Quantum violation of an instrumental test

Wednesday, 13 December, 2017

Quantum violation of an instrumental test
Joint Brazilian-Italian work, reported in Nature Physics, demonstrates unexpected quantum effects in the simplest causal-inference scenario.

Why do things happen? What causes Nature to behave the way we observe it to? These are among the most fundamental questions in science and philosophy. If interventions are available, one can modify a system, observe the effects of such modifications, and, from those effects, infer cause-effect relations. However, often, interventions are not possible and only observed data are available. To make things worse, often, not all causes of a given effect under scrutiny are accessible, i.e. there are relevant factors hidden from our empirical observations. Remarkably, even so, one can still infer causal relations between correlated events if their causal explanation —i.e. causal model— contains a special type of variable called an instrument. The instrumental causal structure is the simplest one where causal influences can be detected in the presence of hidden common causes and without interventions, i.e. from observations alone. This makes this structure crucial for causal inference, with applications ranging from randomized clinical trials to econometrics. All classical instrumental causal models satisfy fundamental constraints known as instrumental inequalities. Up to now, it was believed that the violation of such inequalities unambiguously certified that the causal model is not an instrumental. For these reason, instrumental inequalities were used as a test against “bad instruments”. Defying this well established paradigm, a team of physicists has shown that the presence of quantum entanglement in instrumental causal models can both fake causal influences as well as lead to violations of instrumental inequalities, something impossible classically.

Brazilian-based Profs. Rafael Chaves, from the International Institute of Physics at Natal, and Leandro Aolita, from the Federal University of Rio de Janeiro, together with an Italian-based team led by Prof. Fabio Sciarrino, from Sapienza University Rome, studied quantum instrumental causal models. There, the (unobserved) common cause between two observed events is equipped with quantum entanglement. Entanglement is arguably the most counter-intuitive property of quantum mechanics, responsible for the infamous “spooky action at a distance”, as Einstein and his co-workers used to refer to it. The Brazilian-Italian team theoretically predicted and experimentally confirmed that entanglement can not only enhance the descriptive power of instrumental models but also even lead to violations of instrumental inequalities. The discovery shakes fundamental pillars of the field of causal inference. In addition, it is also relevant to the foundations of quantum theory itself, since the instrumental scenario is simpler than the paradigmatic Bell scenario commonly used to observe the mismatch between quantum and classical predictions. In fact, the instrumental causal structure is the simplest one where a gap between classical and quantum predictions is possible, as the team has now confirmed.

In Bell scenarios, non-classicality tests require the involved parties to be space-like separated, i.e. such that the time period between the measurement events at both labs is shorter than what it would take light to travel from one lab to the other. This constitutes a major experimental challenge. In contrast, in the instrumental scenario, direct causal influences from one party to the other are not prohibited. In the experiment, these influences were implemented by a communication channel that allowed the measurement choice of the second party to depend on the measurement outcome registered by the first one (a mechanism known as “information feed-forward”). The measurements were made on entangled photons by the team in Rome. The experimentalists delayed the detection of the first photon with respect to that of the second one through a long optical fiber, which left time to adapt the second measurement according to the first measurement outcome. Such measurement adaptation required a fast device to switch between the measurement choice. From an experimental point of view, this outcome communication mechanism is a nontrivial feature not present in previous Bell-like experiments interesting in itself. Apart from Prof. Fabio Sciarrino, the Roman team of the Quantum Information Lab at Sapienza University was composed by the two PhD students Gonzalo Carvacho and Iris Agresti, the Masters student Valerio Di Giulio, and the technician Sandro Giacomini.

Quantum violation of an instrumental test
Rafael Chaves, Gonzalo Carvacho, Iris Agresti, Valerio Di Giulio, Leandro Aolita, Sandro Giacomini & Fabio Sciarrino
Nature Physics (2017) doi:10.1038/s41567-017-0008-5