Efficient photon-pair emission from a nanostructured resonator and its theoretical description

Spontaneous parametric down-conversion (SPDC) in subwavelength nanostructures is a promising source of quantum light, owing to its multifunctionality and ability to generate complex quantum states. Nevertheless, the mechanisms governing photon-pair generation in such systems remain only partially understood. In particular, experimental investigations of key emission properties in individual resonators, such as directionality and spectral distribution, are still lacking, and predictive theoretical frameworks have not yet been experimentally validated. Here, we report the first measurement of the directional and spectral distributions of photon pairs generated via SPDC in a lithium niobate bullseye nanostructured resonator. Both distributions exhibit resonant behaviour, which we describe using an extended quasi-normal-mode theory. This comparison is enabled by photon-pair count rates of up to 0.45 Hz/mW-- to our knowledge, the highest reported for a nanostructured resonator. Our results provide new physical insight into nanoscale SPDC and represent an important step toward designing of efficient miniaturized quantum light sources.