  Motivated by the success of group IV colour centres in nanodiamonds (NDs) for
hybrid technology requiring a single photon source, we study single
germanium-vacancy (GeV$^-$) centres in NDs at room temperature with size
rangingfrom 10 to 50 nm and with remarkable spectral properties. We
characterize their zero-phonon line (ZPL), study their internal population
dynamics and compare their emission properties in the framework of a three
level model with intensity dependent de-shelving. Furthermore, we characterize
their lifetime, polarization and brightness. We find amaximum photon emission
count rate of 1.6 MHz at saturation. We also report a polarization visibility
of 92% from the fluorescence light, which potentially makes GeV$^-$ centres
good candidates for quantum key distribution (QKD)requiring polarized single
photons. We show that the GeV$^-$ in NDs presented in this work have a
comparable spectral stability compared to their bulk counterpart which is
needed for future applications using NDs.
