  The silicon-vacancy center in diamond offers attractive opportunities in
quantum photonics due to its favorable optical properties and optically
addressable electronic spin. Here, we combine both to achieve all-optical
coherent control of its spin states. We utilize this method to explore spin
dephasing effects in an impurity-rich sample beyond the limit of phonon-induced
decoherence: Employing Ramsey and Hahn-echo techniques at 12mK base temperature
we identify resonant coupling to a substitutional nitrogen spin bath as the
limiting decoherence source for the electron spin.
