Micro-Coliseums of Light

Using laser light, physicists have created "optical billiards" for gas atoms, traps in which atoms bounce back and forth like balls on a billiard table. With shapes ranging from circles to wedges, such arrangements provide "coliseums" for testing important ideas in physics. Groups at the University of Texas at Austin (Mark Raizen, 512-471-4753, raizen@physics.utexas.edu) and the Weizmann Institute of Science in Israel (Nir Davidson, 011-972-8-9342034, fedavid@wisemail.weizmann.ac.il) use rapidly scanning laser beams in two dimensions to draw out the desired patterns. Laser light induces electric dipoles, or a separation of electrical charge, to occur in the atoms; the dipoles in turn cause the atoms to become repelled from certain regions of the light beams' electrical field which correspond to the "walls" of the coliseum.

By studying how the trajectory of the atomic atoms depends on the shape of the billiard table, both groups tested aspects of classical chaos theory. They probed atomic trajectories indirectly, by creating a little hole in the optical billiard and measuring the escape rate of the atoms. Trapping ultracold cesium atoms in a micron-scale V-shaped wedge whose vertex faced downward toward the ground, the Texas researchers confirmed theoretical predictions that the trajectory of the atoms shifted from stable to chaotic as the angle of the vertex was changed. Confining rubidium atoms in micron-scale billiard tables oriented perpendicular to the ground, the Weizmann group found that circle and ellipse shapes promoted stable, non-chaotic motion, while a "tilted stadium," consisting of two half circles connected by two non-parallel straight lines, caused the atoms to exhibit essentially chaotic motion.

In future studies, both teams plan to use optical billiards to test such things as quantum chaos and the effects of noise on the trajectories of atoms. (Milner et al. and Friedman et al., Physical Review Letters, 19 Feb. 2001; text at Physics News Select).