Quantum violation of an instrumental test

TL;DR

This paper demonstrates that quantum entanglement can violate classical instrumental causal tests, revealing fundamental differences in causal inference when quantum effects are involved, and provides experimental evidence using photonic setups.

Contribution

It introduces a quantum violation of the instrumental test, bridging causal inference and quantum mechanics, with an experimental demonstration using active feed-forward photonic systems.

Findings

Quantum entanglement violates classical instrumental tests.

Experimental proof using photonic setup with active feed-forward.

Implications for causal inference and quantum technology applications.

Abstract

Inferring causal relations from experimental observations is of primal importance in science. Instrumental tests provide an essential tool for that aim, as they allow one to estimate causal dependencies even in the presence of unobserved common causes. In view of Bell's theorem, which implies that quantum mechanics is incompatible with our most basic notions of causality, it is of utmost importance to understand whether and how paradigmatic causal tools obtained in a classical setting can be carried over to the quantum realm. Here we show that quantum effects imply radically different predictions in the instrumental scenario. Among other results, we show that an instrumental test can be violated by entangled quantum states. Furthermore, we demonstrate such violation using a photonic set-up with active feed-forward of information, thus providing an experimental proof of this new form of non-classical behaviour. Our findings have fundamental implications in causal inference and may also lead to new applications of quantum technologies.