  Spin defects in van der Waals materials offer a promising platform for
advancing quantum technologies. Here, we propose and demonstrate a powerful
technique based on isotope engineering of host materials to significantly
enhance the coherence properties of embedded spin defects. Focusing on the
recently-discovered negatively charged boron vacancy center
($\mathrm{V}_{\mathrm{B}}^-$) in hexagonal boron nitride (hBN), we grow
isotopically purified $\mathrm{h}{}^{10}\mathrm{B}{}^{15}\mathrm{N}$ crystals.
Compared to $\mathrm{V}_{\mathrm{B}}^-$ in hBN with the natural distribution of
isotopes, we observe substantially narrower and less crowded
$\mathrm{V}_{\mathrm{B}}^-$ spin transitions as well as extended coherence time
$T_2$ and relaxation time $T_1$. For quantum sensing,
$\mathrm{V}_{\mathrm{B}}^-$ centers in our
$\mathrm{h}{}^{10}\mathrm{B}{}^{15}\mathrm{N}$ samples exhibit a factor of $4$
($2$) enhancement in DC (AC) magnetic field sensitivity. For additional quantum
resources, the individual addressability of the $\mathrm{V}_{\mathrm{B}}^-$
hyperfine levels enables the dynamical polarization and coherent control of the
three nearest-neighbor ${}^{15}\mathrm{N}$ nuclear spins. Our results
demonstrate the power of isotope engineering for enhancing the properties of
quantum spin defects in hBN, and can be readily extended to improving spin
qubits in a broad family of van der Waals materials.
