Non-Abelian Fusion Rules from an Abelian System

Abstract: We demonstrate the emergence of non-Abelian fusion rules for excitations of a two dimensional lattice model built out of Abelian degrees of freedom. It can be considered as an extension of the usual toric code model on a two dimensional lattice augmented with matter fields. It consists of the usual $\mathbb{C}(\mathbb{Z}_p)$ gauge degrees of freedom living on the links together with matter degrees of freedom living on the vertices. The matter part is described by a $n$ dimensional vector space which we call $H_n$. The $\mathbb{Z}_p$ gauge particles act on the vertex particles and thus $H_n$ can be thought of as a $\mathbb{C}(\mathbb{Z}_p)$ module. An exactly solvable model is built with operators acting in the corresponding Hilbert space. The vertex excitations for this model are studied and shown to obey non-Abelian fusion rules. We will show this for specific values of $n$ and $p$, though we believe this feature holds for all $n>p$. We will see that non-Abelian anyons of the quantum double of $\mathbb{C}(S_3)$ are obtained as part of the vertex excitations of the model with $n=6$ and $p=3$. Ising anyons are obtained in the model with $n=4$ and $p=2$. The $n=3$ and $p=2$ case is also worked out as this is the simplest model exhibiting non-Abelian fusion rules. Another common feature shared by these models is that the ground states have a higher symmetry than $\mathbb{Z}_p$. This makes them possible candidates for realizing quantum computation.