  Quantum-degenerate Fermi gases provide a remarkable opportunity to study
strongly interacting fermions. In contrast to other Fermi systems, such as
superconductors, neutron stars or the quark-gluon plasma, these gases have low
densities and their interactions can be precisely controlled over an enormous
range. Here we report observations of vortices in such a gas that provide
definitive evidence for superfluidity. By varying the pairing strength between
two fermions near a Feshbach resonance, one can explore the crossover from a
Bose-Einstein condensate (BEC) of molecules to a Bardeen-Cooper-Schrieffer
(BCS) superfluid of loosely bound pairs whose size is comparable to, or even
larger than, the interparticle spacing. The crossover realizes a novel form of
high-T_C superfluidity and it may provide new insight for high-T_C
superconductors. Previous experiments with Fermi gases have revealed
condensation of fermion pairs. While these and other studies were consistent
with predictions assuming superfluidity, the smoking gun for superfluid
behavior has been elusive. Our observation of vortex lattices directly displays
superfluid flow in a strongly interacting, rotating Fermi gas.
