Strongly anisotropic spin relaxation in the neutral silicon vacancy center in diamond

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 (T-1) of the neutral charge state of the silicon vacancy center in diamond. Above 20 K, T-1 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, T-1 is reduced by over three orders of magnitude. The electron spin coherence time (T-2) follows the same temperature dependence but is drastically shorter than T-1. 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.