Global Experiment Challenges Einstein’s Principle of Local Realism: BIG Bell Test

A Bell test, named for the Northern Irish physicist John Stewart Bell, is a randomized trial that compares observations against the philosophical worldview of local realism, in which the properties of the physical world are independent of our observation of them and no signal travels faster than light. On November 30, 2016, more than 100,000 people contributed to the so-called BIG Bell Test. Using internet-connected devices, volunteers generated more than 90 million binary choices, which were directed via a scalable web platform to 12 labs, where experiments tested local realism using photons, single atoms, atomic ensembles and superconducting devices. The results are reported in the journal Nature.

This picture shows the setup of the experiment. Image credit: Jian-Wei Pan’s Group.

This picture shows the setup of the experiment. Image credit: Jian-Wei Pan’s Group.

“The BIG Bell Test was an incredibly challenging and ambitious project,” said Dr. Carlos Abellán, a researcher at the Institut de Ciencies Fotoniques at the Barcelona Institute of Science and Technology in Spain.

“It sounded impossibly difficult on day zero, but became a reality through the efforts of dozens of passionate scientists, science communicators, journalists and media, and especially the tens of thousands of people that contributed to the experiment during November 30, 2016.”

In a Bell test, pairs of entangled particles such as photons are generated and sent to different locations, where particle properties such as the photons’ colors 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 volunteers 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.

The BIG Bell Test participants contributed 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.

The participants contributed with 97,347,490 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.

Each of the 12 labs carried out a different experiment, to test local realism in different physical systems and to test other concepts related to realism.

“Our team explores the Bell’s inequality with partial perfect randomness input,” said researchers from the CAS Center for Excellence and Synergetic Innovation Center in Quantum Information and Quantum Physics at the University of Science and Technology of China (CAS-USTC).

“Analyzing the random numbers contributed by the volunteers, we may find the human random number are not perfectly random, and tend to produce patterns. However, the human generated randomness is highly attractive because of the element of human free will.”

“True randomness, which is not controlled by hidden variables, exists in between the human choices. Remarkably, it is able to say how well the hidden variable would have to control the human choices.”

“This is made possible by using a special type of Bell inequality, the measurement dependent local (MDL) inequality.”

In the experiment, a 780 nm pump laser focused on a periodically poled potassium titanyl phosphate (PPKTP) crystal to create photon pairs at 1560 nm via spontaneous parametric down conversion. The down-converted photon pairs interfere at the polarizing beam splitter (PBS) in a Sagnac based setup to create entangled pairs.

The entangled state is adjusted to be a special non-maximum entangled state for the inequality.

“The photon pairs are then sent to two measurement stations that are 90 m away for measurement. This spatial separation makes sure the measurement in Alice’s lab will not affect that in Bob’s lab, and vice versa,” the scientists said.

“The random numbers contributed by the participants control the Pockels cell to set the measurement basis for each pair of photons.”

“The photons are finally detected with superconducting nanowire single-photon detectors.”

The violation of the MDL Bell inequality gives the bound of the input human randomness to rule out local realism. With around 80 Mb random numbers contributed by the volunteers, the MDL Bell inequality violation is decided to be l = 0.10 ± 0.05.

“Although there are numerous Bell test experiments, the ‘free will’ loophole is still not closed,” said Professor Jian-Wei Pan, a researcher at CAS-USTC.

“This experiment is a very interesting and important try. In the future, with the help of space station, one may close both ‘collapse locality’ and ‘free will’ loopholes in one experiment.”


C. Abellán et al (The BIG Bell Test Collaboration). 2018. Challenging local realism with human choices. Nature 557: 212-216; doi: 10.1038/s41586-018-0085-3

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