Single germanium vacancy centers in nanodiamonds with bulk-like spectral stability

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