Edge-entanglement spectrum correspondence in a nonchiral topological phase and Kramers-Wannier duality

A two-dimensional quantum system with anyonic excitations can be considered as a quantum computer. Unitary transformations can be performed by moving the excitations around each other. Measurements can be performed by joining excitations in pairs and observing the result of fusion. Such computation is fault-tolerant by its physical nature.

We formulate a universal characterization of the many-particle quantum entanglement in the ground state of a topologically ordered two-dimensional medium with a mass gap. We consider a disk in the plane, with a smooth boundary of length L, large compared to the correlation length. In the ground state, by tracing out all degrees of freedom in the exterior of the disk, we obtain a marginal density operator \rho for the degrees of freedom in the interior. The von Neumann entropy S(\rho) of this density operator, a measure of the entanglement of the interior and exterior variables, has the form S(\rho)= \alpha L -\gamma + ..., where the ellipsis represents terms that vanish in the limit L\to\infty. The coefficient \alpha, arising from short wavelength modes localized near the boundary, is nonuniversal and ultraviolet divergent, but -\gamma is a universal additive constant characterizing a global feature of the entanglement in the ground state. Using topological quantum field theory methods, we derive a formula for \gamma in terms of properties of the superselection sectors of the medium.