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.

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R.C. and L.A. acknowledge financial support from the Brazilian ministries MEC and MCTIC. In addition, L.A. is also grateful to the Brazilian agencies CAPES, CNPq, FAPERJ and INCT-IQ for financial support. This work was supported by the ERC-Starting Grant 3D-QUEST (3D-Quantum Integrated Optical Simulation; grant agreement number 307783): http://www.3dquest.eu, and QUCHIP-Quantum Simulation on a Photonic Chip grant agreement number 641039. G.C. thanks Becas Chile and Conicyt for a doctoral fellowship.

Author information


  1. International Institute of Physics, Federal University of Rio Grande do Norte, 59078-970, PO BOX 1613, Natal, Brazil

    • Rafael Chaves
  2. Dipartimento di Fisica - Sapienza Università di Roma, P.le Aldo Moro 5, 00185, Roma, Italy

    • Gonzalo Carvacho
    • , Iris Agresti
    • , Valerio Di Giulio
    • , Sandro Giacomini
    •  & Fabio Sciarrino
  3. Instituto de Física, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, Rio de Janeiro, RJ, 21941-972, Brazil

    • Leandro Aolita


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G.C., I.A, V.D.G, S.G. and F.S. devised and performed the experiment; R.C. and L.A. developed the theoretical tools; all authors discussed the results and contributed to the writing of the manuscript.

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The authors declare no competing financial interests.

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Correspondence to Rafael Chaves or Fabio Sciarrino.

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