Entanglement involves two quantum particles – photons, for example – forming a single physical system in spite of the distance between them. Every action performed on one of the two photons has an impact on its “twin” photon. This principle of entanglement leads to quantum non-locality: the measurements and statistics of the properties observed on one of the photons are very closely correlated with the measurements made on the other photon. “Quantum non-locality was discovered theoretically by John Stewart Bell in 1964,” begins Nicolas Brunner, associate professor in the Department of Applied Physics in UNIGE’s Faculty of Science. “This showed that photon correlations are exclusively quantum in nature, and so can’t be explained by conventional physics. This principle could be used to generate ultra-secure encryption keys.”
Is it possible to force photons in a network to become entangled?
But what are the implications of this principle of quantum non-locality when several pairs of photons are placed in a network? “To answer this question, we devised an experiment involving three pairs of photons that were then separated and dispersed to three points forming a triangle”, explains Marc-Olivier Renou, who is also a researcher in the Department of Applied Physics. “At each vertex, two photons from a different pair are processed together.” The physicists subsequently forced the two photons at each vertex of the triangle to entangle by making them interact with each other, before measuring them. They finally showed that the statistics arising from these measurements cannot be explained by any local physical theory. In addition, these statistics are so strongly correlated that they could represent a new form of quantum correlations. “It could become a new version of Bell’s theorem, specific to quantum networks”, enthuses Nicolas. This important theoretical discovery underlines the power of quantum correlations in networks, which far exceeds what researchers had originally thought possible. The next step will be to observe these phenomena in the laboratory. “It’s not going to be child’s play, because conducting an experiment like this is still extremely difficult for the time being”, concludes Nicolas Gisin, a professor in UNIGE’s Department of Applied Physics. Reference: “Genuine Quantum Nonlocality in the Triangle Network” by Marc-Olivier Renou, Elisa Bäumer, Sadra Boreiri, Nicolas Brunner, Nicolas Gisin and Salman Beigi, 30 September 2019, Physical Review Letters.DOI: 10.1103/PhysRevLett.123.140401
