Observers of quantum systems cannot agree to disagree

@article{ContrerasTejada2021ObserversOQ,
  title={Observers of quantum systems cannot agree to disagree},
  author={Patricia Contreras-Tejada and Giannicola Scarpa and Aleksander M. Kubicki and Adam Brandenburger and Pierfrancesco La Mura},
  journal={Nature Communications},
  year={2021},
  volume={12}
}
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. We 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. We… 

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