Robustness and classical proxy of entanglement in variants of quantum walk

TL;DR

This paper investigates the robustness of entanglement in various quantum walk models under randomness and introduces a classical measure called overlap as a proxy for entanglement, with implications for experiments.

Contribution

It demonstrates the robustness of entanglement in quantum walks against randomness and proposes a classical overlap measure as an effective proxy for entanglement.

Findings

Entanglement remains robust under time- and space-dependent randomness.

The classical overlap correlates with entanglement entropy in most cases.

Limitations of the overlap as a proxy are identified in high population imbalance scenarios.

Abstract

Quantum walk (QW) utilizes its internal quantum states to decide the displacement, thereby introducing single-particle entanglement between the internal and positional degrees of freedom. By simulating three variants of QW with the conventional, symmetric, and split-step translation operators with or without classical randomness in the coin operator, we show the entanglement is robust against both time- and spatially- dependent randomness, which can cause localization transitions of QW. We propose a classical quantity call overlap, which literally measures the overlap between the probability distributions of the internal states, as a proxy of entanglement. The overlap is associated with the off-diagonal terms of the reduced density matrix in the internal space, which then reflects its purity. Therefore, the overlap captures the inverse behavior of the entanglement entropy in most cases. We test the limitation of the classical proxy by constructing a special case with high population imbalance between the internal states to blind the overlap. Possible implications and experimental measurements are also discussed.