BEC Made From Fermion Molecules

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.