  The controlled creation of defect center---nanocavity systems is one of the
outstanding challenges for efficiently interfacing spin quantum memories with
photons for photon-based entanglement operations in a quantum network. Here, we
demonstrate direct, maskless creation of atom-like single silicon-vacancy (SiV)
centers in diamond nanostructures via focused ion beam implantation with $\sim
32$ nm lateral precision and $< 50$ nm positioning accuracy relative to a
nanocavity. Moreover, we determine the Si+ ion to SiV center conversion yield
to $\sim 2.5\%$ and observe a 10-fold conversion yield increase by additional
electron irradiation. We extract inhomogeneously broadened ensemble emission
linewidths of $\sim 51$ GHz, and close to lifetime-limited single-emitter
transition linewidths down to $126 \pm13$ MHz corresponding to $\sim 1.4$-times
the natural linewidth. This demonstration of deterministic creation of
optically coherent solid-state single quantum systems is an important step
towards development of scalable quantum optical devices.
