NONLINEAR ATOM OPTICS. Four-wave mixing of atom waves, a process in which three matter waves combine to produce a fourth wave while conserving energy and momentum, has been demonstrated by researchers at NIST. This experiment provides the first example in atoms of "non-linear optics" effects which are important in laser beams. In lasers, these effects arise when the light is so intense that it changes the index of refraction of the material through which it traverses. The behavior of the material thereby depends on the intensity of light, and this non-linearity can lead to the self-focusing of light and the creation of new colors of light from a single color. The NIST researchers created three overlapping Bose- Einstein condensates of sodium atoms moving at different velocities relative to one another. The three BECs interfered to create a fourth condensate moving at a different velocity. This four-wave mixing phenomenon, which also occurs in light waves, can be used to explore the uniquely quantum mechanical properties of matter waves. (Deng et al., Nature, 18 March 1999 and report at the Atlanta APS meeting.)