Lifetime Reduction of Single Germanium-Vacancy Centers in Diamond via a Tunable Open Microcavity

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 ($\mathrm{Ge}$V) 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 $4.5\pm 0.3$ and extract coherent-coupling rates up to $360\pm 20$ MHz. Our results indicate that the $\mathrm{Ge}$V defect has favorable optical properties for cavity coupling, with a quantum efficiency of at least $0.34\pm 0.05$ and likely much higher.