  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 $\mathrm{{}^{171}Yb^{3+}}$ 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 readout with fidelity greater than 95%. These results showcase a
solid-state platform based on single coherent rare-earth ions for the future
quantum internet.
