  Coupling between a single quantum emitter and an optical cavity presents a
key capability for future quantum networking applications. Here, we explore
interactions between individual germanium-vacancy (GeV) defects in diamond and
an open microcavity at cryogenic temperatures. Exploiting the tunability of our
microcavity system to characterize and select emitters, we observe a
Purcell-effect-induced lifetime reduction of up to $4.5\pm0.3$, and extract
coherent coupling rates up to $360\pm20$ MHz. Our results indicate that the GeV
defect has favorable optical properties for cavity coupling, with a quantum
efficiency of at least $0.34\pm0.05$ and likely much higher.
