The Big Bell Test collaboration

Researchers at the ARC Centre of Excellence for Quantum Computation and Communication Technology (CQC2T) based at Griffith University have played an important role in a major international collaboration that tested quantum nonlocality—Einstein’s ‘spooky action at a distance’—in a suite of experiments worldwide.

The Big Bell Test (BBT) brought people together in one big worldwide project, using random numbers sourced from people’s free will to rigorously ensure unpredictability in the measurement settings required for such tests. The project used an online game through which members of the public provided random numbers to the experiments in real time.

‘Nonlocal’ effects such as entanglement underlie the quantum computation and communication technologies being pursued in CQC2T, and this collaborative research transitioned the Centre into its new research program from 2017. The joint work of the BBT consortium, published in Nature in 2018, also serves as a flagship for new approaches to citizen involvement in science, and for science outreach.

The Griffith University team, led by Dr Raj Patel and Professor Geoff Pryde, performed a test of ‘quantum steering’ as part of the BBT. Steering is a practical form of quantum non- locality testing that is resistant to real-world device imperfections and has direct application to quantum communication tasks such as verifying that entanglement has been shared between remote parties.

“One of the things that was exciting and really interesting for us was to be part of a big project that required a large amount of coordination,” says Professor Pryde. “From compiling random numbers from the public to disseminating them between the experiments, and receiving and using them in a timely way, the level of collaboration was remarkable. I also particularly enjoyed the outreach and public involvement side; I enjoyed that we gave people an opportunity to do something which influenced how the experiment ran.”

To generate a stream of random bits, the researchers recruited about 100,000 human participants to play an online video game that incentivises fast, sustained input of unpredictable selections. During the course of the experiment, the participants generated 97,347,490 binary choices, which were directed via a scalable web platform to 12 laboratories on five continents, where 13 experiments tested local realism using photons, single atoms, atomic ensembles and superconducting devices.

The researchers used the data flow of human-created randomness, which flowed in at over 1,000 bits per second, to choose each measurement setting for their experiment. Their observations have now been used to close an important loophole (the ‘freedom-of-choice loophole) that stood in the way of experimentally determining one of the fundamental principles of quantum mechanics, known as ‘local realism’.

The project has also been a test case for the use of video-game methods in the rapid collection of human-generated randomness, and the use of networking techniques for global participation in experimental science.

“It will be great to harness this quantum weirdness for new technologies in the future,” say the research team.

Fun facts from the Griffith CQC²T experimental team

• The Quantum Optics and Information Lab at Griffith University operates in semi-darkness and uses gentle, coloured LED lighting to protect the sensitive photon detectors.
• When travelling through glass optical fibres, photons move at about 2/3 of the speed that light travels in air.
• The quantum version of bits (zeros and ones) can be encoded, and measured, in light beams using polarization. It’s the same idea as polarized sunglasses, but at a quantum scale—with single particles of light.
• ‘Quantum steering’ is an experiment that closely resembles a Bell test, but one of the two measurement devices is treated a little differently. It is very closely related to Einstein’s idea that quantum physics produces “spooky action at a distance.”
• If two far-apart people can successfully perform a Bell test, then they can use the random bits they share to send a message with absolute security.
• The first part of the worldwide experiment all happened on a single day. Because Griffith University is the easternmost of all of the collaborators, we started using human-generated bits first!
• Although the experimental data was collected on a single day, the research went on for quite a bit longer. That’s because rigorous analysis, comparison and interpretation of the data are key aspects of scientific enquiry.

Image: Griffith University’s apparatus for accurately measuring the polarisation of photons. Random bits in, photons in, quantum experiment data out! Credit: Raj B. Patel.