Quantum Pileup: Ultracold molecules meld into
oneness
Peter Weiss
Two independent teams of physicists have coaxed
molecules into an extraordinary state of ultracold
matter previously demonstrated only with atoms.
In each of the new experiments, the researchers
created minuscule gas clouds with an amazing property.
All of the constituent two-atom molecules meld to form a
single supermolecule, says Deborah S. Jin of JILA, a
joint institute of the National Institute of Standards
and Technology and the University of Colorado, both in
Boulder.
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ALL FOR ONE. Released from a
trap, an ultracold cloud of potassium molecules
(right) barely disperses—a sign that nearly
motionless molecules have condensed into a new
form of matter. Slightly warmer cloud (left),
which hasn't condensed, spreads after
release.
M.
Greiner |
In an upcoming Nature, she and her colleagues
report magnetically influencing ultracold potassium
atoms to make about 200,000 weakly bound pairs. Of
those, about a tenth coalesce into a supermolecule.
Taking a different ap-proach, Rudolf Grimm and his
colleagues at the University of Innsbruck in Austria
cooled 100,000 two-atom lithium molecules into a
supermolecule. The team reports its success in a future
issue of Science.
Blending the identities of atoms, and now of simple
molecules, relies on a quantum-mechanical process made
possible by wave-like characteristics of the particles.
That merger is the hallmark of so-called Bose-Einstein
condensates (BECs).
Satyendra Nath Bose and Albert Einstein independently
predicted BECs in 1924, and the first was made in 1995
from rubidium atoms (SN: 7/15/95, p. 36). Since then,
many laboratories have created BECs from different
elements and have studied the condensates' properties
(SN: 8/12/00, p. 102: Available to subscribers at http://www.sciencenews.org/20000812/fob5.asp).
To take BEC science further, researchers have been
striving to achieve quantum condensations of simple
molecules, such as the potassium or lithium pairs. The
new condensates are an "important milestone," comments
physicist Christophe Salomon of the École Normale
Supérieure in Paris.
Both teams created long, thin condensate clouds that
measured tens of micrometers in diameter. The Innsbruck
group claims to have made a relatively long-lived
condensate, which lasted more than 20 seconds. The
Boulder condensate stuck around for only about 10
milliseconds.
Both groups say that by creating molecular
condensates, they have devised a new means to
investigate fundamental aspects of physics and
chemistry. Potential topics include how electric charge
is distributed within individual electrons and the ways
in which chemical bonds form and break.
The new accomplishments may also lead to deeper
understanding of superfluidity (SN: 10/25/03, p. 262:
Available to subscribers at http://www.sciencenews.org/20031025/fob8.asp),
which is the flow of fluids without friction, and of
superconductivity (SN: 10/11/03, p .229: Available to
subscribers at http://www.sciencenews.org/20031011/fob5.asp),
the resistancefree flow of electrons, comments Wolfgang
Ketterle of the Massachusetts Institute of Technology.
Ketterle shared the 2001 Nobel Prize in Physics for his
pioneering work on BECs.
Physicists have long recognized that elementary
particles of ordinary matter fall into two broad
classes, bosons and fermions. Chummy by nature, nearby
bosons readily occupy the same quantum state. For
instance, photons will share a particular energy level
in a laser. In contrast, standoffish fermions won't
share a quantum state with even one other fermion.
Although electrons, protons, and neutrons, the
building blocks of atoms, are all fermions, some atoms
are bosons and some are fermions. It's easy to tell
which is which: If an atom's total number of building
blocks is even, the atom is a boson. If the total number
is odd, it's a fermion.
To make BECs, scientists trap and cool bosons to
temperatures just above absolute zero. Attempts to do
the same with molecules made of bosonic atoms failed,
however, because collisions shattered those molecules
before condensation could take place.
Earlier this year, the BEC scientific community
discovered that molecules composed of just two fermionic
atoms are far less vulnerable to such disintegration.
Conveniently, such molecules—because they contain an
even number of fermions—are actually bosons.
The discovery has proved critical in the race for
molecular BECs, Grimm says. Turning to molecules made of
fermionic atoms, the Innsbruck and Boulder groups both
achieved the coveted goal.
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References:
Greiner, M., C.A. Regal, and D.S.
Jin. In press. A molecular Bose-Einstein condensate
emerges from a Fermi sea. Nature. Preprint
available at http://arxiv.org/abs/cond-mat/0311172.
Jochim, S. . . . and R. Grimm. In
press. Bose-Einstein condensation of molecules.
Science. Abstract available at http://dx.doi.org/10.1126/science.1093280.
Further Readings:
Weiss, P. 2003. Super spinner:
Seven-atom speck acts like superfluid. Science
News 164(Oct. 25):262. Available to subscribers at
http://www.sciencenews.org/20031025/fob8.asp.
______. 2003. Nobel Prizes go to
scientists harnessing odd phenomena. Science News
164(Oct. 11):229. Available to subscribers at http://www.sciencenews.org/20031011/fob5.asp.
______. 2001. Device shifts molecules
into slow motion. Science News 159(May 12):295.
Available to subscribers at http://www.sciencenews.org/20010512/fob8.asp.
______. 2000. Attractive atoms pick
up repulsive habits. Science News 158(Aug.
12):102. Available to subscribers at http://www.sciencenews.org/20000812/fob5.asp.
______. 2000. Ultracold molecules
form inside superatom. Science News 157(Feb.
12):104. Available to subscribers at http://www.sciencenews.org/20000212/fob8.asp.
______. 1999. When the other half
gets really cold. Science News 156(Sept. 11):166.
References and sources available at http://www.sciencenews.org/sn_arc99/9_11_99/fob5ref.htm.
______. 1999. Electrons display their
antisocial nature. Science News 155(April
10):230. References and sources available at http://www.sciencenews.org/sn_arc99/4_10_99/fob5ref.htm.
______. 1998. Cold molecules make
long-awaited debut. Science News 153(May
30):342.
Wu, C. 1995. Physics 'Holy Grail'
finally captured. Science News 148(July 15):36.
Sources:
Rudolf Grimm
Institute für
Experimentalphysik
Universitä
Innsbruck
Technikerstrasse 25
A-6020
Innsbruck
Austria
Deborah S. Jin
Joint Institute for
Laboratory Astrophysics
National Institute of
Standards and Technology
Department of
Physics
University of Colorado, Boulder
Boulder,
CO 80309-0440
Wolfgang Ketterle
Research
Laboratory for Electronics
Massachusetts Institute of
Technology
MIT-Harvard Center for Ultracold
Atoms
Department of Physics
77 Massachusetts
Avenue, Room 26-243
Cambridge, MA 02139-4307
Christophe Salomon
Départment de
physique de l'Ecole Normale Supérieure
Laboratoire
Kastler Brossel
24 rue Lhomond
75231
Paris
France
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