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See Inside February 2008

Going with the Persistent Flow

Near absolute zero, exotic states of matter can bizarrely flow without friction. Physicists have known that, in principle, everlasting flows are possible in a Bose-Einstein condensate—a pool of ultracold particles that essentially behave as one superparticle—but have failed to see them. Now National Institute of Standards and Technology researchers have created a doughnut-shaped condensate with persistentcircular superfluid flows lasting up to 10 seconds. Unlike past condensates, which have taken ball or cigar shapes, a doughnut stabilizes persistent flows, because it would require too much energy for the central hole to drift about and disturb the rest of the condensate. These findings, to be published in Physical Review Letters, could yield deeper insights into superfluidity and also lead to especially precise navigation gyroscopes.

The particles in ultracold solids may also be capable of frictionless flow, but some physicists have questioned whether past observations of such “supersolidity” were genuine. In the December 6, 2007, Nature, scientists reveal that solid helium becomes more rigid as the temperature drops closer to absolute zero. This phenomenon could have mimicked the effects of supersolidity in previous experiments—or it may indicate an entirely new property of supersolidity. Figuring out this mystery will take more experiments and cold logic.

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