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Device-independent quantum key distribution (DIQKD) enables the generation of secret keys over an untrusted channel using uncharacterized and potentially untrusted devices 1-9 . The proper and secure functioning of the devices can be certified by a statistical test using a Bell inequality 10-12 . This test originates from the foundations of quantum physics and also ensures robustness against implementation loopholes 13 , thereby leaving only the integrity of the users' locations to be guaranteed by other means. The realization of DIQKD, however, is extremely challenging-mainly because it is difficult to establish high-quality entangled states between two remote locations with high detection efficiency. Here we present an experimental system that enables for DIQKD between two distant users. The experiment is based on the generation and analysis of event-ready entanglement between two independently trapped single rubidium atoms located in buildings 400 metre apart 14 . By achieving an entanglement fidelity of [Formula: see text] and implementing a DIQKD protocol with random key basis 15 , we observe a significant violation of a Bell inequality of S = 2.578(75)-above the classical limit of 2-and a quantum bit error rate of only 0.078(9). For the protocol, this results in a secret key rate of 0.07 bits per entanglement generation event in the asymptotic limit, and thus demonstrates the system's capability to generate secret keys. Our results of secure key exchange with potentially untrusted devices pave the way to the ultimate form of quantum secure communications in future quantum networks., (© 2022. The Author(s).)

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 conversion 9 . The long fibres guide the photons to a Bell-state measurement setup in which a successful photonic projection measurement heralds the entanglement of the atoms 10 . 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., (© 2022. The Author(s).)