Superfluid behaviour of a two-dimensional Bose gas

Owing to thermal fluctuations, two-dimensional (2D) systems cannot undergo a conventional phase transition associated with the breaking of a continuous symmetry(1). Nevertheless they may exhibit a phase transition to a state with quasi-long-range order via the Berezinskii-Kosterlitz-Thouless (BKT) mechanism(2). A paradigm example is the 2D Bose fluid, such as a liquid helium film(3), which cannot condense at non-zero temperature although it becomes superfluid above a critical phase space density. The quasi-long-range coherence and the microscopic nature of the BKT transition were recently explored with ultracold atomic gases(4-6). However, a direct observation of superfluidity in terms of frictionless flow is still missing for these systems. Here we probe the superfluidity of a 2D trapped Bose gas using a moving obstacle formed by a micrometre-sized laser beam. We find a dramatic variation of the response of the fluid, depending on its degree of degeneracy at the obstacle location.