Relative entropy close to the edge

Abstract

We show that the relative entropy between the reduced density matrix of the vacuum state in some region $A$ and that of an excited state created by a unitary operator localized at a small distance $\ell$ of a boundary point $p$ is insensitive to the global shape of $A$, up to a small correction. This correction tends to zero as $\ell/R$ tends to zero, where $R$ is a measure of the curvature of $\partial A$ at $p$, but at a rate necessarily slower than $\sim \sqrt{\ell/R}$ (in any dimension). Our arguments are mathematically rigorous and only use model-independent, basic assumptions about quantum field theory such as locality and Poincare invariance.