Observation of an Inverse Turbulent-Wave Cascade in a Driven Quantum Gas

TL;DR

This study demonstrates an inverse turbulent-wave cascade in a driven 2D Bose gas, revealing nonthermal spectra and universal coarsening phenomena, advancing understanding of turbulence in quantum many-body systems.

Contribution

It provides the first experimental observation of an inverse turbulent-wave cascade in a driven quantum gas, linking turbulence, nonthermal fixed points, and universal coarsening.

Findings

Observation of a power-law spectrum consistent with weak-wave turbulence

Identification of a nonthermal fixed point in the momentum distribution

Qualitative understanding of cascade formation through anisotropic driving

Abstract

We observe an inverse turbulent-wave cascade, from small to large lengthscales, in a driven homogeneous 2D Bose gas. Starting with an equilibrium condensate, we drive the gas isotropically on a lengthscale much smaller than its size, and observe a nonthermal population of modes with wavelengths larger than the drive one. At long drive times, the gas exhibits a steady nonthermal momentum distribution. At lengthscales increasing from the drive one to the system size, this 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 reminiscent of a nonthermal fixed point associated with universal coarsening in an isolated 2D gas. In further experiments, based on anisotropic driving, we reveal the complete qualitative picture of how the steady-state cascade forms.