Published:

Interacting bosons or fermions give rise to some of the most fascinating phases of matter, including high-$T_\text{c}$ 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 offers 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 provides a new theoretical framework for investigating strongly correlated systems.