Long-lived remote ion-ion entanglement for scalable quantum repeaters

Quantum networks, integrating quantum communication, quantum metrology and distributed quantum computing, could provide secure and efficient information transfer, high-resolution sensing and an exponential speed-up in information processing¹. Deterministic entanglement distribution over long distances is a prerequisite for scalable quantum networks^2-5. However, the exponential photon loss in optical fibres prohibits efficient and deterministic entanglement distribution. Quantum repeaters⁶, incorporating entanglement swapping^4,7,8 and entanglement purification^9-11 with quantum memories, offer the most promising means to overcome this limitation in fibre-based quantum networks. Despite numerous pioneering efforts^12-25, a critical bottleneck remains, as remote memory-memory entanglement suffers from decoherence more rapidly than it can be established and purified over long distances. Here we demonstrate memory-memory entanglement between two nodes connected by 10 km of spooled fibre surviving beyond the average entanglement establishment time. This is enabled by the development of long-lived trapped-ion memories, an efficient telecom interface and a high-visibility single-photon entanglement protocol^26,27. As an application, we report a proof-of-principle device-independent quantum key distribution demonstration with finite-size analysis over 10 km and a positive key rate over 101 km in the asymptotic limit, with both distances exceeding previous work by more than 2 orders of magnitude^28-30. Our work provides a critical building block for quantum repeaters and marks an important step towards scalable quantum networks.