Quantum networks based on optically addressable spin qubits promise to enable secure communication, distributed quantum computing, and tests of fundamental physics. Scaling up quantum networks based on solid-state luminescent centers requires coherent spin and optical transitions coupled to photonic resonators. Here we investigate single Yb !"! #$ ions in yttrium orthovanadate coupled to a nanophotonic cavity. These ions possess optical and spin transitions that are first-order insensitive to magnetic field fluctuations, enabling optical linewidths less than 1 MHz and spin coherence times exceeding 30 ms for cavity-coupled ions. The cavity-enhanced optical emission rate facilitates efficient spin initialization and conditional single-shot readoutwith fidelity greater than 95%. These results showcase a solid-state platform based on single coherent rare-earth ions for the future quantum internet. Main text:The distribution of entanglement over long distances using optical quantum networks is an intriguing macroscopic quantum phenomenon with applications in quantum systems for advanced computing and secure communication (1, 2). Solid-state emitters coupled to photonic resonators (3) are promising candidates for implementing quantum light-matter interfaces necessary for scalable quantum networks. A variety of systems have been investigated for this purpose, including quantum dots and defects in diamond or silicon carbide (4-8). So far, the ability to scale up these systems has remained elusive and motivates the development of alternative platforms. A central challenge is identifying emitters that exhibit coherent optical and spin transitions while coupled to photonic cavities that enhance the optical transitions and arXiv:1907.12161v1 [quant-ph] 28 Jul 2019 channel emission into optical fibers. Ensembles of rare-earth ions (REIs) in crystals are known to possess highly coherent 4f-4f optical and spin transitions (9, 10), but only recently have single REIs been isolated (11, 12) and coupled to nanocavities (13, 14). The crucial next steps toward using single REIs for quantum networks are demonstrating long spin coherence and single-shot readout in photonic resonators. Here we demonstrate spin initialization, coherent optical and spin manipulation, and high-fidelity single-shot optical readout of the hyperfine spin state of single Yb !"! #$ ions coupled to a nanophotonic cavity fabricated in an yttrium orthovanadate (YVO) host crystal. The relevant energy level structure of Yb ions are coupled to a photonic crystal cavity with small mode volume ~1( KLM ⁄ ) # and large quality factor (1 × 10 P ) (Fig 1C, D. See SI 1.1). This enhances the emission rate, collection efficiency, and cyclicity of the optical transitions A and E via the Purcell effect (16). The qubit is initialized into |0⟩ / by optical and microwave pumping on F, A,and fe to empty | ⟩ / and |1⟩ / , followed by cavity-enhanced decay into |0⟩ / via E (Fig. 1A). A subsequent microwave pulse applied on / optionally initializes the ion into |1⟩ / . The |1⟩ /