Towards quantum-enhanced nonlinear imaging

In recent years, the study of quantum effects in nonlinear light-matter interactions, such as two-photon absorption (TPA) and second-harmonic generation (SHG) driven by nonclassical light, has garnered renewed interest. Debates persist regarding the nature and validity of the observed enhanced nonlinear signals, and there is scepticism about the applicability of quantum effects in two-photon interactions due to the low signal levels typically generated. To contribute to this topic, we investigated second-harmonic generation pumped by parametric down-conversion (PDC), exploring both the spontaneous regime (photon pairs) and higher photon fluxes. We observed a quantum effect for high-flux PDC, exceeding the one-photon-per-mode limit. The observation of quantum effects beyond the conventional quantum-advantage boundaries in SHG or TPA suggests potential avenues for leveraging quantum two-photon interactions in imaging and metrology.