A strongly interacting degenerate fermi gas

of lithium-6 atoms has been produced. Duke University physicists confined the gas in an optical trap and cooled it to 800 nK. That made the fermions degenerate—their de Broglie wavelengths exceeded the interatomic spacings. The cooling took place in the presence of a 91-mT magnetic field, which induced an extremely large and negative scattering length and caused the atoms to interact strongly. When subsequently released from the trap, the cigar-shaped gas expanded in a decidedly lopsided fashion: The cigar of lithium rapidly got fatter without ever growing longer, and became a thick ellipsoid within 2 ms. When the researchers used a field of 53 mT, the atomic interactions vanished in accordance with theory, and the gas expanded spherically as would any normal gas. The researchers suggest two possible explanations for the anisotropic expansion: Either they were observing a new kind of long-range collision between atoms, or they witnessed so-called “resonance superfluidity” triggered by tuning the interactions between fermions. Either way, the new experimental system can provide a test bed for calculations relevant to all strongly interacting fermions, including the neutrons in neutron stars, the quarks in atomic nuclei, and the electrons in superconductors. (K. M. O’Hara et al., Science 298, 2179, 2002 .)