Cooled to a few billionths of a degree above absolute zero, atoms in a Bose-Einstein condensate represent an extreme state of matter. Now physicists have forced the atoms to perform new kinds of gymnastics, stretching them into lines and flattening them into planes. The experiments, reported in the 24 September issue of Physical Review Letters, open the door to investigating a new regime of physics in which the rules are easier to understand.
Atoms in a Bose-Einstein condensate lose their individuality. Cool a clump of matter enough, damping out the random thermal motions of the particles, and the atoms can merge, becoming, in a quantum-mechanical sense, a single coherent object. For the past 5 years, scientists around the world have been experimenting with the strange properties of these atomic ensembles (Science, 13 February 1998, pp. 986 and 1005). But these experiments all probed three-dimensional condensates. Lower dimensional condensates could be useful for studying phenomena such as solitons--stable waves--that would behave more simply in them.
To put condensates into flatland, a group led by Wolfgang Ketterle of the Massachusetts Institute of Technology (MIT) started with ordinary condensates made of sodium atoms. For a one-dimensional condensate, the group simply trapped the condensate in strong magnetic fields and stretched it into a cigar shape. The extreme fields made it much easier for atoms in the condensate to flow along the cigar's long axis. As a result, atoms were able to move in only one dimension if shoved by an outside force.
Achieving two dimensions took an extra step. Instead of staying in a magnetic trap, the atoms had to be transferred to an optical trap, where the condensate was confined by a sheet of light. The MIT team then watched as the condensates switched over from three dimensions to two or one dimension.
"It's a first step to being able to do interesting physics in a new regime," says physicist Randall Hulet of Rice University in Houston. He says the experiments would be even more informative if the condensates formed in lower dimensions, instead of being squeezed into the required shape.