August 2, 1997
by I. Peterson
For physicists probing the curious quantum behavior of superfluids, "Whistle While You Work" is a highly appropriate theme song.
By detecting a high-pitched whistle, researchers have obtained the first clear indication that helium-3 atoms can shuttle back and forth between two containers separated by a thin membrane perforated with tiny holes. Instead of flowing from one container to the other in response to a slight difference in pressure, the helium-3 superfluid oscillates at a characteristic frequency.
"The discovery is fundamental to our understanding of superfluids and, by analogy, of the phenomena we observe in superconductors," says physicist Richard E. Packard of the University of California, Berkeley. Packard, James C. Davis, and their coworkers report their findings in the July 31 Nature. This represents "a beautiful, direct demonstration of quantum mechanics in action on the macroscopic scale," says Peter McClintock of Lancaster University in England.
Physicists first observed such oscillations in an electric current passing between two superconductors separated by an extremely thin layer of electrically insulating material. Named the Josephson effect for Brian D. Josephson of the University of Cambridge in England, this phenomenon serves as the basis of various electric devices, including the world's most sensitive detectors of magnetic fields.
More than 30 years ago, theorists predicted that analogous oscillations would occur when a minuscule aperture connects two reservoirs of the same superfluid at different pressures. A superfluid is a state of matter in which all atoms belong to the same quantum state and move in a coordinated manner, allowing the liquid to flow without resistance.
Researchers expended considerable effort looking for the predicted effect. Earlier experiments by Packard's team and by Oliver Avenel of the Centre d'Etudes de Saclay in Gif-sur-Yvette, France, and his collaborators failed to furnish unambiguous evidence of oscillations.
Two years ago, the Berkeley team decided to use a barrier with an array of holes instead of just a single aperture. "We thought there was a small chance that all of these holes would quantum mechanically act together," Packard says.
When that experiment didn't work, the researchers tried a membrane in which the holes were more widely spaced than before. Fabricated from silicon nitride, the barrier was 50 nanometers thick, punctured by 4,225 holes about 100 nm wide and 3 micrometers apart.
Special refrigerator used to Chill helium-3 to less than 1 millikelvin. (Photo R. Orr/Univ. of Calif. Berkeley.)
The researchers made a fortuitous decision to connect headphones directly to the detector to listen for the oscillations rather than attempt to view the signal on an instrument. When they did the experiment, they could hear, above the background noise, a distinct, though faint, high-pitched whistle -- the helium-3 oscillations. As the pressure difference decreased, the pitch dropped, just as theory predicted.
"It's amazing how the brain picks out the tone from the background noise, like hearing a faint piccolo against the background of a large orchestra," Davis says.
"It worked fantastically," Packard remarks. "That was something that I had wanted to do for more than 10 years, and I didn't really expect it to happen."
In this drawing of the apparatus, a force causes a slight pressure difference between two reserviors of the superfluid, which induces it to oscillate through a tiny aperature, generating waves that can be picked up by an extremely sensitive detector.
Pereverzev, S.V., et al. 1997. Quantum oscillations between two weakly coupled reservoirs of superfluid helium-3. Nature 388(July 31):449.
McClintock, P. 1997. Whistles from superfluid helium. Nature 388(July 31):421.
Peterson, I. 1997. Superfluid gyro detects Earth's spin. Science News 151(April 12):223.
Additional information about the Josephson effect in superfluid helium-3 is available at http://physics1.berkeley.edu/research/jcdavis/ .
Richard E. Packard
University of California
Berkeley, CA 94720
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