Going Beyond Bell’s Theorem

Anton Zeilinger, Daniel M. Greenberger, Michael Horne

Flavio Del Santo, Nicolas Gisin

We review a series of recent no-go theorems that extend the methodological and conceptual framework introduced by Bell's theorem to more sophisticated operational scenarios involving independent sources, quantum networks, joint measurements, and multiple observers. Many of these results were originally developed within quantum information science and are scattered across the literature, where their technical and application-driven success has partly obscured their broader foundational significance. We organize them around a common Bell-inspired methodology that extends beyond quantum nonlocality, and emphasize how increasingly rich operational structures change which physical assumptions can be tested.

Kaj Börge Hansen, Filosofiska föreningen

Laurens Walleghem, Lorenzo Catani

The renowned Local Friendliness no-go theorem demonstrates the incompatibility of quantum theory with the combined assumptions of Absoluteness of Observed Events - the idea that observed outcomes are singular and objective - and Local Agency - the requirement that the only events correlated with a setting choice are in its future light cone. Granted that the Local Friendliness scenario can be realized, this result is stronger than Bell's theorem because the assumptions of Local Friendliness are weaker than those of Bell's theorem: Local Agency is less restrictive than local causality, and Absoluteness of Observed Events is encompassed within the notion of realism assumed in Bell's theorem. Drawing inspiration from the correspondence between nonlocality proofs in Bell scenarios and generalized contextuality proofs in prepare-and-measure scenarios, we present the Noncontextual Friendliness no-go theorem. This theorem demonstrates the inconsistency of quantum theory with the joint assumptions of Absoluteness of Observed Events and Noncontextual Agency, the latter being a weaker version of noncontextuality, in the same way that Local Agency is a weaker version of local causality. Our result generalizes the Local Friendliness no-go theorem and, granted that the scenario can be realized, is stronger than no-go theorems based on generalized noncontextuality.

F. J. Duarte