Hybrid integrated photonics for on-chip heralded N00N state generation

International audience; Recent progress in the field of quantum information processing has highlighted the prospects of using integrated optical devices for quantum applications. Integrated quantum photonics offers several advantages compared to free-space setups. Not only the miniaturization, which dramatically reduces the size of the building blocks and allows imprinting or cascading several functions on a single substrate, but also the possibility to reproduce the same photonic circuit many times on the same chip. For instance, this has been exploited to spatially multiplex heralded single-photon, leading to an increase of the single-photon emission rate at constant noise level. More strikingly, one can think of combining, on-chip, several synchronised single photon sources towards engineering large photon-number entangled states. Despite this attractive potential, only few examples of spatial mul- tiplexing have been reported in the literature due to the technological challenge related to fabrication processes. On one hand, lithium nio- bate (LN) is very suitable for quantum integrated photonics since it shows both optic-optic and electro-optic non-linearities, as well as the possibility to integrate low-loss waveguides. However, several parallel and/or cascaded optical functions require various lithographic steps leading to reduced yields. On the other hand, femto-second laser direct-writing (FLDW) technique on glass-type substrates, allows fast fabrication of low-loss waveguide circuits requiring no lithographic masks nor chemical exchange. However, no efficient non-linear processes are available in SiO2 waveguides for photon-pair generation. We discuss here an integrated photonic chip able to generate photon-number states which consists of three photonic chips fabricated on either LN for photon generation or on SiO2 substrates for photon manipulation purposes. Our approach takes advantage of the best features of both worlds.