Entanglement distribution over a 96-km-long submarine optical fiber

Significance Entanglement, the existence of correlations in distant systems stronger than those allowed by classical physics, is one of the most astonishing features of quantum physics. By distributing entangled photon pairs over a 96-km-long submarine fiber, which is part of existing infrastructure carrying internet traffic, we demonstrate that polarization entanglement-based quantum key distribution (QKD) can be implemented in real-world scenarios. QKD facilitates secure communication links between two parties, whereby the security is guaranteed by the basic property of quantum mechanics that the quantum state of a photon cannot be duplicated. Quantum entanglement is one of the most extraordinary effects in quantum physics, with many applications in the emerging field of quantum information science. In particular, it provides the foundation for quantum key distribution (QKD), which promises a conceptual leap in information security. Entanglement-based QKD holds great promise for future applications owing to the possibility of device-independent security and the potential of establishing global-scale quantum repeater networks. While other approaches to QKD have already reached the level of maturity required for operation in absence of typical laboratory infrastructure, comparable field demonstrations of entanglement-based QKD have not been performed so far. Here, we report on the successful distribution of polarization-entangled photon pairs between Malta and Sicily over 96 km of submarine optical telecommunications fiber. We observe around 257 photon pairs per second, with a polarization visibility above 90%. Our results show that QKD based on polarization entanglement is now indeed viable in long-distance fiber links. This field demonstration marks the longest-distance distribution of entanglement in a deployed telecommunications network and demonstrates an international submarine quantum communication channel. This opens up myriad possibilities for future experiments and technological applications using existing infrastructure.