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The BIG Bell Test - Global Physics Experiment Challenges Einstein

May 12, 2018     Email"> PrintText Size

· Simultaneous experiments on five continents challenge Einstein’s principle of local realism.

· Participants contributed to the experiment generating more than 90 million bits, unpredictably choosing among measurements to escape a paradox known as the “freedom-of-choice loophole”.

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 with 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 were recently published in 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 scientists 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 the 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 disagreed Einstein’s worldview, closed the freedom-of-choice loophole for the first time, and demonstrated several new methods in the study of entanglement and local realism.

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

The research group led by Prof. PAN Jianwei and Prof. ZHANG Qiang at University of Science and Technology of China (USTC) of Chinese Academy of Sciences (CAS) worked to explore the Bell’s inequality with partial perfect randomness input.

By analysing the random numbers contributed by Bellsters, the researchers found that the human random number may not be perfectly random, and tend to produce patterns. However, the human generated randomness was 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 interfered at the polarizing beam splitter (PBS) in a Sagnac based setup to create entangled pairs. The entangled state was adjusted to be a special non-maximum entangled state for the inequality. The photon pairs were then sent to two measurement stations that are ~90 meters away for measurement. This spatial separation made sure the measurement in Alice’s lab will not affect that in Bob’s lab, and vice versa.

The random numbers contributed by Bellsters controled the Pockels cell to set the measurement basis for each pair of photons. The photons were finally detected with superconducting nanowire single-photon detectors (superconducting-nanowire single-photon detectors, produced by the group led by Dr. YOU Lixing at Shanghai Institute of Microsystem and Information Technology of CAS). The violation of the MDL Bell inequality gave the bound of the input human randomness to rule out local realism. With around 80 Mb random numbers contributed by Bellsters, the MDL Bell inequality violation was decided to be l = 0.10 ± 0.05.

"Although there are numerous Bell test experiments, the “free will” loophole is still not closed. 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,” said PAN Jianwei, a Professor from USTC.

"What is most amazing for me is that the argument between Einstein and Niels Bohr, after more than 90 years of effort to make it rigorous and experimentally testable, still retains a human and philosophical element. We know that the Higgs boson and gravitational waves exist thanks to amazing machines, physical systems built to test the laws of physics. But local realism is a question we can’t fully answer with a machine. It seems we ourselves must be part of the experiment, to keep the Universe honest,” said Morgan Mitchell, the leader of the BIG Bell Test project and a Professor from ICFO.

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(Editor: LIU Jia)

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