Observation of an Inverse Turbulent-Wave Cascade in a Driven Quantum Gas
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.
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- See Supplemental Material for details of the DMD driving and an additional panel for Fig. 5.
- For our strong drive, we observe efficient excitation over about ±120 Hzplus-or-minustimes120hertz\pm$120\text{\,}\mathrm{Hz}$± start_ARG 120 end_ARG start_ARG times end_ARG start_ARG roman_Hz end_ARG from our drive frequency.
- The necessity of the reflected flux is due to the fact that the inverse cascade has a finite particle capacity; in contrast, the direct cascade has an infinite energy capacity [37].
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