# Violation of CHSH inequality and marginal laws in mixed sequential measurements with order effects

## TL;DR

This paper models sequential measurements in Bell-test experiments, showing that order effects can violate marginal laws and increase correlations beyond Tsirelson's bound, offering an explanation for observed violations in experimental data.

## Contribution

It introduces a model of mixed sequential measurements that explains violations of marginal laws and extends the upper bound of quantum correlations beyond Tsirelson's limit.

## Key findings

- Order effects cause violations of marginal laws in sequential measurements.
- Incompatible measurements can raise correlation bounds up to 2√3.
- The model explains observed marginal law violations without implying superluminal communication.

## Abstract

We model a typical Bell-test experimental situation by considering that Alice and Bob perform incompatible measurements in a sequential way, with mixed orders of execution. After emphasizing that order effects will generally produce a violation of the marginal laws, we derive an upper limit for the observed correlations. More precisely, when Alice's and Bob's measurements are compatible, the marginal laws are obeyed and Tsirelson's bound limits the quantum correlations in the Bell-CHSH inequality to $2\sqrt{2}$. On the other hand, when Alice and Bob perform incompatible mixed sequential measurements, the marginal laws are typically violated and the upper limit for the correlations is pushed up to $2\sqrt{3}$. Considering that significant violations of the marginal laws (also called no-signaling conditions) have been observed in the data of numerous Bell-test experiments, the present analysis provides a possible mechanism for their appearance, when the protocols are such that Alice's and Bob's measurements can be assumed to be performed in a mixed sequential way. We however emphasize that this does not imply that a communication with superluminal effective speed would be possible.