Local stability of ground states in locally gapped and weakly interacting quantum spin systems

TL;DR

This paper proves that in weakly interacting quantum spin systems with local gaps, ground state changes due to localized perturbations are confined locally, extending the LPPL principle to broader classes of Hamiltonians.

Contribution

It extends the strong LPPL principle to systems with locally gapped ground states and weak interactions, even when spectral gaps are closed by perturbations.

Findings

Ground state changes are localized under perturbations

Extension of LPPL principle to broader Hamiltonian classes

Implications for physical robustness of ground states

Abstract

Based on a result by Yarotsky (J. Stat. Phys. 118, 2005), we prove that localized but otherwise arbitrary perturbations of weakly interacting quantum spin systems with uniformly gapped on-site terms change the ground state of such a system only locally, even if they close the spectral gap. We call this a strong version of the local perturbations perturb locally (LPPL) principle which is known to hold for much more general gapped systems, but only for perturbations that do not close the spectral gap of the Hamiltonian. We also extend this strong LPPL-principle to Hamiltonians that have the appropriate structure of gapped on-site terms and weak interactions only locally in some region of space. While our results are technically corollaries to a theorem of Yarotsky, we expect that the paradigm of systems with a locally gapped ground state that is completely insensitive to the form of the Hamiltonian elsewhere extends to other situations and has important physical consequences.