Obstacles to State Preparation and Variational Optimization from Symmetry Protection

TL;DR

This paper demonstrates that symmetry-protected topological order prevents low-energy state preparation with shallow circuits, establishes a version of the NLTS conjecture, and proposes a non-local QAOA to improve optimization performance.

Contribution

It proves a version of the NLTS conjecture for symmetry-protected topological order and introduces a non-local QAOA that outperforms standard QAOA on certain models.

Findings

Symmetry protection prevents shallow circuit preparation of low-energy states.

Goemans-Williamson algorithm outperforms QAOA for MaxCut instances.

Non-local QAOA significantly outperforms standard QAOA on frustrated Ising models.

Abstract

Local Hamiltonians with topological quantum order exhibit highly entangled ground states that cannot be prepared by shallow quantum circuits. Here, we show that this property may extend to all low-energy states in the presence of an on-site $\mathbb{Z}_2$ symmetry. This proves a version of the No Low-Energy Trivial States (NLTS) conjecture for a family of local Hamiltonians with symmetry protected topological order. A surprising consequence of this result is that the Goemans-Williamson algorithm outperforms the Quantum Approximate Optimization Algorithm (QAOA) for certain instances of MaxCut, at any constant level. We argue that the locality and symmetry of QAOA severely limits its performance. To overcome these limitations, we propose a non-local version of QAOA, and give numerical evidence that it significantly outperforms standard QAOA for frustrated Ising models on random 3-regular graphs.