  Striving for nanometer-sized solid-state single-photon sources, we
investigate atom-like quantum emitters based on single germanium vacancy (GeV)
centers isolated in crystalline nanodiamonds (NDs). Cryogenic characterization
indicated symmetry-protected and bright (> 10^6 counts/s with off-resonance
excitation) zero-phonon optical transitions with up to 6-fold enhancement in
energy splitting of their ground states as compared to that found for GeV
centers in bulk diamonds (i.e., up to 870 GHz in highly strained NDs vs 150 GHz
in bulk). Utilizing lithographic alignment techniques, we demonstrate an
integrated nanophotonic platform for deterministic interfacing plasmonic
waveguides with isolated GeV centers in NDs that enables 10-fold enhancement of
single-photon decay rates along with the emission direction control by
judiciously designing and positioning a Bragg reflector. This approach allows
one to realize the unidirectional emission from single-photon dipolar sources
introducing a novel method that is alternative to the
propagation-direction-dependent techniques based on chiral interactions or
topological protection. The developed plasmon-based nanophotonic platform opens
thereby new perspectives for quantum nanophotonics in general and for realizing
entanglement between single photons and spin qubits, in particular.
