What a difference is made by one-half quantum unit of spin. Atoms with integer spins—bosons—can collapse into a common ground state when sufficiently cold; the result is a collective state known as a Bose–Einstein condensate (BEC). But atoms with half-integer spins—fermions—are excluded from occupying the same quantum states. The lowest-energy configuration they can reach is to fill all levels up to the Fermi energy—the highest level that would be occupied at absolute zero.
It was formidable enough to form a BEC. Creating a degenerate Fermi gas proved even more difficult, largely because Fermi statistics prevents identical particles from participating in the kind of head-on collisions required for the evaporative cooling that takes a gas to ultracold temperatures. The cooling problem was solved, and a degenerate Fermi gas formed, in 1999 by a team from JILA, NIST and the University of Colorado, all in Boulder (see Physics Today, October 1999,...
