  A fully integrated quantum optical technology requires active quantum systems
incorporated into resonant optical microstructures and inter-connected in three
dimensions via photonic wires. Nitrogen vacancy-centres (NV-centres) in diamond
which are excellent photostable room temperature single-photon emitters are
ideal candidates for that purpose. Extensive research efforts to couple
NV-centres to photonic structures such as optical microresonators,
microcavities, and waveguides have been pursued. Strategies for integration
range from top-down fabrication via etching of diamond membranes to
sophisticated bottom-up assembly of hybrid structures using diamond
nanocrystals where the latter approach allows for deterministic coupling.
Recently, another approach based on the incorporation of nanodiamonds in soft
glass optical fibres via a melting process has been introduced. Here, we
utilize two-photon direct laser writing (DLW) to fabricate fully
three-dimensional (3D) structures from a photoresist mixed with a solution of
nanodiamonds containing NV-centres. For the first time, this approach
facilitates building integrated 3D quantum photonic elements of nearly
arbitrary shapes.
