would potentially allow room temperature operations, and possibly the elevated temperatures required for telecoms devices. Finally, it would only require a coherent pump source such as a standard fiber-coupled telecom laser, thus providing a compact, stable, and relatively low cost device. It is a common perception that ordinary nonlinear media are intrinsically unable to display significant single-photon nonlinear behavior and a very large number of photons is necessary to produce appreciable effects, owing to the small value of the $\chi^{(3)}$ elements in ordinary bulk media [@boyd_book]. However, interesting perspectives might come from a proper nano-structuring of passive nonlinear materials, which could lead to strongly enhanced effective nonlinearities down to the single-photon level [@ferretti2012], especially using photonic crystal type confinement allowing to reach ultra-high quality factors [@high_Q] and ultra-small confinement volumes [@arakawa2011] in a purely dielectric structure. Even if potentially not unrealistic, such goal is still quite challenging with single resonators.

Here we report on an alternative strategy to realize an integrated SPS based on non-resonant Kerr-type nonlinearity, which exploits the effects of quantum interference in coupled photonic resonators [@savona10prl; @bamba]. These effects have been shown to arise even for weak nonlinear response, namely for an anharmonic shift of the two-photon 