Nanowire integration in silica based integrated optical circuits: Limitations and challenges towards quantum computing

Optical integrated circuits suggest a very promising platform for the development of robust and efficient quantum computers. A critical issue to their development and scalability is the integration of multiple single photon sources in the circuits. One major category of single photon sources is based on quantum dots that are embedded in semiconductor optical nanowires (NWQD) that allow their accurate handing and deterministic integration. Successful integrations of such nanowires have been reported in high index platforms like silicon or silicon nitride, with adequate coupling efficiency due the modal characteristics compatibility. On the other hand, Silica-on-Silicon is a major integration platform, which combined with new fabrication approaches like direct laser writing, for the definition of optical structures with refractive index modification, can provide the fabrication of highly optimized and tailor-made circuits by rapid prototyping. Considering for the first time the integration scenario of NWQD in laser written silica-based waveguides it is shown that the low (∼10-3) achievable refractive index contrast imposes strict limitations on the compatibility of such waveguides with NWQD resulting in general in low coupling efficiency. By considering several design and fabrication issues, suitable integration approaches with adequate efficiency are demonstrated, while also the limitations and challenges are revealed thus triggering new research directions.