Now, physicists at the University of California, Berkeley, have shown that this same quantum interference occurs between two samples of superfluid helium-3, a liquid so cold - a thousandth of a degree above absolute zero - that it flows without resistance. One potential application of this quantum interference is in an ultrasensitive superfluid gyroscope.

"The successful demonstration of this effect may enable scientists to measure extremely slight increases or decreases in the rotation of objects, including Earth," said Richard E. Packard, UC Berkeley professor of physics. "This device could even be used to establish an absolute state of rest."

"Our experiment was a proof of principle, but if we can reduce the noise enough and build a much larger version of the device, it is conceivable that we could make a sensor to monitor small changes in the Earth's rotation," said J. C. Séamus Davis, UC Berkeley professor of physics and researcher in the Materials Sciences Division of Lawrence Berkeley National Laboratory. "It's beautiful physics."

Davis, Packard, recent PhD Raymond W. Simmonds, former postdoctoral fellow Alexei Marchenkov and graduate student Emile Hoskinson report their findings in the July 5 issue of Nature.

This quantum interference is identical to the interference between light waves, electrons, atomic beams and electrical currents in solid superconductors. It had never before been observed in a liquid.

The UC Berkeley physicists demonstrated quantum interference by building the first superfluid equivalent of a superconducting quantum interference device, called a dc-SQUID, the most sensitive detector of magnetic fields today.

Just as superconducting dc-SQUIDs can measure minuscule magnetic fields, such as magnetic emanations from the brain, a superfluid SQUID can detect changes in rotation, analogous to a gyroscope. In addition to monitoring the Earth's rotation, a superfluid gyroscope also could be used to test predictions from Einstein's general theory of relativity, such as how spinning objects move in a gravitational field.

Four years ago, Davis, Packard and their colleagues demonstrated one of the basic components of the superfluid SQUID - a superfluid Josephson junction, analogous to the Josephson junctions in superconductors. In superconductors, a thin insulator between two superconductors at different voltages generates a microwave oscillation in the junction. This is in contrast to a classical circuit, where current flows in only one direction - from high to low voltage.

Two Josephson junctions looped together create oscillating electrical currents that interfere, like beats in interfering sound waves. The beat pattern changes as the magnetic field enclosed in the loop changes, allowing an extremely precise field measurement.

In superfluids, pushing ultracold helium-3 through a perforated Silican wall generates a vibration as the fluid sloshes back and forth through the wall's 4,225 holes. Classically, liquids always flow from high to low pressure. The researchers confirmed these quantum oscillations in 1997 by placing a sensitive superconducting SQUID microphone in the fluid and detecting a high-pitched whistle.

For the current experiment, they took two superfluid Josephson junctions and placed them on either side of a doughnut-shaped tube in hopes of detecting a beat pattern produced by interfering superfluid wavefunctions at the two junctions.

Just as a superconducting SQUID is sensitive to magnetic fields, a superfluid SQUID is sensitive to rotation. In their experiment, the rotation of the Earth shifted the relative phase of the fluid oscillating through the two junctions. When these oscillations are combined they produce an interference pattern. The researchers connected a superconducting SQUID microphone to the doughnut-shaped tube to detect the quantum oscillations through the junctions, and heard a clear 273 Hertz tone.

In a vivid demonstration of the phase shifting, as the researchers tilted the loop relative to the rotation axis of the Earth, the loudness changed as predicted.

The researchers had to conduct the experiments over the Christmas and New Year's holidays so they could shut down the heating and cooling systems in UC Berkeley's Birge Hall to reduce extraneous vibrations. The vibrations they detected are 100,000 times smaller than a single atom.

Davis, who in 1984 first began working with Packard on this project as a potential PhD thesis, was ecstatic that it finally worked.

"It's still strange to see quantum interference in a liquid, and to see the effect of the Earth's rotation appear quantum mechanically in a tiny container of liquid," Davis said.

Simmonds, who recently received his physics PhD from UC Berkeley, added, " It's truly amazing how the tiny helium atoms forming the superfluid sense the Earth's rotation and communicate this quantum information over distances as big as my thumb, from one Josephson junction to the other."