Comment on Chiribella et al., Phys. Rev. Lett. 132 (2024) 190201, arXiv:2301.10885

TL;DR

This paper critiques Chiribella et al.'s definition of classical in quantum systems, highlighting its inconsistency with established Hilbert-space quantum theory due to ignoring noncommuting projectors.

Contribution

It provides a critical analysis of a recent quantum-classical boundary definition, emphasizing the importance of noncommuting projectors in quantum theory.

Findings

Chiribella et al.'s definition is inconsistent with quantum theory.

Ignoring noncommuting projectors leads to an incomplete classical-quantum distinction.

The critique clarifies the proper quantum-classical boundary.

Abstract

The article `Bell Nonlocality in Classical Systems Coexisting with Other System Types' by Chiribella et al. defines `classical' in a quantum context in a way that ignores noncommuting quantum projectors, and is hence inconsistent with Hilbert-space quantum theory.