Observation of an inverse turbulent-wave cascade in a driven quantum gas

We observe an inverse turbulent-wave cascade, from small to large lengthscales, in a homogeneous 2D Bose gas driven isotropically on a lengthscale much smaller than its size. Starting with an equilibrium condensed gas, at long drive times we observe a nonthermal steady state. At increasing lengthscales, starting from the forcing one, the steady-state momentum distribution features in turn: (i) a power-law spectrum, with an exponent close to the analytical result for a particle cascade in weak-wave turbulence, and (ii) a spectrum intriguingly reminiscent of a nonthermal fixed point associated with universal coarsening in an isolated 2D gas. In further experiments, based on anisotropic driving, we also reveal the qualitative picture of the cascade-formation dynamics.