A British scientist has linked two of physics's strangest beasts. One is liquid helium cooled to a few degrees above absolute zero. There it becomes a "superfluid," losing all resistance to flow, so that it can creep over the rim of a glass or effortlessly squeeze itself through even the tiniest holes. The other is Bose-Einstein condensate, a state of matter in which many atoms settle into the same quantum state and merge into the equivalent of a large, single particle. In the current issue of Nature,the researchers trace the antics of superfluid helium to a Bose-Einstein condensate formed by its atoms.
Physicists have speculated since the late 1930s that a Bose-Einstein condensate can explain the superfluid properties of liquid helium. Because the atoms in a Bose condensate all occupy exactly the same energy level, they can be thought of as marching in lockstep, which means they do not bump into one another--the source of viscosity. Physicists first demonstrated the existence of Bose-Einstein condensates in 1995 by slowing rubidium atoms and trapping them with magnets. But no one had designed an experiment that could detect the condensate within a superfluid.
Adrian Wyatt of the University of Exeter in the United Kingdom has now devised a clever way to do so. By heating a metallic film, he created supersonic sound waves that traveled through the liquid helium and knocked atoms from its surface. The resulting beam of atoms shot straight up, suggesting that they had no sideways velocity--indeed, no motion at all, except for the kick from the supersonic waves. This, in turn, implied to Wyatt that the atoms were all in the same quantum "ground state."
"This is a historic experiment," says Allan Griffin, a physicist at the University of Toronto. "It settles the problem." Furthermore, the experiment may lead to an atom laser, a column of atoms moving in the same quantum state. Researchers have already created atom lasers from trapped vapors (ScienceNOW, 27 January 1997 ), but the setup developed by Wyatt promises a simpler way to do so.