Boundaries of the compact free boson through edge modes and boundary SymTFT

Abstract

We study boundary conditions of the compact free boson theory by coupling the bulk scalar to one-dimensional topological edge modes. We show that a single compact edge mode realizes the Dirichlet family, while a two field system realizes the Neumann family; more general compact edge theories can describe superpositions of boundary conditions and reproduce the expected boundary operator spectra. We then study this system in the framework of boundary SymTFT. We show that the edge mode theories of Dirichlet and Neumann boundaries are encoded in the degrees of freedom localized on the corners and the choice of polarization on the topological face of the boundary SymTFT. We further show that the T-duality defect exchanges the data associated with the Dirichlet and Neumann configurations. Finally, we explore extensions involving non-compact edge modes and multiple bulk fields, obtaining continuous boundary spectra in the first case and mixed boundary conditions in the second.