  The realization of quantum networks critically depends on establishing
efficient, coherent light-matter interfaces. Optically active spins in diamond
have emerged as promising quantum nodes based on their spin-selective optical
transitions, long-lived spin ground states, and potential for integration with
nanophotonics. Tin-vacancy (SnV$^{\,\textrm{-}}$) centers in diamond are of
particular interest because they exhibit narrow-linewidth emission in
nanostructures and possess long spin coherence times at temperatures above 1 K.
However, a nanophotonic interface for SnV$^{\,\textrm{-}}$ centers has not yet
been realized. Here, we report cavity enhancement of the emission of
SnV$^{\,\textrm{-}}$ centers in diamond. We integrate SnV$^{\,\textrm{-}}$
centers into one-dimensional photonic crystal resonators and observe a 40-fold
increase in emission intensity. The Purcell factor of the coupled system is 25,
resulting in channeling of the majority of photons ($90\%$) into the cavity
mode. Our results pave the way for the creation of efficient, scalable
spin-photon interfaces based on SnV$^{\,\textrm{-}}$ centers in diamond.
