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    Soft symmetries of topological orders

    Ryohei Kobayashi1 and Maissam Barkeshli2

    Phys. Rev. B 113, 115150 – Published 24 March, 2026

    DOI: https://doi.org/10.1103/cwn4-jl57

    Abstract

    (2+1)-dimensional [(2+1)D] topological orders possess emergent ordinary symmetries given by a group Autbr(C), which consists of the braided tensor autoequivalences of the modular tensor category C that describes the anyons. In this paper we discuss cases where Autbr(C) has elements that neither permute anyons nor are associated to any symmetry fractionalization but still acts faithfully on the theory, which we refer to as soft symmetries. We point out that one can construct topological defects corresponding to such exotic symmetry actions by decorating with a certain class of gauged symmetry protected topological states that cannot be distinguished by their torus partition function. This gives a physical interpretation to work by Davydov on soft braided tensor autoequivalences. This has a number of important implications for the classification of gapped boundaries, noninvertible spontaneous symmetry breaking, and the general classification of symmetry-enriched topological phases of matter. We also demonstrate analogous phenomena in higher dimensions, such as (3+1)D gauge theory with gauge group given by the quaternion group Q8.

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