  Quantum emitters, such as the negatively charged nitrogen-vacancy center in
diamond, are attractive for quantum technologies such as nano-sensing, quantum
information processing, and as a non-classical light source. However, it is
still challenging to position individual emitters in photonic structures whilst
preserving the spin coherence properties of the defect. In this paper, we
investigate single and ensemble waveguide-integrated nitrogen-vacancy centers
in diamond fabricated by femtosecond laser writing followed by thermal
annealing. Their spin coherence properties are systematically investigated and
are shown to be comparable to native nitrogen-vacancy centers in diamond. This
method paves the way for the fabrication of coherent spins integrated within
photonic devices.
