Observation of Real-Time Dynamics of a Direct Turbulent Cascade in a Bose Flatland

Turbulence is a universal nonequilibrium phenomenon encountered in various systems including the  early universe, nonlinear optics, classical and quantum fluids. Despite its ubiquity, turbulence is still not fully understood as it requires solving a chaotic many-body problem over different length scales. Here, we follow an alternative approach to study turbulence and its origin by using an ultracold atomic system which is conceptually simpler and provides easy experimental tunability of relevant microscopic parameters like dimensionality, interaction strength, etc. In particular, we study the dynamics of a two-dimensional Bose gas held in a box trap and driven far from equilibrium.

Our experimental protocol starts with exciting the lowest-lying phonon mode with a strong continuous shaking of the atomic cloud via a time-varying magnetic field gradient. This leads to the formation of a direct energy turbulent cascade which is revealed by a power-law momentum distribution. We further study the pre-steady state dynamics, as the cascade’s front propagates to higher momentum states. We observe a spatio-temporal self-similar evolution of the momentum spectrum and compare it to the weak-wave turbulence predictions. Our results also provide a link to recently emerging general classification of out of equilibrium dynamics, to so-called non-thermal fixed points.