Loops in 4+1d Topological Phases

TL;DR

This paper investigates the properties of loop excitations in 4+1d topological phases, revealing trivial self-statistics, nontrivial braiding between different loops, and the role of domain walls and symmetries, with implications for boundary conditions and generalizations.

Contribution

It characterizes loop statistics in 4+1d topological phases, introduces explicit unitary mappings between loop types, and discusses symmetry obstructions and boundary theories.

Findings

Loop self-statistics are trivial in 4+1d toric code.

Braiding between different loops can be nontrivial.

SL(2,Z_2) symmetry cannot be gauged due to obstructions.

Abstract

2+1d topological phases are well characterized by the fusion rules and braiding/exchange statistics of fractional point excitations. In 4+1d, some topological phases contain only fractional loop excitations. What kind of loop statistics exist? We study the 4+1d gauge theory with 2-form $\mathbb{Z}_2$ gauge field (the loop only toric code) and find that while braiding statistics between two different types of loops can be nontrivial, the self `exchange' statistics are all trivial. In particular, we show that the electric, magnetic, and dyonic loop excitations in the 4+1d toric code are not distinguished by their self-statistics. They tunnel into each other across 3+1d invertible domain walls which in turn give explicit unitary circuits that map the loop excitations into each other. The $SL(2,\mathbb{Z}_2)$ symmetry that permutes the loops, however, cannot be consistently gauged and we discuss the associated obstruction in the process. Moreover, we discuss a gapless boundary condition dubbed the `fractional Maxwell theory' and show how it can be Higgsed into gapped boundary conditions. We also discuss the generalization of these results from the $\mathbb{Z}_2$ gauge group to $\mathbb{Z}_N$.