L$\ddot{u}$der rule, von Neumann rule and Cirelson's bound of Bell CHSH inequality

TL;DR

This paper demonstrates that using the von Neumann measurement rule instead of Lüders rule can lead to violations of Cirelson's bound in the CHSH inequality, challenging its validity and highlighting the role of measurement-induced non-locality.

Contribution

The study shows that von Neumann measurement rule can produce quantum bounds exceeding Cirelson's bound in CHSH inequality, questioning its physical validity.

Findings

Von Neumann rule leads to CHSH violations beyond Cirelson's bound.

Violations are due to added quantum non-locality from von Neumann measurements.

Results challenge the acceptance of von Neumann rule as a valid state reduction.

Abstract

In [PRL, 113, 050401 (2014)] the authors have shown that instead of L$\ddot{u}$der rule, if degeneracy breaking von Neumann projection rule is adopted for state reduction, the quantum value of three-time Leggett-Garg inequality can exceed it's L$\ddot{u}$ders bound. Such violation of L$\ddot{u}$ders bound may even approach algebraic maximum of the inequality in the asymptotic limit of system size. They also claim that for Clauser-Horne-Shimony-Holt (CHSH) inequality such violation of L$\ddot{u}$ders bound (known as Cirelson's bound) cannot be obtained even when the measurement is performed sequentially first by Alice followed by Bob. In this paper, we have shown that if von Neumann projection rule is used, quantum bound of CHSH inequality exceeds it's Cirelson's bound and may also reach its algebraic maximum four. This thus provide a strong objection regarding the viability of von Neumann rule as a valid state reduction rule. Further, we pointed out that the violation of Cirelson's bound occurs due to the injection of additional quantum non-locality by the act of implementing von Neumann measurement rule.