Noise-tolerant Detection of $Z_N$ Topological Orders in Quantum Many-body States

TL;DR

This paper introduces the ring degeneracy as a noise-robust quantity for detecting $Z_N$ topological orders in quantum many-body states, demonstrating its effectiveness under noise through simulations.

Contribution

It proposes the ring degeneracy as a novel, noise-tolerant indicator for topological order detection in tensor network states, with a simple relation for $Z_N$ orders.

Findings

Ring degeneracy remains stable under pure noise.

The relation $oxed{ ext{D} = (N + 1)/2 + d}$ holds for $Z_N$ orders.

Simulations confirm robustness in various quantum states.

Abstract

Topologically ordered states are fundamentally important in theoretical physics, which are also suggested as promising candidates to build fault-tolerant quantum devices. However, it is still elusive how topological orders can be affected or detected under noises. In this work, we find a quantity, termed as the ring degeneracy $\mathcal{D}$, which is robust under pure noise to detect both trivial and intrinsic topological orders. The ring degeneracy is defined as the degeneracy of the solutions of the self-consistent equations that encode the contraction of the corresponding tensor network(TN). For the $Z_N$ orders, we find that the ring degeneracy satisfies a simple relation $\mathcal{D} = (N + 1)/2 + d$, with $d = 0$ for odd $N$ and $d = 1/2$ for even $N$. Simulations on several non-trivial states (two-dimensional Ising model, $Z_N$ topological states, and resonating valence bond states) show that the ring degeneracy can tolerate noises up to a strength associated to the gap of the TN boundary theory.