A general quantum circuit framework for Extended Wigner's Friend Scenarios: logically and causally consistent reasoning without absolute measurement events
Résumé
Extended Wigner's Friend Scenarios (EWFSs) go beyond the standard usage of quantum theory where agents are treated classically, and model agents as unitary evolving quantum systems. This has been the subject of several no-go results: Frauchiger and Renner (FR) suggested that quantum agents reasoning using quantum theory will arrive at logical paradoxes, while other works, e.g. the Local-Friendliness theorem, highlight challenges for having an objective notion of measurement events and for causal reasoning in EWFSs. This raises the question: Is it possible to reliably make and test scientific predictions, and consistently reason about the world when applying quantum theory universally, and without assuming that observed measurement outcomes are absolute? We give a positive answer by developing a general quantum circuit framework for EWFSs. We formalise the concept of Heisenberg cuts by mapping them to distinct channels in a quantum circuit, and prove that FR-type paradoxes can be fully resolved by making explicit the conditioning on the quantum channels that are used in the reasoning process. We also provide concrete rules by which quantum agents can reason and make predictions in a logically and causally consistent manner. Our framework describes all perspectives and predictions of an EWFS within a single, well-defined causal structure, although it allows events to be fundamentally subjective. Moreover, we show that an objective notion of measurement events nevertheless emerges in real-world experiments. Our work demonstrates the possibility of a relational yet operational framework overcoming challenges to scientific reasoning in EWFSs, without modifying the Born rule, quantum unitarity or the axioms of classical logic and probability theory applied to measurement outcomes. This enables analysis and comparison of different EWFS arguments and yields a formal platform to extend existing quantum information methods and studies consistently to the domain of Wigner's Friend Scenarios.
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