Entanglement and symmetry resolution in two dimensional free quantum field theories

TL;DR

This paper analytically computes symmetry-resolved entanglement entropies in two-dimensional free quantum field theories with U(1) symmetry, revealing entanglement equipartition and its subleading symmetry-breaking effects.

Contribution

It introduces two analytic methods for calculating charged moments in 2D free QFTs, including explicit formulas and asymptotic expansions, advancing understanding of symmetry resolution in entanglement.

Findings

Charged moments computed explicitly for Dirac and scalar fields.

Symmetry-resolved entropies exhibit entanglement equipartition at leading order.

Subleading terms break the equipartition, as shown by analytical and numerical results.

Abstract

We present a thorough analysis of the entanglement entropies related to different symmetry sectors of free quantum field theories (QFT) with an internal U(1) symmetry. We provide explicit analytic computations for the charged moments of Dirac and complex scalar fields in two spacetime dimensions, both in the massive and massless cases, using two different approaches. The first one is based on the replica trick, the computation of the partition function on Riemann surfaces with the insertion of a flux $\alpha$, and the introduction of properly modified twist fields, whose two-point function directly gives the scaling limit of the charged moments. With the second method, the diagonalisation in replica space maps the problem to the computation of a partition function on a cut plane, that can be written exactly in terms of the solutions of non-linear differential equations of the Painlev\'e V type. Within this approach, we also derive an asymptotic expansion for the short and long distance behaviour of the charged moments. Finally, the Fourier transform provides the desired symmetry resolved entropies: at the leading order, they satisfy entanglement equipartition and we identify the subleading terms that break it. Our analytical findings are tested against exact numerical calculations in lattice models.