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Cause and Effect in Quantum Physics

How come some things happen and others don’t? What determines the events that we observe in nature? These are some of the most frequently recurring and fascinating questions that have been pondered by scientists and philosophers alike, but a definitive answer remains a challenge.

The Sapienza Quantum Lab Research Group, in collaboration with the Universities of Natal and Rio de Janeiro in Brazil, has experimentally verified that, from a theoretical point of view, quantum physics can provide an alternative forecast to cause-effect schemes in classical physics.

In the classical model, instrumental processes provide a cause-effect explanation for two related events. This scheme can be used, for example, to test the efficiency of a new drug by affirming that administering drug X (event 1) can cure patient A (event 2) and no other phenomenon. More specifically, measurements can be taken to verify the events described in an instrumental process and translate the problem into precise mathematical relations known as “instrumental inequalities.”

Until this new study conducted by the Sapienza Quantum Lab Research Group, any violation of such an instrumental inequality implied that two observed events could not be compatible with an instrumental process. The research team, instead, has demonstrated that this paradigm is not necessarily respected when facing quantum phenomena such as entanglement, one of the principal properties that underlie the difference between classic and quantum mechanics.

The research group at the Department of Physics observed this quantum violation by conducting an in-lab instrumental process during which the measurements conducted on two entangled photons represented the events in a scheme. The type of measurement conducted on one photon depended directly on the result of the measurement conducted on the other photon. The technological challenge was to generate a pair of entangled photons and to delay the measurement on the second photon, allowing time to measure the first photon. This result was obtained by transmitting the second photon down a 100-metre optic fibre.

The experiment, which was financed by the European Research Council via the 3D-QUEST Grant, has not only implications related to the importance of instrumental processes in a wide range of fields, but has also allowed the observation of the differences in behaviour in classic and quantum physics with a greater validity over previous tests.

“An instrumental process,” explains Fabio Sciarrino, the research group supervisor, “is an ideal scenario to observe this discrepancy, as the tests used to date, based on “Bell’s Inequalities present a series of technical difficulties related to the total absence of a cause-effect relationship between the two measurements. This condition is very restrictive and hard to obtain due to the fact that the two measurement stations must be separated by a distance greater than that which light can traverse during the time between the two measurements.”

Moreover, the results presented in the article define a new type of violation of classic physics that reach beyond the concept of quantum “non-locality.” Indeed, they shed light on the profound difference between classic and quantum theory and constitute a fundamental element for the development of new applications related to cryptography and the generation of casual numbers.

The research has been published on Nature Physics.


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