorb a photon head-on, thus slowing their motion in that direction. The atoms will eventually re-emit a photon but in random directions. The effect of the laser bombardment is a net slowing of the atoms. This "optical molasses" can slow millions of atoms to temperatures just millionths of a degree above absolute zero. Adding magnetic fields to the laser configuration enables the atoms to be trapped and cooled further.

As a result of these techniques, physicists can cool atoms closer to absolute zero than ever before, to temperatures of nanokelvins in some cases. Reducing the distracting presence of thermal motion permits the study of atomic properties with much greater precision. Furthermore, laser cooling serves as the first stage in reaching the exotic condition known as Bose-Einstein condensation, the new state of matter in which many atoms begin to "overlap," eventually assuming a single common quantum state.

Chu received his PhD in physics from the University of California at Berkeley in 1976 and remained there for two more years as a postdoctoral fellow before joining the technical staff at AT&T Bell Laboratories in 1978. He headed the Quantum Electronics Research Department in 1983, leaving the company in 1987 to join the faculty of Stanford University, where he is currently a professor of physics and applied physics. He received the APS Arthur L. Schawlow Prize in 1994 for his work in laser cooling and trapping of atoms. Chu has used his laser array to split ultracold atoms into separate waves and recombine them to form interference patterns that can provide detailed information on the atoms.

Phillips received his PhD in 1976 from the Massachusetts Institute of Technology and completed a postdoctoral stint there before joining NIST in 1978. Although his work was originally related to precision electrical measurements, he was allowed to perform experiments in laser cooling in his spare time, which eventually led to NIST's internationally recognized research program in this field. He was recently selected as the 1998 recipient of the APS Schawlow Prize, to be presented next fall. With his laser setup, Phillips can create "optical lattices," crystal-like arrays of atoms held in place by light waves. He and his team are continuing to study ultra-cold trapped atoms with spin-off applications for improved accuracy in atomic clocks and in the fabrication of nanostructures.

Born in Algeria and a citizen of France, Cohen-Tannoudji received his PhD in 1962 from the Ecole Normale Suprieure in Paris. He has been a professor at the College de France since 1973 and is a member of the Academie des Sciences. He shared the 1992 APS Lilienfeld Prize with Alan Guth. In a particularly sophisticated form of laser cooling, Cohen-Tannoudji has put helium atoms into a "dark state," whereby the coldest atoms become unable to absorb additional light and fall to temperatures even lower than previously imagined possible. The chilled atoms may also become the basis for extremely precise atomic clocks, accelerometers and gyroscopes.