Down-converted photon pairs in a high-Q silicon nitride microresonator.

Entangled photon pairs from spontaneous parametric down-conversion (SPDC) are central to many quantum applications. SPDC is typically performed in non-centrosymmetric systems with an inherent second-order nonlinearity (χ). We demonstrate strong narrowband SPDC with an on-chip rate of 0.8 million pairs per second in SiN. SiN is the pre-eminent material for photonic integration and also exhibits the lowest waveguide loss (which is essential for integrated quantum circuits). However, being amorphous, silicon nitride lacks an intrinsic χ, which limits its role in photonic quantum devices. We enabled SPDC in SiN by combining strong light-field enhancement inside a high optical Q-factor microcavity with an optically induced space-charge field. We present narrowband photon pairs with a high spectral brightness. The quantum nature of the down-converted photon pairs is verified through coincidence measurements. This light source, based on SiN integrated photonics technology, unlocks new avenues for quantum systems on a chip.