Entanglement is one of the properties specific to quantum particles. When two photons become entangled, for instance, the quantum state of the first will correlate perfectly with the quantum state of the second, even if they are at a distance from one another. But what happens when three pairs of entangled photons are placed in a network? Researchers at the University of Geneva (UNIGE), Switzerland, working in partnership with Tehran’s Institute for Research in Fundamental Sciences (IPM), have proved that this arrangement allows for a new form of quantum correlation in theory. When the scientists forced two photons from separate pairs to become entangled, the connection was also made with their twin photon present elsewhere in the network, forming a highly-correlated triangle. These results, which you can read all about in the journal Physical Review Letters, create the potential for new applications in cryptography while reviving quantum physics at its most fundamental level.

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.”