Can photons communicate?
The fundamental question that the experiments were designed to answer was whether photons can communicate with each other across long distances, at speeds greater than the speed of light. Some experimental results, including results from a research group of which LiU’s Professor Jan-Åke Larsson was a member, suggest the existence of what Albert Einstein called “spooky action at a distance”. At the very instant that the polarisation of one photon of an entangled pair of photons is measured, it appears that the other photon adopts the opposite polarisation, even if the two are separated by many kilometres.Previous experiments have used random number generators to select polarization, but the Big Bell experiment is the first to use the free will of people. This method is used to prevent nature tricking the physicists into using the wrong theory. Every time a direction was selected during the experiments, it was made by people using a game on a mobile phone to generate strings of ones and zeroes. Strings with a total length of just over 95 million ones and zeroes were generated. 109,046 people throughout the world, 3,283 of them in Sweden, took part in the experiment, including the pupils at Katedralsskolan in Linköping.
13 different experiments
The series of ones and zeros was sent to twelve laboratories around the world such that 13 different experiments could be carried out at exactly the same instant.“Using people as random number generators allows us once and for all to prove that there is no other explanation for this phenomenon than quantum mechanics,” says Jan-Åke Larsson, professor in the Division for Information Coding.
Guilherme Xavier, senior lecturer in the same division, was working in the autumn of 2016 at one of the laboratories that participated in the experiments, in Concepción, Chile.
The thirteen experiments tested an inequality formulated and proved by physicist John Bell in 1964. The inequality sets a limit for how large the correlation can be between two systems if they are unable to influence each other at a distance, i.e. if there is no action at a distance.
“The various experiments used slightly different methods and tools to test the Bell inequality, and it would have required an enormous conspiracy to be able to control the choices of 100,000 people, and the thirteen different experiments that used differently sized parts of the code generated. In Chile we used 27 million bits of the code,” says Guilherme Xavier.
Jan-Åke Larsson agrees: “Any theory able to explain this is so unlikely that we quite simply don’t believe it.” He continues:
Two possibilities
“There are two philosophical theories that form the basis of the experiment: one is “realism” – that photon polarizations exist even when we don’t measure them, while the other is “locality” – that no influence can travel faster than the speed of light. The results of the experiments show that we must reject one of these theories: they depend on each other, but both cannot be valid at the same time.”
The 106 authors of the article in Nature conclude that the results are highly incompatible with local realism.
“We still don’t know which of the two theories we must reject: quantum physicists and classical physicists have different preferences. But we are closing in on the truth , this is what science is about,” says Jan-Åke Larsson.
This research may at first sight appear to be solely philosophical, but it does have applications in information security, the interface between quantum physics and the field of telecommunications.
“I’m in the process of building up a laboratory here at LiU for research into quantum communication in the next generation of optical fibers for the telecommunications industry,” says Guilherme Xavier.
Read more at The Big Bell test homepage
The article: Challenging local realism with human choices, The Big Bell Test Collaboration, Nature 2018, doi:10.1038/s41586-018-0085-3
Translation George Farrants
• CQC2T and Griffith University (Brisbane-Australia),
• EQuS and University of Queensland (Brisbane-Australia)
• The node CEFOP/Department of Electrical Engineering of the Universidad de Concepción (Concepción-Chile), together with the Department of Electrical Engineering - Linköping University, the University of Sevilla and the Dipartimento di Fisica—Sapienza Università di Roma,
• The Quantum Information Lab of the Dipartimento di Fisica - Sapienza Università di Roma (Rome, Italy) with the International Institute of Physics del Federal University of Rio Grande do Norte (Natal, Brazil),
• CAS --University of Science and Technology of China (Hefei-China),
• CITEDEF/Universidad de Buenos Aires (Buenos Aires, Argentina),
• ICFO (Barcelona, Spain),
• IQOQI/OEAW (Vienna-Austria),
• LMU-Ludwig-Maximilian University (Munich, Germany),
• INPHYNI – Université Côte d’Azur/CNRS (Nice-France),
• NIST (Boulder- USA),
• QUDEV- ETH Zurich (Zurich).
