Light-matter entanglement over 50 km of optical fibre

When shared between remote locations, entanglement opens up fundamentally new capabilities for science and technology.Envisioned quantum networks use light to distribute entanglement between their remote matter-based quantum nodes. Here wereport on the observation of entanglement between matter (a trapped ion) and light (a photon) over 50 km of opticalfibre: twoorders of magnitude further than the state of the art and a practical distance to start building large-scale quantum networks. Ourmethods include an efficient source of ion–photon entanglement via cavity-QED techniques (0.5 probability on-demandfibre-coupled photon from the ion) and a single photon entanglement-preserving quantum frequency converter to the 1550 nm telecomC band (0.25 device efficiency). Modestly optimising and duplicating our system would already allow for 100 km-spaced ion–ionheralded entanglement at rates of over 1 Hz. We show therefore a direct path to entangling 100 km-spaced registers of quantum-logic capable trapped-ion qubits, and the optical atomic clock transitions that they contain.