Work

Aumann's theorem beyond ontology: quantum, postquantum, and indefinite causal order

Year: 2026

Type: preprint

Abstract: Agreement theorems are no-go results about rational disagreement: if two agents start from a common prior and their posterior beliefs are common knowledge, they cannot assign different probabilities to the same event. Standard treatments of the result have the agents reason about an underlying state of the world, which has lead some to ask whether the result can extend to quantum or postquantum phenomena, where such a description may no longer be appropriate. We derive an operational version of Aumann's agreement theorem without assuming an objective state of the world and instead focusing only on what is observed. This allows us to establish the theorem's validity in quantum theory and even in situations with indefinite causal order or involving hypothetical postquantum phenomena. We comment on seemingly contradictory results in the literature and point to the one place where the theorem might fail: Wigner's friend-type situations.

Source: arXiv (Cornell University)

DOI: 10.48550/arxiv.2603.23595

Authors: Cepollaro, Carlo, Di Biagio, Andrea

Institutions: Austrian Academy of Sciences, Vienna Center for Quantum Science and Technology

Topic: Quantum Mechanics and Applications

Subfield: Atomic and Molecular Physics, and Optics

Field: Physics and Astronomy

Domain: Physical Sciences

SDG: Peace, justice, and strong institutions

Open Access status: green

Funders: John Templeton Foundation, European Commission, Austrian Science Fund, QuantERA

Awards: G4157035715, G5302217128, G575169067, QuantERA II ERA-NET Cofund in Quantum Technologies, Verification of quantum technologies, applications & systems, Spin structures in pure and rare earth doped thin ferrites

OpenAlex ID: W7140680840

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