THE BIG BELL TEST Global physics experiment challenges Einstein with the help of 100,000 volunteers

On November 30th, 2016, more than 100,000 people around the world contributed to a suite of first-of-a-kind quantum physics experiments known as The BIG Bell Test. Using smartphones and other internet-connected devices, participants contributed unpredictable bits, which determined how entangled atoms, photons, and superconducting devices were measured in twelve laboratories around the world. Scientists used the human input to close a stubborn loophole in tests of Einstein’s principle of local realism. The results have now been analysed, and are reported in this week’s Nature.

In a Bell test (named for the physicist John Stewart Bell), pairs of entangled particles such as photons are generated and sent to different locations, where particle properties such as the photons’ colours or time of arrival are measured. If the measurement results tend to agree, regardless of which properties we choose to measure, it implies something very surprising: either the measurement of one particle instantly affects the other particle (despite being far away), or even stranger, the properties never really existed, but rather were created by the measurement itself. Either possibility contradicts local realism, Einstein’s worldview of a universe independent of our observations, in which no influence can travel faster than light. The BIG Bell Test asked human volunteers, known as Bellsters, to choose the measurements, in order to close the so-called “freedom-of-choice loophole” – the possibility that the particles themselves influence the choice of measurement. Such influence, if it existed, would invalidate the test; it would be like allowing students to write their own exam questions. This loophole cannot be closed by choosing with dice or random number generators, because there is always the possibility that these physical systems are coordinated with the entangled particles. Human choices introduce the element of free will, by which people can choose independently of whatever the particles might be doing. Led by ICFO-The Institute of Photonic Sciences, in Barcelona, the BIG Bell Test recruited participants worldwide to contribute unpredictable sequences of zeros and ones (bits) through an online video game. The bits were routed to state-of-the-art experiments in Brisbane, Shanghai, Vienna, Rome, Munich, Zurich, Nice, Barcelona, Buenos Aires, Concepción Chile and Boulder Colorado, where they were used to set the angles of polarizers and other laboratory elements to determine how entangled particles were measured. Participants contributed with more than 90 million bits, making possible a strong test of local realism, as well as other experiments on realism in quantum mechanics. The obtained results strongly disagree Einstein’s worldview, close the freedom-of-choice loophole for the first time, and demonstrate several new methods in the study of entanglement and local realism.

Within the Big Bell Test, the group led by Prof. Fabio Sciarrino in the University of Rome in collaboration with Prof. Rafael Chaves from the International Institute of Physics in Natal (Brazil), experimentally observed, for the first time, a new type of non-locality arising in a quantum network akin to the famous entanglement swapping experiment. The team implemented a network with three nodes interconnected by two independent sources of quantum states, each producing a pair of photons entangled in their polarization. The experiment exploited the human randomness for the choice of measurement settings performed at each of these nodes.

The Big Bell Test initiative also allowed a collaboration between “La Sapienza” Università di Roma and the Universidad de Concepcion (Chile). Prof. Fabio Sciarrino, Prof. Paolo Mataloni and the PhD. Students Gonzalo Carvacho and Alvaro Cuevas members of the Quantum Optics Group of “La Sapienza” collaborated actively in the preparation of the Big Bell Test experiment carried out in Chile (Universidad de Concepcion) where the loophole of the post-selection was closed by exploiting the so-called “Hug” configuration.

Challenging local realism with human choices, The BIG Bell Test Collaboration
Nature 557, pages 212–216 (2018)

Prof. Fabio Sciarrino