  Instrumental variables allow the estimation of cause and effect relations
even in presence of unobserved latent factors, thus providing a powerful tool
for any science wherein causal inference plays an important role. More
recently, the instrumental scenario has also attracted increasing attention in
quantum physics, since it is related to the seminal Bell's theorem and in fact
allows the detection of even stronger quantum effects, thus enhancing our
current capabilities to process information and becoming a valuable tool in
quantum cryptography. In this work, we further explore this bridge between
causality and quantum theory and apply a technique, originally developed in the
field of quantum foundations, to express the constraints implied by causal
relations in the language of graph theory. This new approach can be applied to
any causal model containing a latent variable. Here, by focusing on the
instrumental scenario, it allows us to easily reproduce known results as well
as obtain new ones and gain new insights on the connections and differences
between the instrumental and the Bell scenarios.

Quantum Physics