  Integrated photonics has emerged as one of the most promising platforms for
quantum applications. The performances of quantum photonic integrated circuits
(QPIC) necessitate a demanding optimization to achieve enhanced properties and
tailored characteristics with more stringent requirements with respect to their
classical counterparts. In this study, we report on the simulation,
fabrication, and characterization of a series of fundamental components for
photons manipulation in QPIC based on silicon nitride. These include crossing
waveguides, multimode-interferometer-based integrated beam splitters (MMIs),
asymmetric integrated Mach-Zehnder interferometers (MZIs) based on MMIs, and
micro-ring resonators. Our investigation revolves primarily around the Visible
to Near-Infrared spectral region, as these devices are meticulously designed
and tailored for optimal operation within this wavelength range. By advancing
the development of these elementary building blocks, we aim to pave the way for
significant improvements in QPIC in a spectral region only little explored so
far.
