  Spin impurities in diamond can be versatile tools for a wide range of
solid-state-based quantum technologies, but finding spin impurities which offer
sufficient quality in both photonic and spin properties remains a challenge for
this pursuit. The silicon-vacancy center has recently attracted a lot of
interest due to its spin-accessible optical transitions and the quality of its
optical spectrum. Complementing these properties, spin coherence is essential
for the suitability of this center as a spin-photon quantum interface. Here, we
report all-optical generation of coherent superpositions of spin states in the
ground state of a negatively charged silicon-vacancy center using coherent
population trapping. Our measurements reveal a characteristic spin coherence
time, T2*, exceeding 250 nanoseconds at 4 K. We further investigate the role of
phonon-mediated coupling between orbital states as a source of irreversible
decoherence. Our results indicate the feasibility of all-optical coherent
control of silicon-vacancy spins using ultrafast laser pulses.
