The Age reports on recent breakthroughs in ultracold molecule
experiments.
The Age "Cold Waves" article: 31 Aug 09
Cold Waves
GEOFF MASLEN
August 31, 2009
At temperatures less than a millionth of a degree above absolute
zero, or about minus 273 degrees, atoms in a gas behave in the most
peculiar fashion. They enter the quantum realm, becoming less like
matter and more like light waves. This is a world of the ultra-cold,
where certain atoms show superfluid characteristics, flowing like
the electrons in a superconducting solid, in which a current could
flow indefinitely without meeting any resistance. In this weird
world, physicists talk of fermions and bosons, of Cooper pairs and
Bose-Einstein condensates. (The latter is a reference to a state
of matter first predicted by Albert Einstein in 1925: building
on the previous work of Satyendra Nath Bose: although it would
be 70 years before physicists produced the first gaseous condensate.)
Now another team of physicists, at Swinburne University of Technology's
Centre for Atom Optics and Ultrafast Spectroscopy and headed by
Professor Peter Hannaford, is pushing the frontier of science. Professor
Hannaford, Dr Chris Vale, and their colleagues at the centre have
established a "cold molecules lab", the only one of its kind in
the southern hemisphere. The Swinburne team is part of an Australian
Research Council Centre of Excellence for Quantum-Atom Optics that
includes not just Swinburne but physicists at the Australian National
University and the University of Queensland. Without such a centre
of excellence, Professor Hannaford says the Swinburne team would
not have been able to achieve the results it has.
Yet it is in the cold molecules laboratory where the Swinburne
physicists have discovered new insights into the nature of matter
by conducting experiments in an almost total vacuum at temperatures
hovering a tiny fraction above absolute zero. In one experiment,
they use a laser beam fired into a glass cell containing a few million
atoms of an isotope of the element lithium, Li6, which is a fermion.
The atoms first absorb then re-emit the radiation, but, because
they eject more energy than they receive, they rapidly cool. "This
happens so quickly, the temperature plummets close to minus 273.15
degrees or absolute zero," Dr Vale says. "We start with a quantity
of lithium metal, vaporise it and inject a beam of atoms into the
vacuum chamber, then use the laser to cool the atoms to very low
temperatures." As he says, usually any gaseous substance when cooled
first condenses to a liquid and then becomes a solid. But because
there are so few atoms of lithium and they are relatively far apart,
they remain as a gas suspended in the glass cell.
Lasers play two roles in this experiment: a first laser cools the
gas to just above absolute zero, then a second laser is used to
form a trap to hold the very cold atoms suspended in the vacuum
and not in contact with anything else. "We can take a picture of
the atoms by shining laser light on them, obtaining a precise image
bright enough to see with the naked eye," Dr Vale says. "With the
atoms in the laser trap, we can conduct different experiments, watch
how they pair up, like Cooper pairs of electrons in a superconductor,
and how they form a superfluid."
The idea of studying these ultra-cold gases is that the scientists
can control all the interactions, "all the microscopic parameters",
and can study how superfluids function. "If you have a flow without
resistance, you can have transport without loss of energy. In a
way, this is where the analogy with superconductivity comes in:
the electrons in a superconductor are also a superfluid and flow
without any loss of energy that would otherwise appear as heat."
Dr Vale says the physics of superconductivity in solids, where electrons
flow without meeting any resistance at temperatures a few degrees
above absolute zero, is well understood. Yet there is no complete
explanation of why the phenomenon can also occur in other more exotic
materials at the temperature of liquid nitrogen or minus 196, which
the scientists classify as a "high temperature".
"With these very cold gases, we can replicate in some ways what
is happening in a superconductor. So we are trying to mimic the
complex behaviours that are happening in these high-temperature
superconductors." One of the main reasons researchers study these
very cold gases, Dr Vale says, is because they display "quantum
mechanical behaviour" where particles begin to behave like waves.
"We can see these behaviours and we can control the way a lot of
things happen, how the particles of the gas interact with each other,
and that plays a critical role in the sorts of behaviours we see."
He says the goal is to understand complex quantum systems by conducting
experiments on simple atoms and studying their interactions to build
up a picture of more complex behaviours.
"At present we are probing very simple superfluids that are similar
to what happens in conventional superconductors. In effect, we
are exploring the way particles behave in neutron stars, although,
rather than having to fly out to deep space, we can probe what is
happening in our tabletop lab experiments." Dr Vale says that if
scientists could work out how high-temperature superconductors work,
then engineers and technologists could use that knowledge to develop
devices such as loss-free power transmission, or other unimagined
possibilities. "While real applications are still a long way down
the track, we are doing the research to try to understand superfluids,
and if we can find out how they work, this could lead on to understanding
how high-temperature superconductors work." The latest discoveries
made by the Swinburne team have been published in the leading physics
journal Physical Review Letters.
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