  Quantum emitters coupled to plasmonic nanoantennas produce single photons at
unprecedentedly high rates in ambient conditions. This enhancement of quantum
emitters' radiation rate is based on the existence of optical modes with highly
sub-diffraction volumes supported by plasmonic gap nanoantennas. Nanoantennas
with gap sizes on the order of few nanometers have been typically produced
using various self-assembly or random assembly techniques. Yet, the difficulty
of controllably fabricate nanoantennas with the smallest mode sizes coupled to
pre-characterized single emitters until now has remained a serious issue
plaguing the development of quantum plasmonic devices. We demonstrate the
transfer of nanodiamonds with single nitrogen-vacancy (NV) centers to an
epitaxial silver substrate and their subsequent deterministic coupling to
plasmonic gap nanoantennas. Through fine control of the assembled nanoantenna
geometry, a dramatic shortening of the NV fluorescence lifetime was achieved.
We furthermore show that by preselecting NV centers exhibiting a photostable
spin contrast, a coherent spin dynamics can be measured in the coupled
configuration. The demonstrated approach opens unique applications of
plasmon-enhanced quantum emitters for integrated quantum information and
sensing devices.
