Decoherence through Ancilla Anyon Reservoirs

We explore the decoherence of the critical (gapless) boundary of a topological order, through interactions with the bulk reservoir of "ancilla anyons." We take the critical boundary of the 2d Toric code as an example. The intrinsic nonlocal nature of the anyons demands the strong and weak symmetry condition for the ordinary decoherence problem be extended to the strong or weak gauge invariance conditions. We demonstrate that in the doubled Hilbert space, the partition function of the boundary is mapped to two layers of the 2d critical Ising model with an interlayer line defect that depends on the species of the anyons causing the decoherence. The line defects associated with the tunneling of bosonic e and m anyons are relevant, and result in long-range correlations for either the e or m anyon, respectively, on the boundary in the doubled Hilbert space. In contrast, the defect of the f anyon is marginal and leads to a line of fixed points with varying effective central charges, and power-law correlations having continuously varying scaling dimensions. We also demonstrate that decoherence-analogs of Majorana zero modes are localized at the spatial interface of the relevant e and m anyon decoherence channels, which leads to a universal logarithmic scaling of the Rényi entropy of the boundary.