"The two-dimensional Bose gas: Thermodynamics and beyond"

A two-dimensional Bose fluid is a remarkably rich many-body system, which allows one to revisit several features of quantum statistical physics. Firstly, the role of thermal fluctuations is enhanced compared to the 3D case, which destroys the ordered state associated with Bose–Einstein condensation. However interactions between particles can still cause a superfluid transition, thanks to the Berezinskii–Kosterlitz–Thouless mechanism. Secondly, the weakly interacting Bose gas in 2D must be scale-invariant, a remarkable feature that manifests itself in the very simple form taken by the equation of state of the fluid. In this talk I will summarize the recent experimental progress in the investigation of 2d atomic gases, and draw some comparisons with other planar fluids such as exciton-polariton gases, which also provide spectacular illustrations of low dimensional many-body physics.