The BIG Bell Test: 100.000 volunteers collaborate in a quantum physics experiment that challenges Einstein
On November 30, 2016, more than 100,000 people around the world contributed to a series of cutting-edge quantum physics experiments, known as The BIG Bell Test. Using an online video game, participants generated sequences of zeroes and one (bit), in the most random way, which determines what measurements to perform on quantum objects such as atoms, photons and superconducting devices. The results have been now analyzed and published in the journal Nature.
In a Bell test (named after the physicist John Stewart Bell), pairs of entangled particles are generated and sent to different places where their properties, such as the polarization of photons, are measured. In the BIG Bell Test, the bits produced by the participants were communicated via internet to the experiments conducted in Rome, Brisbane (Australia), Shanghai (China), Vienna (Austria), Munich (Germany), Zurich (Switzerland), Nice (France), Barcelona (Spain), Buenos Aires (Argentina), Concepción (Chile) and Boulder (USA), where they were used to set the angles of polarizers and other laboratory elements with the aim of determining what measurements to perform on the entangled particles.
"In the laboratory we observe that the results of measurements on two physically separate particles are correlated, regardless of which properties we choose to measure - Fabio Sciarrino, Group leader of Quantum Information Lab Sapienza, explains - It implies something very surprising: the measurement over one particle instantly influences the other one (although it is very far away), or the observed properties never existed, but rather were created by the measure itself. Both these possibilities contradict the local realism, Einstein's worldview, according to which universe’s nature is independent from our observations".
Under the guidance of the ICFO-The Institute of Photonic Sciences in Barcelona, the BIG Bell Test has requested the participation of volunteers from around the world, known as Bellsters, to choose what measures to carry out in order to close the so-called "Loophole of freedom choice ", i.e the possibility that the particles themselves influence the measuremen's choice. The existence of this kind of influence would invalidate the Bell test: it would be as if students were allowed to write their own exam questions.
"This Loophole - Sciarrino says - cannot be eliminated using dice or random number generators, because there is always the possibility that these physical systems are, in some form, connected with the entangled particles. Human choices introduce the element of free will, through which people can choose independently of what the particles could do. Thus, for the first time, human unpredictability played a key role in experiments in quantum physics, some of which have never been done before".
In particular, the group headed by Fabio Sciarrino of the Physics Department of Sapienza, the only Italian partner directly involved in the BIG Bell Test project, has experimentally observed, for the first time, a new type of non-locality within a quantum network. The team, made up of young researchers, has implemented a network with three interconnected nodes and two independent sources of quantum states, each of which produces a pair of entangled photons in their polarization. The experiment exploited human randomness for the choice of the settings in the measurements performed on each of these nodes.
This collective experiment allowed to perform a Bell test and other similar tests on realism and locality in quantum mechanics, free from some strong assumptions previously needed. "The results - Sciarrino concludes - are not in line with Einstein's worldview, they close the Loophole of freedom of choice for the first time and demonstrate several new methods in the study of the entanglement and local realism. Furthermore, this global experiment represents a demonstration of the maturity of emerging quantum technologies with implications from cryptography to quantum networks".
Challenging local realism with human choices - The BIG Bell Test Collaboration - C. Abellán, A. Acín, A. Alarcón, O. Alibart, C. K. Andersen, F. Andreoli, A. Beckert, F. A. Beduini, A. Bendersky M. Bentivegna, P. Bierhorst, D. Burchardt, A. Cabello, J. Cariñe, S. Carrasco, G. Carvacho, D. Cavalcanti, R. Chaves, J. Cortés-Vega, A. Cuevas, A. Delgado, H. de Riedmatten, C. Eichler, P. Farrera, J. Fuenzalida, M. García-Matos, R. Garthoff, S. Gasparinetti, T. Gerrits, F. Ghafari Jouneghani, S. Glancy, E. S. Gómez, P. González, J.-Y. Guan, J. Handsteiner, J. Heinsoo, G. Heinze, A. Hirschmann, O. Jiménez, F. Kaiser, E. Knill, L. T. Knoll, S. Krinner, P. Kurpiers, M. A. Larotonda, J.-Å. Larsson, A. Lenhard, H. Li, M.-H. Li, G. Lima, B. Liu, Y. Liu, I. H. López Grande, T. Lunghi, X. Ma, O. S. Magaña-Loaiza, P. Magnard, A. Magnoni, M. Martí-Prieto, D. Martínez, P. Mataloni, A. Mattar, M. Mazzera, R. P. Mirin, M. W. Mitchell, S. Nam, M. Oppliger, J.-W. Pan, R. B. Patel, G. J. Pryde, D. Rauch, K. Redeker, D. Rieländer, M. Ringbauer, T. Roberson, W. Rosenfeld, Y. Salathé, L. Santodonato, G. Sauder, T. Scheidl, C. T. Schmiegelow, F. Sciarrino, A. Seri, L. K. Shalm, S.-C. Shi, S. Slussarenko, M. J. Stevens, S. Tanzilli, F. Toledo, J. Tura, R. Ursin, P. Vergyris, V. B. Verma, T. Walter, A. Wallraff, Z. Wang, H. Weinfurter, M. M. Weston, A. G. White, C. Wu, G. B. Xavier, L. You, X. Yuan, A. Zeilinger, Q. Zhang, W. Zhang & J. Zhong - Nature volume 557, pages212–216 (2018) doi:10.1038/s41586-018-0085-3