Title: Enstrophy Cascade in Decaying Two-Dimensional Quantum Turbulence,
Abstract: We report evidence for an enstrophy cascade in large-scale point-vortex
simulations of decaying two-dimensional quantum turbulence. Devising a method
to generate quantum vortex configurations with kinetic energy narrowly
localized near a single length scale, the dynamics are found to be
well-characterised by a superfluid Reynolds number, $\mathrm{Re_s}$, that
depends only on the number of vortices and the initial kinetic energy scale.
Under free evolution the vortices exhibit features of a classical enstrophy
cascade, including a $k^{-3}$ power-law kinetic energy spectrum, and steady
enstrophy flux associated with inertial transport to small scales. Clear
signatures of the cascade emerge for $N\gtrsim 500$ vortices. Simulating up to
very large Reynolds numbers ($N = 32, 768$ vortices), additional features of
the classical theory are observed: the Kraichnan-Batchelor constant is found to
converge to $C' \approx 1.6$, and the width of the $k^{-3}$ range scales as
$\mathrm{Re_s}^{1/2}$. The results support a universal phenomenology
underpinning classical and quantum fluid turbulence.