Holography for bulk-boundary local topological order

TL;DR

This paper extends the framework of local topological order to systems with topological boundaries, establishing a holographic correspondence between bulk topological order and boundary algebraic structures, with explicit models and new algebraic insights.

Contribution

It generalizes local topological order axioms to include topological boundaries, linking bulk and boundary topological orders via boundary algebras and categorical nets, with detailed analysis of Levin-Wen and Walker-Wang models.

Findings

Boundary algebras recover topological boundary order.

Type I von Neumann algebras in Walker-Wang models.

Superselection sectors reflect the underlying unitary braided fusion category.

Abstract

In our previous article [arXiv:2307.12552], we introduced local topological order (LTO) axioms for quantum spin systems which allowed us to define a physical boundary manifested by a net of boundary algebras in one dimension lower. This gives a formal setting for topological holography, where the braided tensor category of DHR bimodules of the physical boundary algebra captures the bulk topological order. In this article, we extend the LTO axioms to quantum spin systems equipped with a topological boundary, again producing a physical boundary algebra for the bulk-boundary system, whose category of (topological) boundary DHR bimodules recovers the topological boundary order. We perform this analysis in explicit detail for Levin-Wen and Walker-Wang bulk-boundary systems. Along the way, we introduce a 2D braided categorical net of algebras built from a unitary braided fusion category (UBFC). Such nets arise as boundary algebras of Walker-Wang models. We consider the canonical state on this braided categorical net corresponding to the standard topological boundary for the Walker-Wang model. Interestingly, in this state, the cone von Neumann algebras are type I with finite dimensional centers, in contrast with the type II and III cone von Neumann algebras from the Levin-Wen models studied in [arXiv:2307.12552]. Their superselection sectors recover the underlying unitary category of our UBFC, and we conjecture the superselection category also captures the fusion and braiding.