  The silicon monovacancy in 4H-SiC is a promising candidate for solid-state
quantum information processing. We perform high-resolution optical spectroscopy
on single V2 defects at cryogenic temperatures. We find favorable low
temperature optical properties that are essential for optical readout and
coherent control of its spin and for the development of a spin-photon
interface. The common features among individual defects include two narrow,
nearly lifetime-limited optical transitions that correspond to $m_s{=}\pm 3/2$
and $m_s{=}\pm 1/2$ spin states with no discernable zero-field splitting
fluctuations. Initialization and readout of the spin states is characterized by
time-resolved optical spectroscopy under resonant excitation of these
transitions, showing significant differences between the $\pm 3/2$ and $\pm
1/2$ spin states. These results are well-described by a theoretical model that
strengthens our understanding of the quantum properties of this defect.
