Boundary-bulk relation for topological orders as the functor mapping higher categories to their centers

TL;DR

This paper establishes a mathematical framework linking topological orders and their gapped boundaries, showing that the bulk corresponds to the mathematical notion of a center, and introduces a functorial classification of local topological orders.

Contribution

It introduces a functorial classification of local topological orders using higher categories and proves the bulk-boundary relation as a universal property akin to the mathematical center.

Findings

Bulk is unique for a given boundary theory.

Bulk corresponds to the mathematical center in higher categories.

The boundary-bulk relation is functorial and hierarchical.

Abstract

In this paper, we study the relation between topological orders and their gapped boundaries. We propose that the bulk for a given gapped boundary theory is unique. It is actually a consequence of a microscopic definition of a local topological order, which is a (potentially anomalous) topological order defined on an open disk. Using this uniqueness, we show that the notion of "bulk" is equivalent to the notion of center in mathematics. We achieve this by first introducing the notion of a morphism between two local topological orders of the same dimension, then proving that the bulk satisfying the same universal property as that of the center in mathematics. We propose a classification (formulated as a macroscopic definition) of $n+$1D local topological orders by unitary multi-fusion $n$-categories, and explain that the notion of a morphism between two local topological orders is compatible with that of a unitary monoidal $n$-functor in a few low dimensional cases. We also explain in some low dimensional cases that this classification is compatible with the result of "bulk = center". In the end, we explain that above boundary-bulk relation is only the first layer of a hierarchical structure which can be summarized by the functoriality of the bulk (or center). This functoriality also provides the physical meanings of some well-known mathematical results on fusion 1-categories. This work can also be viewed as the first step towards a systematic study of the category of local topological orders, and the boundary-bulk relation actually provides a useful tool for this study.