Spin-photon entanglement of a single Er^3+ ion in the telecom band

Long-distance quantum communication using quantum repeaters is an enabling technology for secure communication, distributed quantum computing and quantum-enhanced sensing and metrology. As a building block of quantum repeaters, spin-photon entanglement has been demonstrated with both atomic and solid-state qubits. However, previously demonstrated qubits with long spin coherence do not directly emit photons into the low-loss telecom band that is needed for long-distance communication. Here, we demonstrate spin-photon entanglement using a single Er^3+ ion in a solid-state crystal, integrated into a silicon nanophotonic circuit. Direct emission into the telecom band enables an entanglement rate of 1. 48 Hz over 15. 6 km of optical fiber, with a fidelity of 73 (3) \%. This opens the door to large-scale quantum networks based on scalable nanophotonic devices and many spectrally multiplexed Er^3+ ions.