Observers of quantum systems cannot agree to disagree

Authors: Patricia Contreras-Tejada (ICMAT-CSIC), Giannicola Scarpa, Aleksander M. Kubicki, Adam Brandenburger & Pierfrancesco La Mura

Source: Nature Communications vol. 12

Date of publication: 2 December 2021



Is the world quantum? An active research line in quantum foundations is devoted to exploring what constraints can rule out the postquantum theories that are consistent with experimentally observed results. In this article, the authors explore this question in the context of epistemics, and ask whether agreement between observers can serve as a physical principle that must hold for any theory of the world. Aumann’s seminal Agreement Theorem states that two observers (of classical systems) cannot agree to disagree.

Now, the researchers propose an extension of this theorem to no-signaling settings. In particular, they establish an Agreement Theorem for observers of quantum systems, while they construct examples of (postquantum) no-signaling boxes where observers can agree to disagree. The PR box is an extremal instance of this phenomenon. These results make it plausible that agreement between observers might be a physical principle, while they also establish links between the fields of epistemics and quantum information that seem worthy of further exploration.