A Whistle for Their Work

After a Decade's Effort, Davis and Packard
Hear the Elusive Sound of a Superfluid

by Robert Sanders

In a dramatic confirmation of predictions made more than 30 years ago, Berkeley physicists have detected quantum vibrations-in this case a high-pitched whistle-in a superfluid analogous to the Josephson effect in superconductors.

The confirmation of this fundamental prediction of quantum mechanics-the theory that describes interactions on the atomic scale-culminates more than 10 years of effort by a Berkeley team led by low temperature physicists James C. "Seamus" Davis and Richard E. Packard.

Over the past three decades numerous laboratories around the world have searched for the effect, but no conclusive evidence for it had been found.

"This has been a Holy Grail of physics," says Packard, a professor of physics. "The discovery is fundamental to our understanding of superfluids and by analogy of the phenomena we observe in superconductors."

The Berkeley team, which includes post-doctoral scientist Sergey Pereversev and graduate students Scott Backhaus and Alex Loshak, reported their results in the July 31 issue of the British journal Nature.

The effect they observed is the oscillation of superfluid between two containers of superfluid helium-3 at slightly differing pressures, connected only by a microscopic hole. Nobel laureates Philip Anderson of Princeton, Brian Josephson of Cambridge and the late Richard Feynman of Caltech all independently predicted that in such a situation the superfluid would slosh back and forth at a frequency determined by the pressure difference.

The predictions came not long after confirmation in 1961 of an analogous electrical oscillation between two superconductors separated by an extremely thin film of insulator. Detection of the so-called Josephson effect in superconductors garnered the Nobel Prize for Josephson in 1973, and the phenomenon is used today in various electronic devices, including the world's most sensitive detectors of magnetic fields, superconducting quantum interference devices (SQUIDs).

To detect the quantum oscillations in superfluid helium-3, the Berkeley scientists suspended a highly sensitive SQUID microphone pickup next to the microscopic aperture. For a long time they detected only noise. But on Sunday afternoon, May 10, Pereversev, responding to a suggestion of Davis and Packard, plugged headphones into the SQUID output and picked out a faint tone decreasing in pitch in perfect concert with the decrease in pressure across the quantum holes.

"I was absolutely elated, astonished, amazed, ecstatic-I was over the moon," recalls Davis, an assistant professor of physics. "I never hoped to hear the sound so clearly. 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."

The first thing he did was corner two colleagues in the hallway and force them to listen to the sounds coming out of the headphones. Then the whole team popped open a bottle of champagne.

"It takes several months to cool down the experiment from room temperature to near zero Kelvin, so we only get two or three shots at this each year," he says. "But here was evidence of something amazing-- a demonstration from quantum mechanics in one of the most exotic materials in the universe, helium-3."

The quantum oscillations could be used to establish an absolute pressure standard in the same way that Josephson junctions have been used to establish an absolute voltage standard accepted worldwide.

The effect may also allow sensitive detection of rotation, as in superfluid gyroscope. The projected sensitivity of the quantum gyroscope would allow it to become an important tool in studies of the Earth's rotation, which are significant to the field of geodesy as well as to climate studies. In addition the effect provides a new tool for fundamental studies of superfluidity.

"These quantum oscillations are well-known for microscopic quantum systems like atoms and nuclei," Packard says, "but have never been observed in fluids in the macroscopic world, that is, things big enough to see and observe with human senses."

At the time of the initial theoretical predictions, the only superfluid known was helium-4-the kind of helium used in balloons-cooled to 2.17 degrees above absolute zero, or minus 456 degrees Fahrenheit. For helium-4 the hole required to see oscillations is about the size of an atom, too small for anyone to create even today. Packard says that even if someone had succeeded in creating such tiny holes, it is unlikely that fluid flow through the passage would have been detectable.

Nevertheless people tried, most hoping that by focusing the right frequency of sound on a larger hole they could stimulate fluid oscillations. By the 1980s, nearly everyone had given up.

With the discovery in 1972 that helium-3-a lighter isotope of helium-4-can also become superfluid, the possibility again reared its head, since the hole size required for oscillations in superfluid helium-3 is 100 times greater, on the order of 50 nanometers. For low pressure differences the oscillations through the hole would be on the order of kilohertz, within the range of human hearing.

Using state-of-the-art microfabrication techniques developed for the semiconductor industry, Davis and Packard constructed minute apertures in incredibly thin silicon nitride membranes supported by a silicon chip, and attempted various configurations. This year graduate student Alex Loshak made an array of 4,225 microscopic holes, each about 1,000 atoms across (100 nanometers), in the hopes that the greater amount of fluid moving back and forth would be more easily detected.

"Each 100 nanometer hole is still so small that fluid moving through it would take about 1,000 years to fill a thimble," Davis says. "It was quite a technical accomplishment to make an array of such small holes in such a fragile membrane."

They placed the array between two small containers of helium-3 and chilled the entire experiment to 300,000 times colder than room temperature. The pressure difference between the two helium-3 reservoirs was about one ten-millionth of atmospheric pressure, which would produce an oscillation in the 200 to 6,000 Hertz range.

Because of vibrational background noise, even in the dead of night, the researchers were initially unable to see any evidence of oscillations through the array of holes. Only when the output of the SQUID was connected to a headphone were they able to pick out a descending pitch that correlated precisely with the pressure changes across the array of holes. The human brain has the amazing ability to detect this faint sound against a loud background noise in a few seconds, where sophisticated electronics had failed, Packard says.

Davis says that the helium atoms in the superfluid probably slosh back and forth about a millionth of a meter during each oscillation, moving in perfect synchrony through all 4,000 holes.

"We took a gamble that the fluid would vibrate in phase through all the holes, like 4,000 violins playing at the same time," he says. "But it worked. No one else has attempted an experiment like this."

The work was sponsored in part by the National Science Foundation, the Office of Naval Research and the Packard Foundation.

Editorial note: Tom Pedersen. A research and development toolmaker in the Physics Machine Shop, Pedersen helped build the equipment used in this important experiment.



Copyright 1997, The Regents of the University of California.
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