Non-perturbative dynamics of the operator size distribution in the Sachdev-Ye-Kitaev model

TL;DR

This paper establishes non-perturbative bounds on how the operator size distribution evolves over time in the Sachdev-Ye-Kitaev model, revealing finite Lyapunov exponents and a distribution shape consistent with perturbative predictions.

Contribution

It provides the first non-perturbative bounds on operator size dynamics in the SYK model using probabilistic methods, independent of Feynman diagram resummation.

Findings

Finite many-body Lyapunov exponent at infinite temperature.

Operator size distribution shape matches perturbative predictions in certain limits.

Distribution evolution can be modeled as a quantum walk with bounded rates.

Abstract

We prove non-perturbative bounds on the time evolution of the probability distribution of operator size in the $q$-local Sachdev-Ye-Kitaev model with $N$ fermions, for any even integer $q>2$ and any positive even integer $N>2q$. If the couplings in the Hamiltonian are independent and identically distributed Rademacher random variables, the infinite temperature many-body Lyapunov exponent is almost surely finite as $N\rightarrow\infty$. In the limit $q \rightarrow \infty$, $N\rightarrow \infty$, $q^{6+\delta}/N \rightarrow 0$, the shape of the size distribution of a growing fermion, obtained by leading order perturbation calculations in $1/N$ and $1/q$, is similar to a distribution that locally saturates our constraints. Our proof is not based on Feynman diagram resummation; instead, we note that the operator size distribution obeys a continuous time quantum walk with bounded transition rates, to which we apply concentration bounds from classical probability theory.