  Nitrogen-vacancy (NV) centers feature outstanding properties like a spin
coherence time of up to one second as well as a level structure offering the
possibility to initialize, coherently manipulate and optically read-out the
spin degree of freedom of the ground state. However, only about three percent
of their photon emission are channeled into the zero phonon line (ZPL),
limiting both the rate of indistinguishable single photons and the
signal-to-noise ratio (SNR) of coherent spin-photon interfaces. We here report
on the enhancement of the SNR of the optical spin read-out achieved by tuning
the mode of a two-dimensional photonic crystal (PhC)cavity into resonance with
the NV-ZPL. PhC cavities are fabricated by focused ion beam (FIB) milling in
thin reactive ion (RIE) etched ultrapure single crystal diamond membranes
featuring modes with Q-factors of up to 8250 at mode volumes below one cubic
wavelength. NV centers are produced in the cavities in a controlled fashion by
a high resolution atomic force microscope (AFM) implantation technique. On
cavity resonance we observe a lifetime shortening from 9.0ns to 8.0ns as well
as an enhancement of the ZPL emission by almost one order of magnitude.
Although on resonance the collection efficiency of ZPL photons and the
spin-dependent fluorescence contrast are reduced, the SNR of the optical spin
read-out is almost tripled for the cavity-coupled NV centers.
