26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

Extended Wigner's Friend Scenario in the Instrumental Network

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral AIP | Quantum Science and Technology (QST)

Speaker

Aaditya Datar (Griffith University)

Description

Bell inequality violations have been a paradigmatic framework to quantify how strongly nature departs from assumptions that seem reasonable from a classical perspective. More precisely, they show that no theory can simultaneously uphold the assumptions of absoluteness of observed events, freedom of choice, relativistic causality, and classical cause-effect relations. Furthermore, while it is known that more generalised notions of causality can accommodate for Bell correlations, whether they are appropriate beyond these scenarios remains unclear.

Recently, the Extended Wigner's Friend scenario has been used to clarify this by connecting nonclassical correlations with the measurement problem. We use this narrative to show theoretically, and via a proof-of-principle photonic experiment, that even when the classical casual constraints are relaxed, they do not reproduce quantum correlations when we keep the remaining metaphysical assumptions (namely, the absoluteness of observed events and the freedom of choice). Interestingly, this inconsistency arises without space-like separation, that is, in a scenario where there are no restrictions due to relativistic causality. This is afforded by studying an Extended Wigner's Friend scenario embedded in the instrumental network — a well-known casual structure that is used to distinguish between direct and common causes — which allows for direct communication between the involved parties, and thus, relaxes the constraint of relativistic causality between them. We use a single-photon path as the "friend observer", and observe that the correlations thus found are incompatible with any model that assumes absoluteness of observed events, and generalised causal relations.

The nonclassicality obtained is thus stronger than the one identified in previous violations of classical causal relations in the instrumental scenario, and can possibly inform new types of correlations that can be implemented in a growing class of quantum technologies.