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Game Over for “Spooky Action” Loophole

Over a 48-hour period, more than 100,000 citizen scientists generated random num

Over a 48-hour period, more than 100,000 citizen scientists generated random numbers while playing a video game. Credit: ICFO

By Tara Roberson & Martin Ringbauer

More than 100,000 citizen scientists have taken part in the world’s first global quantum physics experiment to test Einstein’s concept of “local realism”.

Our everyday experience tells us that physical objects have properties independent of whether or not we measure them. As Albert Einstein put it: the Moon is there, whether you look at it or not. However, this is different in the quantum world; the mere act of observing this world seems to change it.

Einstein wasn’t a fan of this view of physics, and suggested there must be something missing from the quantum description. This view became most apparent in his treatment of quantum entanglement. When two particles are entangled they behave as if they are one. No matter how far apart they are in time or space, observing one of them seems to have an instantaneous influence over the other. Einstein called this effect “spooky action at a distance” and suggested that other hidden variables were at play.

In 1964, CERN physicist John Bell precisely formalised Einstein’s view as the concept of “local realism”. Here “realism” says that objects have well-defined properties even when we are not looking at them, while “locality” states that objects can only be influenced by causes in their immediate vicinity.

Bell also put forth a powerful argument to show that Einstein’s view, even when including his suggested missing pieces, cannot explain the results of experiments performed on entangled quantum particles. In doing so he made it possible to test Einstein’s arguments in the lab.

A Bell test sends a pair of entangled particles to two far-apart laboratories, one belonging to “Alice” and the other to “Bob” (Fig. 1). Alice and Bob then randomly perform one of two measurements and compare their results.

According to Einstein’s view, since Alice and Bob are far apart, the particles cannot coordinate their measurement results (locality), and the results should then only depend on the information carried by the particle (realism). On the other hand, quantum mechanics predicts that Alice and Bob will get the same measurement results much more often than can be explained in a world governed by local realism. Hence, if we find in a Bell test that the results agree more often than they disagree, local realism is ruled out as a possible explanation.

Loopholes in the Bell Test

The first Bell tests date back as far as 1972. When the tests began, it quickly became apparent that there were many challenges involved in making a Bell test watertight. Researchers must carefully ensure that all conditions of the test are obeyed so that loopholes don’t open the way for alternative explanation of the results. In particular, the experiment must be precisely timed and almost every run must be successfully detected. In addition, Alice and Bob need to be sufficiently separated so they cannot communicate, and their measurement choices must be random and unpredictable.

It took four decades for researchers to satisfy these requirements and enable the first “loophole-free” Bell tests at the Institute for Quantum Optics and Quantum Information Vienna, and the National Institute of Standards and Technology in Boulder, Colorado. These experiments in 2015 strongly support the existence of “spooky action at a distance” without the pitfalls of the usual loopholes. However, by using physical random number generators to produce unpredictable measurement choices, these experiments inevitably relied on assumptions about the same quantum physics the researchers were trying to test.

This opens yet another loophole in existing Bell tests – the “freedom of choice” loophole. The devices used to produce the random measurement choices are themselves described by quantum mechanics. This opens us up to the possibility that there is a hidden mechanism coordinating the measurement choices with the measurement results obtained by the particles, while appearing random to the experimenter.

One way to get a handle on this effect is by using random number generators that are very far apart from each other. Some experiments have gone so far as to use the random light of neuron stars from opposite sides of the Milky Way galaxy to choose measurement settings. Due to their distance, the light from these stars has not been able to interact for at least 600 years. For a hidden mechanism to be influencing these measurements, nature must have been plotting to mess with current Bell test experiments for at least as long.

Future experiments could continue with this approach and push this even further back. However, the BIG Bell Test Collaboration followed a very different approach.

Citizen Science Gamers

Published in Nature (, the BIG Bell Test set out to close the “freedom-of-choice” loophole by using human free will to generate random measurement settings. However, no single human could choose measurement settings fast enough and randomly enough to make this feasible.

To overcome this obstacle, lead author Carlos Abellan of the Institute of Photonic Sciences in Barcelona suggested that the project could recruit people around the world to play a game on their computer, tablet or smartphone. To collect enough random numbers, at least 30,000 people needed to contribute data that was as random as possible.

Over a 48-hour period, more than 100,000 citizen scientists contributed by generating random numbers – ones and zeros – while playing a video game. The six-level game used machine learning to analyse each participants’ initial entries, and tried to predict their next entries. To improve their score, players had to become increasingly unpredictable in their number selection.

Through this game, nearly 100 million entries were generated and sent to labs across the world. The numbers controlled experiments by determining the measurement settings used in each laboratory.

A very bright pulsed laser is used to generate pairs of entangled photons. Credit: Patrick Self

The Experiments

Each of the 13 experiments conducted on 30 November 2016 strongly contradicted local realism or other realist positions using a variety of different physical systems. These included various types of entangled pairs of light particles or photons, entangled photon–atom pairs, entanglement between a photon and a cloud of atoms, and entangled superconducting circuits. Some of the experiments were also able to close other loopholes simultaneously by separating Alice and Bob enough to prevent communication or detecting almost all events to close the so-called “detection loophole”.

Going far beyond traditional Bell tests, the experiments of the BIG Bell Test showed that quantum systems are not only in conflict with local realism and bi-local realism, but also tested concepts such as entanglement in time, quantum steering, and high-dimensional quantum entanglement.

Of the two Australian teams, Griffith University tested quantum steering to show that Alice’s measurement can affect Bob’s quantum system. This implies that Bob’s system cannot be described by a quantum state independent of Alice’s measurement even though, according to local realism, the two could not influence each other.

Our Australian Research Council Centre of Excellence for Engineered Quantum Systems team at the The University of Queensland used the human-generated random numbers to test quantum entanglement in time. Where Alice and Bob would normally share pairs of entangled particles, we used only a single quantum particle that is passed from Alice to Bob and then on to a third player, Charlie. The numbers we received from the Bell gamers were used to set the measurement directions for Alice, Bob and Charlie randomly. This allowed us to perform a Bell test to demonstrate the presence of entanglement in time.

The UQ experiment allowed us to demonstrate a significant difference to the usual entanglement in space, where entanglement between Alice and Bob would preclude entanglement between Alice and Charlie. In contrast, we find that for entanglement in time, Bob can, at the same time, be maximally entangled to Alice and to Charlie. This opens the road for many further studies and potential applications of entanglement in quantum technologies and even quantum blockchain.

The BIG Bell Test allowed researchers to close the “freedom of choice” loophole by crowdsourcing random data from citizen scientists around the world. As well as providing a novel approach for testing local realism, the BIG Bell Test was an opportunity for gamers to engage in quantum theory and has hopefully opened the door to more global quantum experiments.

Martin Ringbauer is currently a Research Fellow at The University of Innsbruck. Tara Roberson is completing her PhD in science communication at the Australian National University and is the Communications Officer for the ARC Centre of Excellence for Engineered Quantum Systems. They are co-authors of the Nature paper published by The BIG Bell Test Collaboration (