Large-c BCFT Entanglement Entropy with Deformed Boundaries from Emergent JT Gravity

TL;DR

This paper investigates how boundary deformations in 2D BCFTs influence entanglement entropy, linking it to island entropy in a coupled JT gravity setup, and demonstrates this correspondence at large central charge.

Contribution

It establishes a connection between boundary deformations in BCFTs and island entropy in JT gravity, expanding understanding beyond holographic BCFT assumptions.

Findings

Boundary deformations affect von Neumann entropy in BCFTs.

The entropy with boundary deformations matches island entropy in JT gravity.

Analysis applies under mild assumptions, including many light operators.

Abstract

We study the effect of boundary deformations on the von Neumann entropy of subregions in two-dimensional boundary conformal field theories (BCFTs) at zero and finite temperature. The deformations considered are infinitesimal global conformal transformations that move the boundary and can equivalently be viewed as the leading-order effect of certain BCFT perturbations with the boundary displacement operator. We demonstrate that at large central charge the von Neumann entropy in the presence of a deformed boundary is reproduced by the island entropy of the same interval in an undeformed BCFT acting as a bath and coupled to a gravitating spacetime. Here, the BCFT is joined to an AdS$_2$ region governed by Jackiw-Teitelboim (JT) gravity via transparent boundary conditions. The boundary condition for the dilaton field is set by the boundary deformation in the BCFT computation. Our analysis relies on mild assumptions about the spectrum and OPE coefficients of the BCFT. Notably, these conditions are consistent with an exponentially large number of light operators and are therefore weaker than those required for holographic BCFTs. This is possible since the BCFT result is not reproduced by a gravitational computation in three dimensions with an O(1) number of matter fields, but instead by a computation in two dimensions and with an O(c) number of light fields.