Optimal free descriptions of many-body theories

TL;DR

This paper introduces a non-perturbative framework to identify the closest free theory to a given interacting many-body system, aiding understanding of complex quantum phases.

Contribution

It develops a general, non-perturbative method to find the optimal free theory approximating strongly correlated systems based on ground state correlations and entanglement spectrum.

Findings

Provides a quantitative measure of the distance between interacting and free theories.

Offers a new approach for effective low-energy descriptions of complex quantum systems.

Facilitates analysis of strongly correlated phases beyond perturbation theory.

Abstract

Interacting bosons or fermions give rise to some of the most fascinating phases of matter, including high-temperature superconductivity, the fractional quantum Hall effect, quantum spin liquids and Mott insulators. While these systems are promising for technological applications, they also present conceptual challenges as they require approaches beyond mean-field and perturbation theory. Here we develop a general framework for identifying the free theory that is closest to a given interacting model in terms of their ground state correlations. Moreover, we quantify the distance between them using the entanglement spectrum. When this interaction distance is small, the optimal free theory provides an effective description of the low energy physics of the interacting model. Our construction of the optimal free model is non-perturbative in nature, thus it offers a new theoretical framework for investigating strongly correlated systems.