Rank-2 Toric Code in Two Dimensions

TL;DR

This paper introduces an exactly solvable two-dimensional spin model derived from a Higgsed rank-2 U(1) lattice gauge theory, revealing novel excitations, ground state degeneracy, and unique braiding statistics.

Contribution

It presents a new rank-2 toric code model with exact solvability, novel excitations, and distinctive braiding properties, expanding the understanding of topological quantum codes.

Findings

Ground state degeneracy depends on lattice size and prime p.

Identifies two types of dipole excitations with unique mobility.

Monopole-monopole braiding phase is non-trivial and related to Aharonov-Bohm effect.

Abstract

We study a two-dimensional spin model obtained by "Higgsing" the rank-2 U(1) lattice gauge theory (LGT) with scalar or vector charges on the L_x * L_y square lattice under the periodic boundary condition (PBC). There are p degrees of freedom per orbital and three orbitals per unit cell in the spin model. The resulting spin model is a stabilizer code consisting of three mutually commuting projectors that are, in turn, obtained by Higgsing the mutually commuting Gauss's law operators and the magnetic field operators in the underlying LGT. The spin model thus obtained is exactly solvable, with the ground state degeneracy (GSD) D given by log_p D=2+(1+delta_{L_x mod p,0})(1+delta_{L_y mod p,0}) when p is a prime number. Two types of dipole excitations, pristine and emergent, are identified. Both the monopoles and the dipoles are free to move, with restrictions on monopoles to hop only by p lattice spacing along with certain directions. The monopole-monopole braiding phase depends on the separation of the x or y coordinates of the initial monopole positions, making it distinct from the ordinary anyon braiding statistics. The monopole-dipole braiding obeys the usual anyonic statistics. Despite the oddity, the monopole-monopole braiding phase can be understood as the Aharonov-Bohm phase of some emergent vector potentials.