  The silicon-vacancy ($\mathrm{SiV}^-$) color center in diamond has attracted
attention due to its unique optical properties. It exhibits spectral stability
and indistinguishability that facilitate efficient generation of photons
capable of demonstrating quantum interference. Here we show high fidelity
optical initialization and readout of electronic spin in a single
$\mathrm{SiV}^-$ center with a spin relaxation time of $T_1=2.4\pm0.2$ ms.
Coherent population trapping (CPT) is used to demonstrate coherent preparation
of dark superposition states with a spin coherence time of $T_2^\star=35\pm3$
ns. This is fundamentally limited by orbital relaxation, and an understanding
of this process opens the way to extend coherences by engineering interactions
with phonons. These results establish the $\mathrm{SiV}^-$ center as a
solid-state spin-photon interface.
