We investigate anomalously slow coarsening in a dilute two-dimensional superfluid closed with respect to particle and energy exchange with the environment. The dynamics is demonstrated to be closely connected both to a nonthermal fixed point (NTFP) in a far-from-equilibrium quantum system and to Kraichnan-Kolmogorov turbulence. During a universal dynamical regime associated with an anomalous NTFP, vortex dynamics are understood to be governed by three-vortex collisions that trigger vortex-antivortex annihilation events, leading to a subdiffusive decay of the vortex density and thus growth of the characteristic interdefect length scale, ${\ell }_{\text{v}}\sim t^{\beta }$ with $\beta \approx 1/5$. It is found that, during the same time when this power law in time is seen, the moments of the superfluid velocity circulation $\mathrm{\Gamma }$ around an area of spatial extent $r$ exhibit power-law scaling ${\mathrm{\Gamma }}^2\left(r\right)\sim r^{8/3}$, in agreement with Kraichnan-Kolmogorov predictions for an inverse energy cascade in the inertial range, in a driven-open setting. Moreover, in high-order moments, intermittent deviations from linear scaling ${\mathrm{\Gamma }}^{2p}\left(r\right)\sim {\left[{\mathrm{\Gamma }}^2\left(r\right)\right]}^p$ are observed that are consistent with bifractal intermittency corrections previously measured in fully developed classical turbulence. These results establish a quantitative link between decaying quantum turbulence in a closed superfluid and universal dynamics near a nonthermal fixed point. Notably, the subdiffusive decay exponent $\beta \approx 1/5$ deviates significantly from values reported for classical systems.

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• Bose-Einstein condensates
• Compressible turbulence
• Dynamic critical phenomena
• Energy cascade
• Intermittency
• Two-dimensional turbulence
• Vortices in superfluids