Frigid atoms settle into surprising states.

When poet Robert Service wrote of the "strange things done in the midnight sun" of the frigid Arctic, he wasn't referring to atoms chilled to temperatures near absolute zero. Nonetheless, such atomic assemblages turn out to display a host of surprising characteristics.

In the latest twist in the saga of the coldest atoms, researchers have discovered a way to cool a single cloud of rubidium atoms in a magnetic trap so that each atom ends up in either of two subtly different quantum states. Unexpectedly, the two sets of atoms spontaneously separate into barely overlapping clouds.

Eric A. Cornell, Carl E. Wieman, and their coworkers at the University of Colorado and the National Institute of Standards and Technology, both in Boulder, report their findings in the Jan. 27 Physical Review Letters. "This is the first time that anybody has gotten two condensates of different types in the same trap," Wieman says.

Atoms that have been cooled to a sufficiently low temperature collectively enter the same quantum state and behave as a single unit. They are known as a Bose-Einstein condensate. Recently, a group at the Massachusetts Institute of Technology demonstrated that the array's atoms are coherent and constitute an atom laser (SN: 2/1/97, p. 71).

The Colorado-NIST team has now developed a cooling technique and a new type of apparatus that allows them to create condensates of neutral atoms in distinct atomic states. The two states differ by whether the electrons and nucleus of each atom spin in the same or opposite directions.

"What's surprising is that there's so little overlap of the two clouds in the case we studied," Wieman says. "Instead of mixing like normal gases, they separate like oil and water."

The new cooling method opens up the possibility of studying mixtures of two Bose-Einstein condensates. Theorists have already predicted a rich variety of interesting behaviors in such combinations.

In the Oct. 14, 1996 Physical Review Letters, Tin-Lun Ho and V.B. Shenoy of Ohio State University in Columbus showed theoretically that overlapping mixtures of two different condensates would, under certain conditions, display vortices and other structures that occur when liquids become superfluids.

"The current experiments have already produced surprises," Ho says. With further improvements in the techniques, "one has a chance to see a new kind of superfluidity."