(August 5, 2015) UC Berkeley
physicists have cooled a gas to the quietest state ever achieved, hoping to
detect faint quantum effects lost in the din of colder but noisier fluids.
While the ultracold gas’s temperature – a billionth of a
degree above absolute zero – is twice as hot as the record cold, the gas has
the lowest entropy ever measured. Entropy is a measure of disorder or noise in
a system; a record low temperature gas isn’t necessarily the least noisy.
“This ‘lowest entropy’ or ‘lowest noise’ condition means
that the quantum gas can be used to bring forth subtle quantum mechanical
effects which are a main target for modern research on materials and on
many-body physics,” said co-author Dan Stamper-Kurn, a UC Berkeley professor of
physics. “When all is quiet and all is still, one might discern the subtle
music of many-body quantum mechanics.”
The quantum gas, a so-called Bose-Einstein condensate,
consisted of about a million rubidium atoms trapped by a beam of light,
isolated in a vacuum and cooled to their lowest energy state. The entropy and
temperature were so low that the researchers had to develop a new type of
thermometer to measure them.
While achieving extremely low temperatures may make the
record books, UC Berkeley graduate student Ryan Olf said, what scientists aim
for today are low-entropy states they can study to understand more interesting
but difficult-to-study materials.
The UC Berkeley team’s ability to manipulate ultracold,
low-entropy gases will allow them to study these quantum systems, including
quantum magnets – potentially useful in quantum computers – and
high-temperature superconductors. High-temperature superconductors are
experimental materials that display superconductivity – electrical flow without
resistance – at relatively high temperatures compared to the 3 or 4 degrees
Celsius above absolute zero typical of today’s conventional superconductors.
“One of the holy grails of modern physics is to understand
these exotic materials well enough to design one that is superconducting
without requiring any cooling at all,” Olf said. “By studying the properties of
low-entropy gases in various configurations, our community of researchers hope
to learn what makes these fascinating materials work the way they do.”