Entangling single atoms over 33 km telecom fibre

Quantum networks promise to provide the infrastructure for many disruptive applications, such as efficient long-distance quantum communication and distributed quantum computing^1,2. Central to these networks is the ability to distribute entanglement between distant nodes using photonic channels. Initially developed for quantum teleportation^3,4 and loophole-free tests of Bell's inequality^5,6, recently, entanglement distribution has also been achieved over telecom fibres and analysed retrospectively^7,8. Yet, to fully use entanglement over long-distance quantum network links it is mandatory to know it is available at the nodes before the entangled state decays. Here we demonstrate heralded entanglement between two independently trapped single rubidium atoms generated over fibre links with a length up to 33 km. For this, we generate atom-photon entanglement in two nodes located in buildings 400 m line-of-sight apart and to overcome high-attenuation losses in the fibres convert the photons to telecom wavelength using polarization-preserving quantum frequency conversion⁹. The long fibres guide the photons to a Bell-state measurement setup in which a successful photonic projection measurement heralds the entanglement of the atoms¹⁰. Our results show the feasibility of entanglement distribution over telecom fibre links useful, for example, for device-independent quantum key distribution^11-13 and quantum repeater protocols. The presented work represents an important step towards the realization of large-scale quantum network links.