Physics News Update, Number 422

Number 422, April 7, 1999 by Phillip F. Schewe and Ben Stein

A CONTINUOUS ATOM LASER BEAM has been demonstrated for the first time by researchers at the Max Planck Institute for Quantum Optics in Munich (Bloch, Haensch, and Esslinger, Physical Review Letters, 12 April 1999). The first atom laser (Update 305) produced pulses of atoms, rather than continuous beams. Secondly, the atoms quickly spread out in a moon-like crescent, instead of forming a more desirable narrow beam. At the APS Centennial Meeting, Theodor Haensch (011-49-893-290-5702) described a design that produces a continuous stream of atoms lasting as long as 100 milliseconds. In addition, the beam can potentially have a radius of a nanometer, thousands of times narrower than the focus of an optical laser beam. Using a specially designed magnetic trap, placed inside a magnetic shield enclosure, the researchers created a Bose-Einstein condensate (Updates 233, 362) of rubidium in a well-defined magnetic field which experienced a minimal amount of fluctuations from the environment. In such a low-noise setup, a radio wave can address a specific magnetic-field region of the condensate. Since every atom acts like a wave and is spread out over the entire condensate, each atom eventually gets affected by the radio wave, becoming converted from a trapped to an untrapped state. The conversion process occurs gradually, with a beam emerging at a slower and more controlled rate than in previous atom- laser demonstrations. The escaping rubidium atoms fall under the influence of gravity. Also at the APS Centennial, Bill Phillips of NIST discussed an atom laser that can produce beams in any direction, not just downwards. (Hagley et al., Science, 12 Mar. 1999). The researchers applied a pair of optical laser beams to a sodium BEC; the atoms absorbed some of the momentum of the beams and could be kicked in the desired direction to form a beam. Their design actually generates a "quasi-continuous beam," pulses of atoms so close together in succession that they overlap. In addition, the beam is narrowly collimated, spreading out just a tenth of a degree, comparable to a laser pointer. These beams provide an excellent source of atoms for devices which can measure tiny amounts of rotation (Update 306) and precision measurements of gravitational acceleration (Update 384) and time; they may also allow researchers to make very sophisticated nanostructures. Interestingly, even the pioneers in this new frontier do not know how physics concepts such as coherence will be redefined for the case of these laser-like matter waves. (See figure at Physics News Graphics; see also Physics Today, April 1999)

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.)