Mitigating indistinguishability issues in photon pair sources by delayed-pump intermodal four wave mixing

Large arrays of independent, pure and identical heralded single photon sources are an important resource for linear optical quantum computing protocols. In the race towards the development of increasingly ideal sources, delayed-pump intermodal four wave mixing (IFWM) in multimode waveguides has recently emerged as one of the most promising approaches. Despite this, fabrication imperfections still spoil the spectral indistinguishability of photon pairs from independent sources. Here we show that by tapering the width of the waveguide and by controlling the delay between the pump pulses, we add additional spectral tunability to the source while still inheriting all the distinctive metrics of the IFWM scheme. This feature is used to recover spectral indistinuishability in presence of fabrication errors. Under realistic tolerances on the waveguide dimensions, we predict >99.5% indistinguishability between independent sources on the same chip, and a maximum degradation of the heralded Hong-Ou-Mandel visibility <0.35%.