Exciton-Enhanced Spontaneous Parametric Down-Conversion in Two-Dimensional Crystals

We show that excitonic resonances and interexciton transitions can enhance the probability of spontaneous parametric down-conversion, a second-order optical response that generates entangled photon pairs. We benchmark our ab initio many-body calculations using experimental polar plots of second harmonic generation in ${\mathrm{NbOI}}_{2}$, clearly demonstrating the relevance of excitons in the nonlinear response. A strong double-exciton resonance in 2D ${\mathrm{NbOCl}}_{2}$ leads to giant enhancement in the second order susceptibility. Our work paves the way for the realization of efficient ultrathin quantum light sources.