The study of quantum gases, gases that display spectacular quantum
effects, has come under sharp scrutiny over the past decade, partly
because they offer the chance to study a model quantum system in which
the interaction among atoms can possibly be tuned at will by the researcher.
Chilled gases are not all alike. Cold clouds of boson atoms (atoms with
an overall spin with a whole-number value) can fall into a single quantum
state known as a Bose Einstein condensate (BEC). BEC was first observed
in 1995 for the case of bosonic rubidium atoms (at
NIST/Colorado), lithium atoms (Rice
Univ), and sodium atoms (MIT).
Meanwhile, fermion atoms (with half-integral overall spin) must avoid
consorting with each other in any unified quantum state (a behavior
enforced by the Pauli exclusion principle, which also dictates how electrons
in atoms group into discrete shells---a grouping with implications for
all chemical relationships). This means condensation is out of the question.
Fermi atoms can, however, show off their quantum nature by piling up
into all possible quantum energy levels allowed by the ambient temperature
inside an atom trap. This feat was achieved in 1999 by another NIST
group.
In 2002, BECs were formed from molecules of bosonic
rubidium atoms. Now, in the latest chapter in the saga of quantum
gases, two research groups have succeeded in producing a BEC of molecules
made from pairs of fermion atoms. Note that the atoms are fermions but
considered as pairs they are bosons and therefore able to condense in
Bose-Einstein fashion. The two groups involved: Rudolf Grimm and his
colleagues at the University of Innsbruck (publishing last week online
in Science) used lithium atoms, and Deborah Jin and her colleagues at
NIST (publishing online in Nature) used potassium atoms.
Researchers will next want to tinker with the force between the
pairs of atoms. At the one extreme is the strong interaction
typical of the atomic BECs. At the other extreme is an interaction
in which the atoms forming the pair are correlated but essentially
unbound (in the chemical sense). The best example of this fragile
arrangement is the special correlation, "Cooper pairing" between
electrons, forming the essence of superconductivity. Such Cooper
pairing of fermion atoms (at work in bringing about the superfluid
state in liquid helium-3) does not seem to have occurred yet in the
present BEC experiments with gases. elation, "Cooper pairing" between
electrons, forming the essence of superconductivity. Such Cooper
pairing of fermion atoms (at work in bringing about the superfluid
state in liquid helium-3) does not seem to have occurred yet in the
present BEC experiments with gases.