  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.
