Emergence of Classicality in Wigner's Friend Scenarios

Abstract: The Wigner's Friend (WF) thought experiment concerns quantum measurements by a 'superobserver' of an observer measuring a quantum system. Variations on the setup and its extended versions have seen a resurgence in recent years, in light of a series of no-go theorems that reveal new quantum effects and question the existence of absolute events. But most theoretical and experimental studies of WF scenarios have restricted themselves to a 'friend' composed of a single qubit with idealised measurement settings in an idealised lab. In this work, we consider a specific, unitary model of the interaction between the Friend and the system in the presence of a decohering environment. In particular, we study WF scenarios from the perspective of quantum Darwinism (QD). The QD framework is well-suited to studying the questions of observations and agents in quantum theory that WF scenarios raise, as it is concerned with how observers record objective information about a system with access only to its surroundings. Here we describe how to add environments to simple and extended WF scenarios in the QD framework, and present numerical results that study the emergence of classicality, in the form of the Friend's measurement result becoming more objective. In both the simple and extended cases, we also find that the model and the environment obfuscate genuine WF effects and introduce strong restrictions on them. However, we also find a novel form of WF effect that exploits coherence between the Friend and the environment.