Researchers at the University of Geneva (UNIGE), working with Tehran’s Institute for Research in Fundamental Sciences (IPM), have shown theoretically that a new, ultrastrong quantum correlation is formed when three pairs of entangled photons are placed in a network. This discovery could one day be used to generate ultrasecure encryption keys. In his 1964 theory of quantum nonlocality, John Stewart Bell showed that photon correlations are exclusively quantum in nature. The UNIGE team investigated the implications of Bell’s theory when it is applied to several pairs of photons that are placed in a network. A quantum network with a triangular structure allows for a fundamentally novel type of quantum correlation. Courtesy of UNIGE. The researchers devised an experiment involving three pairs of photons that were separated and dispersed to three points, forming a triangle. At each vertex of the triangle, two photons from a different pair were processed together. The scientists forced the two photons to entangle by causing them to interact with each other before measuring them. When the scientists forced two photons from separate pairs to become entangled, a connection was also made with each photon’s twin, which was present elsewhere in the network. The researchers said that the statistics resulting from their measurements could not be explained by any local physical theory, and that the statistics were so strongly correlated they could represent a new form of quantum correlations. “It could become a new version of Bell’s theorem, specific to quantum networks,” professor Nicolas Brunner said. This theoretical discovery underlines the power of quantum correlations in networks, which far exceeds what researchers had originally thought possible. The next step for the team will be to observe these phenomena in the laboratory. The research was published in Physical Review Letters (https://doi.org/10.1103/PhysRevLett.123.140401).