Non-local emergent hydrodynamics in a long-range quantum spin system

Generic short-range interacting quantum systems with a conserved quantity exhibit universal diffusive transport at late times. We employ non-equilibrium quantum field theory and semi-classical phase-space simulations to show how this universality is replaced by a more general transport process in a long-range XY spin chain at infinite temperature with couplings decaying algebraically with distance as $r^{-\alpha}$. While diffusion is recovered for $\alpha>1.5$, longer-ranged couplings with $0.5<\alpha\leq 1.5$ give rise to effective classical L\'evy flights; a random walk with step sizes drawn from a distribution with algebraic tails. We find that the space-time dependent spin density profiles are self-similar, with scaling functions given by the stable symmetric distributions. As a consequence, for $0.5<\alpha\leq1.5$ autocorrelations show hydrodynamic tails decaying in time as $t^{-1/(2\alpha-1)}$ and linear-response theory breaks down. Our findings can be readily verified with current trapped ion experiments.