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

Superfluidity in its various forms has fascinated scientists since the observation of frictionless flow in liquid helium II. In three spatial dimensions (3D), it is conceptually associated with the emergence of long-range order (LRO) at a critical temperature $T_{\text{c}}$. One of its hallmarks, 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 systems, thermal fluctuations preclude LRO, but superfluidity nevertheless emerges at a nonzero $T_{\text{c}}$ via the infinite-order Berezinskii-Kosterlitz-Thouless (BKT) transition, which is associated with a universal jump in the superfluid density $n_{\text{s}}$ without any discontinuities in the fluid's thermodynamic properties. BKT superfluids are also predicted to support two sounds, but the observation of this 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 extract its superfluid density. Our results agree with BKT theory, including the prediction for the universal superfluid-density jump.