Identification of optically active quartet spin centers based on a Si vacancy in SiC promising for quantum technologies

Optically active (bright) and optically inactive (dark) quartet $S = 3/2$ spin color centers including a negatively charged Si vacancy have been identified in silicon carbide using high-frequency electron nuclear double resonance on the nuclei of the $^{13}$C isotope, enhanced by a tenfold increase in its content. The alignment of populations of spin levels is optically induced in a bright center promising for quantum technologies, whereas the populations of spin levels in a dark center, which is an isolated negatively charged Si vacancy V$_{\text{Si}}^-$, correspond to a Boltzmann distribution and do not change under optical excitation.