Smoking gun. Array of vortices proves paired atoms form a superfluid.

Ultracold Atoms Turn a Cool Trick

In recent years, physicists have been racing to prove that ultracold atoms can pair up and flow like the freely flowing electrons in a superconductor. Now, subtle swirls in a cloud of atoms provide proof positive that researchers have reached that elusive goal.

For more than a decade, physicists have used laser beams and magnetic fields to chill gases of atoms to within billionths of a degree of absolute zero, producing a variety of weird quantum effects. Atoms of some isotopes can crowd into a single quantum wave, or "state," to create a Bose-Einstein condensate, or BEC. First produced in 1995, a BEC is a "superfluid," a weird quantum soup that flows without any resistance and refuses to rotate. Atoms of other isotopes cannot squeeze into a single quantum state because of the way they spin. However, physicists have long expected that such atoms can still form a superfluid, although in a more complicated way. To do so, they must form pairs, as the electrons do in a superconductor. The pairs join together to form the superfluid.

Or so the theory goes. Researchers have been striving to produce such a superfluid, and last year, they finally coaxed ultracold atoms into pairing (ScienceNOW, 28 January, 2004). But experimenters had not proved that the paired-up "Fermi condensate" was a superfluid.

To clinch the case, physicists Martin Zwierlein, Wolfgang Ketterle, and colleagues at the Massachusetts Institute of Technology in Cambridge tried to rotate a Fermi condensate of the isotope lithium-6 by tickling it with a laser, much as one might set a golf ball spinning by brushing around its circumference with a feather. Had the lithium-6 been an ordinary fluid, the cloud would have rotated as a whole, like water in a slowly turning drinking glass. A superfluid resists rotation, however, because it is a quantum wave that can possess only quantized amounts of rotation. Turn its container fast enough, and a superfluid admits one quantum of rotation in the form of a tiny whirlpool, or vortex. Turn faster still, and the vortices proliferate and form a triangular array, which is what Zwierlein and Ketterle observed in the cloud of atoms, as they report 23 June in Nature.

The results leave no doubt that a Fermi condensate is a superfluid, says Rudolf Grimm, an experimental physicist at the University of Innsbruck, Austria. "Everybody was waiting for the smoking gun," he says, "and there it is."

Related sites
Ketterle's group Web page
Grimm's group Web page