Nets of Subfactors

TL;DR

This paper applies subfactor theory to quantum field theory nets, analyzing the canonical endomorphism to understand and reconstruct local extensions of observable theories, with several non-trivial examples.

Contribution

It introduces a method to characterize and reconstruct local extensions of quantum field theory nets using subfactor theory and the canonical endomorphism.

Findings

Canonical endomorphism extends to the field net

Method characterizes local extensions from observables

Provides multiple non-trivial examples

Abstract

A subtheory of a quantum field theory specifies von~Neumann subalgebras $\aa(\oo)$ (the `observables' in the space-time region $\oo$) of the von~Neumann algebras $\bb(\oo)$ (the `fields' localized in $\oo$). Every local algebra being a (type $\III_1$) factor, the inclusion $\aa(\oo) \subset \bb(\oo)$ is a subfactor. The assignment of these local subfactors to the space-time regions is called a `net of subfactors'. The theory of subfactors is applied to such nets. In order to characterize the `relative position' of the subtheory, and in particular to control the restriction and induction of superselection sectors, the canonical endomorphism is studied. The crucial observation is this: the canonical endomorphism of a local subfactor extends to an endomorphism of the field net, which in turn restricts to a localized endomorphism of the observable net. The method allows to characterize, and reconstruct, local extensions $\bb$ of a given theory $\aa$ in terms of the observables. Various non-trivial examples are given.