  Engineering coherent systems is a central goal of quantum science. Color
centers in diamond are a promising approach, with the potential to combine the
coherence of atoms with the scalability of a solid state platform. However, the
solid environment can adversely impact coherence. For example, phonon- mediated
spin relaxation can induce spin decoherence, and electric field noise can
change the optical transition frequency over time. We report a novel color
center with insensitivity to both of these sources of environmental
decoherence: the neutral charge state of silicon vacancy (SiV0). Through
careful material engineering, we achieve over 80% conversion of implanted
silicon to SiV0. SiV0 exhibits excellent spin properties, with spin-lattice
relaxation times (T1) approaching one minute and coherence times (T2)
approaching one second, as well as excellent optical properties, with
approximately 90% of its emission into the zero-phonon line and near-transform
limited optical linewidths. These combined properties make SiV0 a promising
defect for quantum networks.
