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

Entangled photon pairs from spontaneous parametric down-conversion (SPDC)¹ are central to many quantum applications^2-6. SPDC is typically performed in non-centrosymmetric systems⁷ with an inherent second-order nonlinearity (χ⁽²⁾)^8-10. We demonstrate strong narrowband SPDC with an on-chip rate of 0.8 million pairs per second in Si₃N₄. Si₃N₄ 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⁽²⁾, which limits its role in photonic quantum devices. We enabled SPDC in Si₃N₄ 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 Si₃N₄ integrated photonics technology, unlocks new avenues for quantum systems on a chip.