Full characterization of a spin liquid phase: from topological entropy to robustness and braid statistics

TL;DR

This paper fully characterizes the topological order of a $ ext{Z}_2 imes ext{Z}_2$ spin liquid in the topological color code using topological entanglement entropy, fusion rules, and modular matrices, and studies its robustness under perturbations.

Contribution

It introduces a comprehensive method to analyze the topological phase of the TCC using TEE, fusion rules, and modular matrices, including ground state dependence and robustness analysis.

Findings

TEE accurately characterizes the topological phase.

Multiple minimum entropy states are identified.

Topological order persists until a critical magnetic field.

Abstract

We use the topological entanglement entropy (TEE) as an efficient tool to fully characterize the Abelian phase of a $\mathbb{Z}_2 \times \mathbb{Z}_2$ spin liquid emerging as the ground state of topological color code (TCC), which is a class of stabilizer states on the honeycomb lattice. We provide the fusion rules of the quasiparticle (QP) excitations of the model by introducing single- or two-body operators on physical spins for each fusion process which justify the corresponding fusion outcome. Beside, we extract the TEE from Renyi entanglement entropy (EE) of the TCC, analytically and numerically by finite size exact diagonalization on the disk shape regions with contractible boundaries. We obtain that the EE has a local contribution, which scales linearly with the boundary length in addition to a topological term, i.e. the TEE, arising from the condensation of closed strings in the ground state. We further investigate the ground state dependence of the TEE on regions with non-contractible boundaries, i.e. by cutting the torus to half cylinders, from which we further identify multiple independent minimum entropy states (MES) of the TCC and then extract the U and S modular matrices of the system, which contain the self and mutual statistics of the anyonic QPs and fully characterize the topological phase of the TCC. Eventually, we show that, in spite of the lack of a local order parameter, TEE and other physical quantities obtained from ground state wave function such as entanglement spectrum (ES) and ground state fidelity are sensitive probes to study the robustness of a topological phase. We find that the topological order in the presence of a magnetic field persists until the vicinity of the transition point, where the TEE and fidelity drops to zero and the ES splits severely, signaling breakdown of the topological phase of the TCC.