Testing quantum causal influences with photons*

Bell’s theorem and violation of Bell’s inequalities represent the first evidence of the incompatibility between quantum theory and the classical notion of local causality. Bell’s tests rely only on input/output correlations, and allow to infer the non-classical nature of physical systems with minimal assumptions on the adopted apparatus in the so-called device-independent approach. The certification of quantum non-classicality is of crucial importance for several applications in quantum information tasks, but sometimes this task can be very demanding or even impossible in some experimental causal structures.

Here, we take a novel route based on the field of causal inference and show that one can witness quantum effects even in situations where no violation of a Bell inequality is ever possible. We quantify the causal influence of a variable on another by using instrumental variables and interventions. In particular, we use a photonic platform for the implementation of the instrumental causal structure. Polarization entangled bipartite states are generated being able to show violations of classical bound for causal influence. Importantly, the observed correlations, while would appear classical from a standard perspective, are proved to be truly quantum by using our new causal approach.