Wigner's Friend paradoxes: consistency with weak-contextual and weak-macroscopic realism models

• URL: http://arxiv.org/abs/2211.02877v1
• Date: Sat, 5 Nov 2022 11:13:12 GMT
• Title: Wigner's Friend paradoxes: consistency with weak-contextual and weak-macroscopic realism models
• Authors: Ria Joseph, Manushan Thenabadu, Channa Hatharasinghe, Jesse Fulton, Run-Yan Teh, P. D. Drummond and M. D. Reid
• Abstract summary: Wigner's friend paradoxes highlight contradictions between measurements made by Friends inside a laboratory and superobservers outside a laboratory. The contradictions lead to no-go theorems for observer-independent facts, thus challenging concepts of objectivity. We demonstrate consistency with a contextual weak form of macroscopic realism.
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• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Wigner's friend paradoxes highlight contradictions between measurements made by Friends inside a laboratory and superobservers outside a laboratory, who have access to an entangled state of the measurement apparatus. The contradictions lead to no-go theorems for observer-independent facts, thus challenging concepts of objectivity. Here, we examine the paradoxes from the perspective of establishing consistency with macroscopic realism. We present versions of the Brukner-Wigner-friend and Frauchiger-Renner paradoxes in which the spin-$1/2$ system measured by the Friends corresponds to two macroscopically distinct states. The local unitary operations $U_{\theta}$ that determine the measurement setting $\theta$ are carried out using nonlinear interactions, thereby ensuring measurements need only distinguish between the macroscopically distinct states. The macroscopic paradoxes are perplexing, seemingly suggesting there is no objectivity in a macroscopic limit. However, we demonstrate consistency with a contextual weak form of macroscopic realism (wMR): The premise wMR asserts that the system can be considered to have a definite spin outcome $\lambda_{\theta}$, at the time after the system has undergone the unitary rotation $U_{\theta}$ to prepare it in a suitable pointer basis. We further show that the paradoxical outcomes imply failure of deterministic macroscopic local realism, and arise when there are unitary interactions $U_{\theta}$ occurring due to a change of measurement setting at both sites, with respect to the state prepared by each Friend. In models which validate wMR, there is a breakdown of a subset of the assumptions that constitute the Bell-Locality premise. A similar interpretation involving a weak contextual form of realism exists for the original paradoxes.

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