”Why do things happen?”, “What causes Nature to behave the way we observe it to?” are among the most fascinating and fundamental questions that science and philosophy have always been struggling to answer.
This innovative work, lead by researchers from Sapienza, Natal and Rio de Janeiro universities, proved that Quantum Mechanics can alter even the simplest classical cause-effect relations.

According to classical physics, specific cause-effect schemes, such as the one underlying instrumental processes, allow to fully retrace causal relations between two observed events. For instance, the aforementioned scheme is adopted within new drugs’ trials, with the aim of verifying whether the prescription of drug “X” (event #1) on patient “A” is the actual reason for their recovery (event #2). Specifically, some appropriate measurements can be performed to scientifically verify that two events belong to an instrumental process. These measurements are mathematically correlated through the so-called “instrumental inequalities”.

A classical violation of these inequalities means implies that the observed process is certainly not an instrumental process. Stunningly, this model has been proved untrustworthy when quantum mechanics, and more precisely quantum entanglement (a quantum effect with no classical counterpart), is involved.

Quantum Lab research group was able to observe this quantum violation, implementing an instrumental process whose events were represented by measurements on polarization-entangled photon pairs. In this scheme, for each pair, the measurement to be performed on one photon was selected according to the outcome of the measurement on the other. Therefore, after the generation of entangled photon pairs, the experimental challenge was to delay one photon per pair through a 150 m long optical fiber, so to adapt the measurement according to the one performed on its twin.

This experiment was supported by the European Research Council, through the 3D-QUEST grant, and it sheds light on a novel form of discrepancy between classical and quantum physics, which goes beyond quantum “non-locality”.
“An instrumental process - states Fabio Sciarrino (supervisor of Quantum Lab) - is the ideal scenario to observe the gap between quantum and classical physics. Indeed, previous quantum non-locality tests, based on the so-called “Bell inequalities”, imposed absolutely no causal relations between the two measurements to be performed. This is a very strict condition, which requires the two measurement stations to be further apart than the distance covered by light in the time between the two measurements”.

This result, besides the fundamental relevance, opens a brand-new venue for theoretical and experimental exploration in a relatively simple scenario, finding potential applications in practical information-processing tasks, like in cryptographic or randomness-generation protocols, since there is no need of large spatial separations.