  We demonstrate that silicon carbide (SiC) with natural isotope abundance can
preserve a coherent spin superposition in silicon vacancies over unexpectedly
long time approaching 0.1 seconds. The spin-locked subspace with drastically
reduced decoherence rate is attained through the suppression of heteronuclear
spin cross-talking by applying a moderate magnetic field in combination with
dynamic decoupling from the nuclear spin baths. We identify several
phonon-assisted mechanisms of spin-lattice relaxation, ultimately limiting
quantum coherence, and find that it can be extremely long at cryogenic
temperature, equal or even longer than 8 seconds. Our approach may be extended
to other polyatomic compounds and open a path towards improved qubit memory for
wafer-scale quantum techmologies.
