  Negatively charged boron vacancies ($V_B^-$) in hexagonal boron nitride (hBN)
exhibit a broad emission spectrum due to strong electron-phonon coupling and
Jahn-Teller mixing of electronic states. As such, the direct measurement of
zero-phonon line (ZPL) of $V_B^-$ has remained elusive. Here, we measure the
room-temperature ZPL wavelength to be $773\pm2$ nm by coupling the hBN layer to
the high-Q nanobeam cavity. As the wavelength of cavity mode is tuned, we
observe a pronounced intensity resonance, indicating the coupling to $V_B^-$.
Our observations are consistent with the spatial redistribution of $V_B^-$
emission. Spatially resolved measurements show a clear Purcell effect maximum
at the midpoint of the nanobeam, in accord with the optical field distribution
of the cavity mode. Our results are in good agreement with theoretical
calculations, opening the way to using $V_B^-$ as cavity spin-photon
interfaces.
