Quantum violation of an instrumental test

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.

Main manuscript (10 pages) + Supplementary Information (8 pages)

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• Quantum speedup in the identification of cause-effect relations
• Why initial system-environment correlations do not imply the failure of complete positivity: a causal perspective
• Single-photon nonlocality in quantum networks
• Concentration phenomena in the geometry of Bell correlations
• Resource theories of multi-time processes: A window into quantum non-Markovianity
• Quantum violations in the Instrumental scenario and their relations to the Bell scenario
• Quantifying causal influences in the presence of a quantum common cause
• Entanglement in prepare-and-measure scenarios: many questions, a few answers
• Device independent certification of a quantum delayed choice experiment
• Experimental nonclassicality in a causal network without assuming freedom of choice
• Quantum communication complexity beyond Bell nonlocality
• Bipartite post-quantum steering in generalised scenarios
• Quantum violation of local causality in urban network with hybrid photonic technologies
• Bell inequalities with one bit of communication
• Quantum steering beyond instrumental causal networks
• Certified Random Number Generation from Quantum Steering
• Analysing causal structures in generalised probabilistic theories
• Quantum Causal Inference with Extremely Light Touch
• Witnessing Non-Classicality in a Simple Causal Structure with Three Observable Variables
• Quantum chicken-egg dilemmas: Delayed-choice causal order and the reality of causal non-separability
• No nonlocal advantage of quantum coherence beyond quantum instrumentality
• Quantifying Quantum Causal Influences
• Modified quantum delayed-choice experiment without quantum control
• Causal reappraisal of the quantum three box paradox
• Combining contextuality and causality: a game semantics approach
• Multistage games and Bell scenarios with communication
• Causal Networks and Freedom of Choice in Bell's Theorem
• Detection Efficiency Bounds in (Semi-)Device-Independent Scenarios
• Experimental test of quantum causal influences
• On safe post-selection for Bell tests with ideal detectors: Causal diagram approach
• A minimal example of quantum non-classicality without freedom of choice
• Fast tests for probing the causal structure of quantum processes