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

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 $N\to \infty$. In the limit $q\to \infty$, $N\to \infty$, $q^{6+\delta }/N\to 0$, 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.

arXiv comment23 pages. v2: published version

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• Absence of operator growth for average equal-time observables in charge-conserved sectors of the Sachdev-Ye-Kitaev model
• Locality bounds for quantum dynamics at low energy