[microsound] Quantum Whistles - Superfluid helium-4 whistles just the right tune

[Dan Heidebrecht <dan.heidebrecht@xxxxxxxxx>]

If you just want to listen:

http://www.berkeley.edu/news/media/releases/2005/01/images/He4QuantumWhistle.wav


Superfluid helium-4 whistles just the right tune

By Robert Sanders, Media Relations | 27 January 2005

BERKELEY â University of California, Berkeley, physicists can now tune
in to and hear normally inaudible quantum vibrations, called quantum
whistles, enabling them to build very sensitive detectors of rotation
or very precise gyroscopes.
Quantum whistle
Hear the synchronized vibrations from a chorus of more than 4,000
nano-whistles, created when physicists pushed superfluid helium-4
though an array of nanometer-sized holes. Note that the pitch drops as
the pressure drops.

A quantum whistle is a peculiar characteristic of supercold condensed
fluids, in this case superfluid helium-4, which vibrate when you try
to push them through a tiny hole. Richard Packard, professor of
physics at UC Berkeley, and graduate student Emile Hoskinson knew that
many other researchers had failed to produce a quantum whistle by
pushing helium-4 through a tiny aperture, which must be no bigger than
a few tens of nanometers across - the size of the smallest viruses and
about 1,000 times smaller than the diameter of a human hair.

To their surprise, however, a chorus of thousands of nano-whistles
produced a wail loud enough to hear. This is the first demonstration
of whistling in superfluid helium-4. According to Packard and
Hoskinson, the purity of the tone may lead to the development of
rotation sensors that are sufficiently sensitive to be used for Earth
science, seismology and inertial navigation.

"You could measure rotational signals from an earthquake or build more
precise gyroscopes for submarines," Packard speculated.

Four years ago, Packard and his coworkers built and successfully
tested a gyroscope based on quantum whistling in superfluid helium-3.
But that required cooling the device to a few thousandths of a degree
above absolute zero, a highly specialized and time-consuming process.
Because the new phenomenon exists at 2 Kelvin - a temperature
achievable with off-the-shelf cryo-coolers - the proposed sensors also
will be user-friendly to scientists unfamiliar with cryogenic
technology. A temperature of 2 Kelvin is the equivalent of minus 456
degrees Fahrenheit.

"Because these oscillations appear in helium-4 at a temperature 2,000
times higher than in superfluid helium-3, it may be possible to build
sensitive rotation sensors using much simpler technology than
previously believed," the researchers wrote in a brief communication
appearing in the Jan 27 issue of the journal Nature.

Packard noted that sensitive rotation or spin detectors could have
application in numerous fields, from geodesy, which charts changes in
the spin and wobble of the Earth, to navigation, where gyroscopes are
used to guide ships. Though little is now know about the rotational
signals from earthquakes, having a sensitive rotation detector might
reveal new and interesting phenomena.

Quantum whistling is analogous to a phenomenon in another macroscopic
quantum system, a superconductor, which develops an oscillating
current when a voltage is applied across a non-conducting gap. Nobel
Laureates Philip Anderson, Brian Josephson and Richard Feynman
predicted in 1962 that the same would happen in superfluids. In the
case of superfluids, however, a pressure difference across a tiny hole
would cause a vibration in the superfluid at a frequency - the
Josephson frequency - that increases as the pressure increases. The
fact that the fluid oscillates back and forth through the hole rather
than flows from the high-pressure side to the low-pressure side, as a
normal liquid would, is one of the many weird aspects of quantum
systems like superfluids.

Eight years ago, Packard and fellow UC Berkeley physicist Seamus
Davis, now at Cornell University, heard such vibrations when pushing
superfluid helium-3 through a similar array of 4,225 holes, each 100
nanometers across. Though no simple feat - it took them 10 years to
make their experiment whistle, working at one thousandth of a degree
Kelvin - it's theoretically easier than with helium-4.

For helium-4 to whistle, physicists predicted that the holes either
had to be much smaller, pushing the limits of today's technology, or
the temperature had to be within a few hundred thousandths of a degree
of the temperature at which helium-4 becomes a superfluid, that is, 2
Kelvin. While working with an array of holes 70 nanometers across,
essentially testing the apparatus with helium-4 before using it to
conduct a helium-3 experiment, Hoskinson was surprised when he put on
earphones and heard the characteristic pennywhistle sound as the pitch
dropped with the pressure in the device.

"Predictions on where the Josephson oscillations would occur put them
much closer to the transition temperature than I could hope to go,"
Hoskinson said. "The fact that I could detect the oscillations with
the set-up I had was amazing in itself, and something we're very
interested in exploring."

He and Packard calculated that the tones were due to a different
mechanism, phase slippage, than that producing the whistle in
helium-3, though it follows the same relationship between frequency
and driving pressure. Phase slippage shouldn't have produced a pure
tone at all. The vibrations at the holes should shift randomly and get
lost in the noise. Even if phase slippage did produce a constant tone
in a single hole, the whistles from the array of 4,225 holes should
have been out of phase and the resulting sound less than 100 times
louder than that from a single hole.

Apparently, Packard said, the vibrating holes somehow achieved
synchrony, like crickets chirping in unison on a summer evening,
amplifying the sound 4,000 times higher - loud enough to be heard
above the background noise of the experiment.

"For 40 years, people have been trying to see something like this, but
it has always been with single apertures," Hoskinson said. "Maybe it's
true that you don't get coherent oscillations with a single aperture,
but somehow, with an array of apertures, the noise is suppressed and
you hear a coherent whistle."

"There was no reason to expect that. I still think it's amazing," Packard added.

The research by Packard, Hoskinson and post-doctoral fellow Thomas
Haard is supported by the National Science Foundation and by the
National Aeronautics and Space Administration.

---------------------------------------------------------------------
To unsubscribe, e-mail: microsound-unsubscribe@xxxxxxxxxxxxx
For additional commands, e-mail: microsound-help@xxxxxxxxxxxxx
website: http://www.microsound.org