Figure 1: Schematic of the on-chip quantum interference measurement set-up.

A 775-nm continuous wave (cw) diode laser is used as a pump for generating orthogonally polarized 1,550 nm photon pairs via type-II spontaneous parametric down conversion in a 10.5-mm-long fibre-coupled periodically poled KTP waveguide. Input polarizations to the ppKTP source and a polarizing fibre beam splitter (PBS) are adjusted with fibre polarization controllers (PC) for optimal SPDC efficiency and deterministic splitting of photon pairs into separate PBS-output modes, respectively. Temporal delay between photons in separate output modes is set with a fibre-free-space-fibre optical delay line. Light is then guided into a closed-cycle cryostat where it is coupled from an optical fibre array into on-chip SiN photonic circuits via optical grating couplers at In 1 and In 2 (optical micrograph). The alignment of the chip to the fibre array with low-temperature nanopositioners (attocube) is aided by monitoring the optical transmission at the auxiliary ports Out 1 and Out 2. Photons interfere at a 33-μm-long directional coupler (beam splitter, BS) with 400 nm gap between waveguides in the coupling region. SSPDs on top of the beam splitter’s output waveguides are supplied with a 10–15 μA bias from a low-noise current source and read out via a rf-probe connected to a bias-T outside the cryostat. After signal amplification (PPL 5828 & RF-Bay LNA-4050), photon statistics are recorded with a time-correlated single-photon counting unit (PicoHarp 300).