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.
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