THE 1997 NOBEL PRIZE FOR PHYSICS has been won by Steven Chu of Stanford, Claude Cohen-Tannoudji of the Ecole Normale Superieure in France, and William Phillips of NIST for their development of laser cooling for neutral atoms. In this case "cooling" means reducing the relative velocities of atoms. 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 one to trap the atoms and cool them 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. With his laser setup, Phillips can create "optical lattices," crystal-like arrays of atoms held in place by light waves. 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. 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. What else can be done with the chilled atoms? They may become the basis for extremely precise atomic clocks, accelerometers, and gyroscopes. (Background articles: Scientific American, March 1987, trapping atoms, William Phillips; July 1993, accurate time measurements, Norman Ramsey; February 1992, Steven Chu, trapping neutral particles; Physics Today, October 1990, Phillips and Cohen-Tannoudji, laser cooling; also see Official 1997 Nobel Prize in Physics site)

THE FIRST SOLID MATERIAL THAT CAN REVERSIBLY SWITCH BETWEEN METAL AND INSULATOR at room temperature and pressure, and without changing its chemical makeup, has been created by researchers at UCLA (James Heath, heath@chem.ucla.edu). The researchers prepare a Langmuir film, an ultrathin layer of material on a water surface. The film consists of a 2-D hexagonal pattern of silver nanocrystals (only nm in size) with each nanocrystal's surface capped by compressible organic molecules. Applying pressure to the film can decrease the distance between adjacent nanocrystals from 12 to 5 angstroms. When compressed, the film becomes shiny and its optical properties match those of a thin metal film. Prior to compression, the film has the optical properties of an insulator: in this state, the nanocrystalsbehave as semi-isolated particles and they do not share electrons. As the separation between nanocrystals decreases, the researchers observe a transition from "classical coupling" (adjacent nanocrystals induce the movement of charge in each other and thereby transfer energy) to "quantum coupling" (nanocrystals begin to share electrons simultaneously and electrons delocalize, or cease to occupy a specific position in the material). (C. P. Collier et al, Science, 26 September 1997)