Events in quantum mechanics are maximally non-absolute

TL;DR

This paper explores the non-absoluteness of events in quantum mechanics, introducing measures to quantify it and demonstrating that quantum correlations can be maximally non-absolute using Bell inequalities.

Contribution

It introduces two novel measures of non-absoluteness of events and shows their maximal quantum violation in Wigner's friend scenarios.

Findings

Quantum correlations can be maximally non-absolute.

New measures quantify non-absoluteness of events.

Bell inequalities constrain non-absoluteness in quantum scenarios.

Abstract

The notorious quantum measurement problem brings out the difficulty to reconcile two quantum postulates: the unitary evolution of closed quantum systems and the wave-function collapse after a measurement. This problematics is particularly highlighted in the Wigner's friend thought experiment, where the mismatch between unitary evolution and measurement collapse leads to conflicting quantum descriptions for different observers. A recent no-go theorem has established that the (quantum) statistics arising from an extended Wigner's friend scenario is incompatible when one try to hold together three innocuous assumptions, namely no-superdeterminism, parameter independence and absoluteness of observed events. Building on this extended scenario, we introduce two novel measures of non-absoluteness of events. The first is based on the EPR2 decomposition, and the second involves the relaxation of the absoluteness hypothesis assumed in the aforementioned no-go theorem. To prove that quantum correlations can be maximally non-absolute according to both quantifiers, we show that chained Bell inequalities (and relaxations thereof) are also valid constraints for Wigner's experiment.