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A degeneracy bound for homogeneous topological order
by Jeongwan Haah
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Submission summary
Authors (as registered SciPost users):  Jeongwan Haah 
Submission information  

Preprint Link:  https://arxiv.org/abs/2009.13551v3 (pdf) 
Date accepted:  20210113 
Date submitted:  20210109 00:28 
Submitted by:  Haah, Jeongwan 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We introduce a notion of homogeneous topological order, which is obeyed by most, if not all, known examples of topological order including fracton phases on quantum spins (qudits). The notion is a condition on the ground state subspace, rather than on the Hamiltonian, and demands that given a collection of balllike regions, any linear transformation on the ground space be realized by an operator that avoids the balllike regions. We derive a bound on the ground state degeneracy $\mathcal D$ for systems with homogeneous topological order on an arbitrary closed Riemannian manifold of dimension $d$, which reads \[ \log \mathcal D \le c \mu (L/a)^{d2}.\] Here, $L$ is the diameter of the system, $a$ is the lattice spacing, and $c$ is a constant that only depends on the isometry class of the manifold, and $\mu$ is a constant that only depends on the density of degrees of freedom. If $d=2$, the constant $c$ is the (demi)genus of the space manifold. This bound is saturated up to constants by known examples.
Author comments upon resubmission
I would like to thank the referees for their generous comments. One of the referees had specific questions, which I answer as follows.
1) The referee asked to include discussions for models with local order parameters. I restructured the section on nonexamples to discuss such models.
2) The referee asked to include proofs for simple specific fracton models that they have homogeneous topological order. I wrote a new subsection to give a full discussion for topologically ordered (in a conventional sense) translation invariant Pauli stabilizer codes.
3) The referee wondered to what extent the technique in the manuscript can constrain shape of logical operators. This is an important question and deserves deeper look.
3) A suggestion either for an additional comment in the paper (if the answer is straightforward) or followup work (if not): if a homogeneously topologically ordered subspace is used as a topological errorcorrecting code, to what extent can the technique used to prove the main theorem be used to restrict the form of logical operators?
 This deserves deeper investigation.
List of changes
 Strengthened the statement of the theorem for d=2.
 Mentioned hyperbolic surface of constant curvature.
 Gave one more reference for the asymptotic volume formula of $a$neighborhood of $(d2)$skeleton.
 Restructured the section on nonexamples, which now includes $\Pi$ with local observables.
 Wrote a new subsection 3.1 to show that translation invariant exact code Hamiltonians (a certain class of Pauli stabilizer code) have our homogeneous topological order.
Published as SciPost Phys. 10, 011 (2021)