Quantum interference demonstrated for first time in liquids as physicists make superfluid analog of superconducting SQUID, a potential ultrasensitiv

In the quantum world, waves can act like particles and particles like waves,
interfering like overlapping ripples in a pond.

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 Emily 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.”

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The work was supported by the National Aeronautics and Space Administration, the
Office of Naval Research and the National Science Foundation.

NOTE: J. C. Séamus Davis can be reached at 510-642-4505 or
jcdavis@socrates.berkeley.edu. Davis’s Web site is
http://socrates.berkeley.edu/~davisgrp/. Richard Packard is out of town until August.