This thesis studies rare-earth ions doped in solid-state hosts for scalable quantum interconnects. Rare-earth ions are an ideal candidate for long-distance entanglement between physically distant and disparate quantum systems due to their long spin coherence times and narrow optical linewidths. Among the family of rare-earth elements, trivalent erbium (Er3+) is particularly interesting due to its optical emission in the low-loss telecom C-band used in existing fiber-optic networks. However, prior demonstrations of erbium qubits are limited to dopants in bulk host crystals with either degraded optical coherence or short spin coherence times, which prevent further advancement of this emerging quantum technology. We develop a new rare-earth qubit platform based on epitaxially grown, single crystal thin films to address these challenges. Specifically, using cavity-enhanced, spin and optical spectroscopy, we demonstrate milliseconds spin coherence times of Er3+ qubits in yttrium oxide (Y2O3) thin films and gain an understanding of the environment-induced decoherence mechanisms with suitable techniques to mitigate them. We also study the role played by symmetry in protecting erbium from environmental noise and report kHz optical dephasing rate of erbium telecom qubits. These results combined demonstrate a significant prospect of rare-earth qubits in epitaxial single-crystal films as a spin-photon interface for quantum network applications.