  We report on the coherent internal-state control of single crystalline
nanodiamonds, containing on average 1200 nitrogen-vacancy (NV) centers,
embedded in three-dimensional direct-laser-written waveguides. We excite the NV
centers by light propagating through the waveguide, and we show that emitted
fluorescence can be efficiently coupled to the waveguide modes. We find an
average coupling efficiency of 21.6% into all guided modes. Moreover, we
investigate optically-detected magnetic-resonance spectra as well as Rabi
oscillations recorded through the waveguide-coupled signal. Our work shows that
the system is well suited for magnetometry and remote read-out of spin
coherence in a freely configurable waveguide network, overcoming the need for
direct optical access of NV centers in nanodiamonds. These waveguide-integrated
sensors might open up new applications, like determining magnetic field
distributions inside opaque or scattering media, or photosensitive samples,
like biological tissue.
