Page Curve for Local-Operator Entanglement from Free Probability

Abstract

The local-operator entanglement (LOE) measures the classical simulability of a Heisenberg operator and is conjectured to witness many-body chaos in locally interacting systems. Using tools from free probability, we analytically compute its value for Haar random dynamics for all R\'enyi indices. We find that it asymptotically reproduces the Page curve for random states in the case of traceless operators, with exponentially deviating corrections. In contrast to higher-order out-of-time ordered correlators, which depend on operator correlations via free cumulants, the leading-order LOE is independent of the initial operator. Guided by our Haar result, we therefore argue that the long-time value of the LOE entropies in chaotic systems will depend only on autocorrelation functions of the initial operator up to exponentially small corrections, suggesting that the higher-order structure of the full Eigenstate Thermalization Hypothesis is not necessary to describe it.