  Color centers in diamond are a promising platform for quantum technologies,
and understanding their interactions with the environment is crucial for these
applications. We report a study of spin- lattice relaxation (T1) of the neutral
charge state of the silicon vacancy center in diamond. Above 20 K, T1 decreases
rapidly with a temperature dependence characteristic of an Orbach process, and
is strongly anisotropic with respect to magnetic field orientation. As the
angle of the magnetic field is rotated relative to the symmetry axis of the
defect, T1 is reduced by over three orders of magnitude. The electron spin
coherence time (T2) follows the same temperature dependence but is drastically
shorter than T1. We propose that these observations result from phonon-mediated
transitions to a low lying excited state that are spin conserving when the
magnetic field is aligned with the defect axis, and we discuss likely
candidates for this excited state.
