Magic and Wormholes in the Sachdev-Ye-Kitaev Model

TL;DR

This paper investigates the statistical properties of expectation values of Majorana fermion strings in the SYK model, revealing Gaussian behavior in chaotic cases and non-Gaussian in integrable cases, with implications for quantum magic and holography.

Contribution

It provides a detailed analysis of operator expectation values in SYK, linking chaos, randomness, and wormhole geometries to quantum magic measures, and offers a dual gravity perspective.

Findings

Expectation values are Gaussian in chaotic SYK and non-Gaussian in integrable SYK.

Variance of operator strings relates to wormhole geometries in dual gravity.

Results connect quantum magic, chaos, and holography in a concrete setting.

Abstract

Any quantum state is fully specified by the expectation values of a complete set of Hermitian operators. For a system of Majorana fermions, such as the Sachdev-Ye-Kitaev (SYK) model, this set of observables can be taken to be all possible strings of Majorana fermion operators. The expectation values of these fermion strings in a thermal state depend erratically on the microscopic couplings that specify the SYK Hamiltonian, and we study their statistical properties directly in the thermodynamic limit using path integral techniques. When the underlying SYK Hamiltonian is chaotic, we find that these expectation values are well-modeled as real Gaussian random variables with zero mean and a variance that we compute. In contrast, for the integrable variant of SYK, we find that the expectation values are actually non-Gaussian. We then use these results to study measures of magic in the SYK thermal state, including the robustness of magic and the stabilizer R\'enyi entropy. We also show that our results can be quantitatively reproduced with a dual gravity calculation in the chaotic case at sufficiently low temperature. In this dual gravity model the variance of a given microscopic operator string is related to a wormhole geometry stabilized by a massive particle which is dual to the operator string. Our results thus provide a concrete and quantitative setting in which to study the relationship between randomness, wormholes, and closed universes as well as a holographic dual of quantum magic.