Fermionic atoms can team up in distinctly different ways. In one regime, the atoms can bind tightly together to form molecules. Such molecules, being bosons, can collapse at low enough temperature into a common ground state—a Bose—Einstein condensate (BEC). In another regime, the fermions can associate in widely spaced pairs. At low temperature, the pairs may lock in phase with one another, falling into a coherent many-body state analogous to the electronic superconductor described by the Bardeen-Cooper-Schrieffer theory. The paired state, like the BEC, would be a superfluid.
Fortuitously, experimenters can take a gas of ultracold fermionic atoms smoothly from the BEC regime to the BCS regime simply by tuning a magnetic field. More than a year ago, experimenters saw a BEC in molecules made of fermionic atoms (see Physics Today, October 2003, page 18,). Soon afterward Deborah Jin’s group from JILA, NIST, and the University of Colorado,...
