Topological Entanglement Entropy of Fracton Stabilizer Codes. (arXiv:1710.01744v3 [cond-mat.str-el] UPDATED)
Entanglement entropy provides a powerful characterization of two-dimensional
gapped topological phases of quantum matter, intimately tied to their
description by topological quantum field theories (TQFTs). Fracton topological
orders are three-dimensional gapped topologically ordered states of matter, but
the existence of a TQFT description for these phases remains an open question.
We show that three-dimensional fracton phases are nevertheless characterized,
at least partially, by universal structure in the entanglement entropy of their
ground state wave functions. We explicitly compute the entanglement entropy for
two archetypal fracton models --- the `X-cube model' and `Haah's code' --- and
demonstrate the existence of a topological contribution that scales linearly in
subsystem size. We show via Schrieffer-Wolff transformations that the
topological entanglement of fracton models is robust against arbitrary local
perturbations of the Hamiltonian. Finally, we argue that these results may be
extended to characterize localization-protected fracton topological order in
excited states of disordered fracton models.