Fermi gases with resonant interactions have been available in the laboratory for ten years, and experiments performed in many laboratories all over the world continue to produce exciting results on the quantum many-body physics of strongly interacting fermion systems. The superfluid behavior of Fermi gases is now a well-established fact, and many aspects of superfluidity have been investigated. Quite surprisingly, the observation of a key phenomenon has so far been an elusive goal: `Second sound’ is a striking manifestation of the two-component nature of a superfluid, well known in context with liquid helium II. This form of sound, which only exists below the critical temperature, corresponds to an entropy wave, where the superfluid and the non-superfluid components oscillate in opposite phase. This is different from ordinary sound (`first sound'), where the two components oscillate in phase. We report on the first observation of second sound in an atomic Fermi gas. Our results can be interpreted in terms of Landau’s famous two-fluid theory, and the measured second-sound speeds allow us to extract the temperature dependence of the superfluid fraction, which in strongly interacting quantum gases has been an inaccessible quantity so far.

(*) Work performed in collaboration with Leonid Sidorenkov and Meng Khoon Tey (Innsbruck) and Yan-Hua Hou, Lev Pitaevskii, and Sandro Stringari (Univ. Trento, Italy)