Orbifold completion of defect bicategories

Orbifolds of two-dimensional quantum field theories have a natural formulation in terms of defects or domain walls. This perspective allows for a rich generalisation of the orbifolding procedure, which we study in detail for the case of topological field theories. Namely, a TFT with defects gives rise to a pivotal bicategory of "world sheet phases" and defects between them. We develop a general framework which takes such a bicategory \mathcal B as input and returns its "orbifold completion" \mathcal B_{\mathrm {orb}} . The completion satisfies the natural properties \mathcal B \subset \mathcal B_{\mathrm {orb}} and (\mathcal B_{\mathrm {orb}})_{\mathrm{orb}} \cong \mathcal B_{\mathrm {orb}} , and it gives rise to various new equivalences and nondegeneracy results. When applied to TFTs, the objects in \mathcal B_{\mathrm {orb}} correspond to generalised orbifolds of the theories in \mathcal B . In the example of Landau–Ginzburg models we recover and unify conventional equivariant matrix factorisations, prove when and how (generalised) orbifolds again produce open/closed TFTs, and give nontrivial examples of new orbifold equivalences.