Classical information and collapse in Wigner's friend setups

TL;DR

This paper explores how classical communication between Wigner and his friend influences the Wigner's friend paradox and the violation of local friendliness inequalities, providing a continuum between quantum superpositions and classical collapse.

Contribution

It introduces the role of classical communication channels in Wigner's friend scenarios, enabling control over the emergence of paradoxes and inequality violations.

Findings

Classical communication can regulate the amount of outcome information revealed.

Adjusting communication properties transitions between paradoxical and collapsed descriptions.

The framework allows a smooth interpolation between quantum superpositions and classical outcomes.

Abstract

The famous Wigner's friend experiment considers an observer -- the friend -- and a superobserver -- Wigner -- who treats the friend as a quantum system and her interaction with other quantum systems as unitary dynamics. This is at odds with the friend describing this interaction via collapse dynamics, if she interacts with the quantum system in a way that she would consider a measurement. These different descriptions constitute the Wigner's friend paradox. Extended Wigner's friend experiments combine the original thought experiment with non-locality setups. This allows for deriving local friendliness inequalities, similar to Bell's theorem, which can be violated for certain extended Wigner's friend scenarios. A Wigner's friend paradox and the violation of local friendliness inequalities require that no classical record exists, which reveals the result the friend observed during her measurement. Otherwise Wigner agrees with his friend's description and no local friendliness inequality can be violated. In this article, I introduce classical communication between Wigner and his friend and discuss its effects on the simple as well as extended Wigner's friend experiments. By controlling the properties of a (quasi) classical communication channel between Wigner and the friend one can regulate how much outcome information about the friend's measurement is revealed. This gives a smooth transition between the paradoxical description and the possibility of violating local friendliness inequalities, on the one hand, and the effectively collapsed case, on the other hand.