  We demonstrate an exceptionally bright photon source based on a single
nitrogen- vacancy center (NV-center) in a nanodiamond (ND), placed in the
nanoscale gap between two monocrystalline silver cubes in a dimer
configuration. The system is operated near saturation at a stable photon rate
of 850 kcps, while we further achieve strongly polarized emission and high
single photon purity, evident by the measured auto-correlation with a
g(2)(0)-value of 0.08. These photon source features are key parameters for
quantum technological applications, such as secure communication based on
quantum key distribution. The cube antenna is assembled with an atomic force
microscope, which allows us to predetermine the dipole orientation of the
NV-center and optimize cube positioning accordingly, while also tracking the
evolution of emission parameters from isolated ND to the 1 and 2 cube
configuration. The experiment is well described by finite element modelling,
assuming an instrinsic quantum efficiency of 0.35. We attribute the large
photon rate of the assembled photon source, to increased quantum efficiency of
the NV-center and high antenna efficiency.

Optics