Work on quantum violation of an instrumental test in Nature Physics.

Quantum weirdness alters basic cause-effect relations

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. 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. 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 one. Defying this well established paradigm, a young 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 and ICTP/SAIFR São Paulo, together with an Italian-based team led by Prof. Fabio Sciarrino, from Sapienza University Rome, studied quantum instrumental causal models. 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.

Besides the fundamental relevance, the results may also find applications in practical information-processing tasks. This is due to the fact that the discovered quantum violations of instrumental inequalities define a novel form of quantum nonlocality, stronger than nonlocal hidden-variable models that include one-way communication of measurement outomes between the parties involved. This may be relevant, e.g., in cryptographic or randomness-generation protocols without the need for experimentally ensuring large spatial separations. A brand-new venue for theoretical and experimental exploration in a relatively simple scenario is opened.

To end up with, it is important to highlight that the research was carried out during one of the worst science-budget cuts in Brazil’s recent history. The collaboration with the Italian experimental team was crucial in this sense. This is a story of success in difficult times, and shows that it is still possible to produce top-quality science in Brazil if resources are given. Unfortunately, the Brazilian government has proposed to deepen the budget cut even more. If the current situation is not reverted, this type of success stories is in serious danger.













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