The first half of this dissertation focuses on the coherent interactions of single nitrogen-vacancy (NV) center spins in diamond with light and uses this interaction to demonstrate a plethora of single spin control and measurement techniques. Based on the Faraday effect, a new and non-destructive technique to measure the spin state of individual NV centers via the polarization of interacting light was demonstrated. With this same spin-light interaction, light-generated unitary rotations of the spin along the energy eigenstate basis was also demonstrated, a result of the AC Stark effect. Through a theoretical framework, both Faraday and AC Stark responses are described and compared directly with each other, giving insight on unknown experimental conditions and sources of decoherence.

By then forming a $\Lambda$ system in the NV center optical excited state and coherently driving both optical transitions, the NV center spin could be directly polarized into any quantum superposition via coherent population trapping dynamics. By equally-detuning the optical driving fields from resonance, spin rotations about any axis were performed as a result of stimulated Raman transitions. Finally, by measuring the luminescence of the NV center during the CPT interaction, measurement of the NV center spin along any quantum basis was performed, a process which we call dark/bright-state projection. These basis-tunable techniques of optically-based spin control were then applied in succession to perform all-optical measures of NV center spin coherence. The second half of this dissertation focuses on the initial studies of and coherent interactions between new optically-addressable defect spin ensembles in silicon carbide.

After detailing the optical and spin spectroscopy of six such new defects in 4H SiC discovered by Will Koehl, various measurements of spin coherence (T1, T2, T2*) for these defects are described, as well as their temperature dependence. Excitingly, the spin coherence times measured are comparable to those of diamond NV centers, motivating their study as quantum-coherent systems in solids. Optically-addressable, spin-coherent defects are then observed and studied in both 6H and 3C polytypes of SiC, and the results compared between polytypes. Enabled by the existence of multiple spin species in the 6H polytype, double electron-electron resonance (DEER) was performed between pairs of distinct spin species. Analysis of the DEER response set limits on the otherwise unknown spin concentrations and optically-induced spin polarization fidelity for these defects.