Since quantum theory was first proposed over a century ago, it has swiftly gone from an attempt to understand the world around us to complex technological proposals that exploit its behavior. But, in the meantime, scientists are still trying to convince themselves that nature is in fact quantum.

It’s often said that the world’s most precise clocks (atomic clocks, which have enabled GPS technology) work thanks to quantum physics. The successful experiments in particle accelerators also rely heavily on this theory. However, some of the predictions of quantum theory are not that simple to test.

One of these is nonlocality, an effect by which observing one particle can reveal information about other particles potentially located thousands of kilometers away. Detecting this phenomenon is not straightforward, and the first convincing tests of nonlocality for two particles only came in 2015. Now, a group of scientists in China and Spain have carried out a very strict test of nonlocality in a network of particles.

Such a network could one day allow people in different cities to be connected by fast, hyper-secure communication channels and thus form the basis of a quantum internet. For the capabilities of the network to go beyond those of our current internet, it is essential that it can yield nonlocality. Thus, the results of this study, published in Physical Review Letters, could be used to certify equipment used to build a quantum internet.

“We have made one of the strongest, strictest tests of quantum phenomena in networks,” said Alejandro Pozas-Kerstjens, co-lead author of the paper and researcher at the Institute of Mathematical Sciences in Madrid.

The basis of the quantum internet

To build a quantum internet connecting different cities, for example, one would need to place photon emitters in intermediate locations between every pair of cities so that they could distribute photons (the particles light is made of) to the cities they serve. The photons would need to be entangled in order to establish quantum links between the cities.

However, in a real-world scenario where one does not have full control over all of the emitters, one could imagine a faulty emitter sending non-entangled photons instead, making that link classical and compromising the functioning of the whole network.

Pozas-Kerstjens wondered whether there could be a way of certifying that all of the links in such a network were indeed quantum, i.e., that none of the emitters were taking shortcuts and sending non-entangled photons.  He figured this would be an essential test for future networks to make sure they are working as they should.

That is why in 2022, he and two other colleagues coined the concept of “full network nonlocality”. For a setup to give rise to full network nonlocality, all of the emitters, or sources, need to distribute entangled photons. “If there exists a classical [i.e., non-quantum] source in your network, you can’t observe this anymore. If you have 500 sources and one distributes classical systems, then you can’t observe full network nonlocality,” Pozas-Kerstjens explained.

Shortly after their proposal came out, Xue-Mei Gu from the University of Science and Technology of China contacted Pozas-Kerstjens. “She told me by email, ‘We read your article, we have this implementation already set up and we’d like to observe your phenomenon. Do you think you could guide us?'” he recalled.

A paranoid experiment

Pozas-Kerstjens knew that carrying out a convincing experiment would not be easy. The setup would need to be “paranoid [in order to] eliminate any chance that the results we observe can be simulated in a classical, or non-quantum, way,” he said. But he immediately accepted Gu’s offer.

The experiment involved a network of three machines standing in as different cities, dubbed Alice, Bob, and Charlie. One emitter would establish a link between Alice and Bob by sending them a pair of entangled photons, while another would similarly establish a link between Bob and Charlie. In order to test for full network nonlocality, Alice, Bob and Charlie would need to make certain measurements on their photons and note down the results. This procedure would be repeated several times and the overall results would be compared at the end.

For the network to be fully network nonlocal, the links between both Alice and Bob and Bob and Charlie needed to be nonclassical. However, the scientists knew that it was essential that they prevent the parties and/or emitters from coordinating – in as “paranoid” a way as possible. Otherwise, the results they obtained would not reliably display full network nonlocality.

That is why they decided to separate Alice, Bob and Charlie by around 200 meters, with the emitters approximately halfway between each pair, and carry out the rounds of the experiment within nanoseconds. This way, it would be impossible for any information held by one party to travel to the other parties before they made their measurements and for the emitters to collude among themselves.

“Since information can’t travel faster than light, measurements are made fast enough so that light does not have time to get from one element of the experiment to the other,” Pozas-Kerstjens explained, thus preventing parties and sources from coordinating.

“Closing all possible loopholes is an extremely difficult task,” said Ivan Šupić, researcher at the Laboratoire d’Informatique Sorbonne Université/CNRS in Paris who was not involved in the study. Carrying out this experiment so fast and at such a distance “is quite an amazing thing”.

A big step for quantum networks

While there are still a few more details to consider in order to be utterly certain that no classical links exist in this network, this experiment constitutes a decisive step forward for the study of nonlocality in quantum networks.

“On a fundamental level, to think that [our results] could not be replicated in any way if you had classical systems is quite shocking,” Pozas-Kerstjens said, adding that it is one more piece of evidence that the world is in fact quantum.

In Europe alone there are a number of proposals for a quantum internet. The European Quantum Communication Infrastructure Initiative aims to link cities across the continent, with projects already underway in Madrid and the Netherlands for city- and country-wide networks.

Once those networks are built, it will be essential to test that they work properly, and especially that all of the links are quantum. “Testing network nonlocality is the way to actually test that you have nonclassical links [in the context of a long-distance quantum internet]”, said Šupić.

“Out of the applications of quantum technologies, people usually focus on quantum computers. But I think that applications of quantum communications will arrive before a quantum computer, because the technology is better understood and in fact some pilots of networks to distribute quantum information among several nodes at considerable distances are already being tested,” Pozas-Kerstjens said.

Reference: Xue-Mei Gu et al., Experimental full network nonlocality with independent sources and strict locality constraints, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.190201

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