  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.
