Observation of first and second sound in a Berezinskii-Kosterlitz-Thouless superfluid

Superfluidity in two dimensions (2D), unlike its three dimensional counter-part, is associated with the pairing of vortices of opposite circulation as described by the Berezinskii-Kosterlitz-Thouless (BKT) theory, rather than the emergence of true long-range order. BKT superfluidity is characterized by a universal jump in the superfluid density at a critical temperature without any discontinuities in the fluid's thermodynamic properties. One of the hallmarks of superfluidity, predicted by the highly successful two-fluid model and observed in both liquid helium and ultracold atomic gases, is the existence of two kinds of sound excitations, the first and second sound. In 2D superfluids the observation of these sound modes has remained elusive. Here we observe first and second sound in a homogeneous 2D atomic Bose gas, and from the two temperature-dependent sound speeds deduce its superfluid density. Our results agree with BKT theory, including the prediction for the universal superfluid-density jump.