Order from chaos: Observation of large-scale flow from turbulence in a two-dimensional superfluid

TL;DR

This study experimentally confirms Onsager's model of turbulence in a two-dimensional superfluid, demonstrating the formation of large-scale vortices through vortex evaporation and inverse energy cascade.

Contribution

First experimental validation of Onsager's statistical mechanics model of turbulence in a quantum superfluid system.

Findings

Observation of inverse energy cascade in a superfluid

Formation of large-scale vortices with negative temperature states

Vortex evaporation drives steady-state large-scale structures

Abstract

Interacting systems driven far from equilibrium tend to evolve to steady states exhibiting large-scale structure and order. In two-dimensional turbulent flow the seemingly random swirling motion of a fluid can evolve towards persistent large-scale vortices. Lars Onsager proposed a model based on statistical mechanics of quantized vortices to explain such behavior. Here we report the first experimental confirmation of Onsager's model of turbulence. We drag a grid barrier through an oblate superfluid Bose--Einstein condensate to generate non-equilibrium distributions of vortices. We observe an inverse energy cascade driven by the evaporative heating of vortices, leading to steady-state configurations characterized by negative temperatures. Our results open a pathway for quantitative studies of emergent structures in interacting quantum systems driven far from equilibrium.