The moments of the spectral form factor in SYK

TL;DR

This paper analyzes the spectral form factor in the SYK model, revealing how it mimics random matrix behavior at low orders and deviates at high orders, with implications for quantum chaos and gravity.

Contribution

It identifies saddle points in SYK that describe moments during the ramp and analyzes deviations from RMT, including effects of sparsification and the q=2 case.

Findings

SYK mimics RMT for low order moments

Deviations at high order moments due to spectral edge fluctuations

Sparsification amplifies corrections, studied numerically

Abstract

In chaotic quantum systems the spectral form factor exhibits a universal linear ramp and plateau structure with superimposed erratic oscillations. The mean signal and the statistics of the noise can be probed by the moments of the spectral form factor, also known as higher-point spectral form factors. We identify saddle points in the SYK model that describe the moments during the ramp region. Perturbative corrections around the saddle point indicate that SYK mimics random matrix statistics for the low order moments, while large deviations for the high order moments arise from fluctuations near the edge of the spectrum. The leading correction scales inversely with the number of random parameters in the SYK Hamiltonian and is amplified in a sparsified version of the SYK model, which we study numerically, even in regimes where a linear ramp persists. Finally, we study the $q=2$ SYK model, whose spectral form factor exhibits an exponential ramp with increased noise. These findings reveal how deviations from random matrix universality arise in disordered systems and motivate their interpretation from a bulk gravitational perspective.