Spectral photon-number distribution of parametric down-conversion and generation of heralded Fock states

Abstract Photon pairs with large nondegeneracy have recently attracted increasing interest, which gives rise to an urgent demand for revealing their complete and accurate spectral distribution. By thoroughly analyzing parametric down-conversion (PDC), we put forward a model to directly describe the spatial-spectral distribution of these photon pairs, which is experimentally demonstrated by a 532-nm pumped type-I PDC in a beta barium borate (BBO) crystal. The measured spectral curves show good agreement with the theoretical predictions over the entire spectral range. We further demonstrate that, as signal wavelength increases, the photon pairs are initially spectrally distinguishable, then partly indistinguishable, finally completely indistinguishable with a maximum bandwidth of approximately 500 nm. Utilizing photon-number-resolving single-photon detectors (SPD), we observe the average photon number decreases significantly more slowly than the spectral intensity as the wavelength deviates from the peak, and the photon numbers follow a quasi-Poisson distribution well for wavelengths around the peak, but a thermal distribution better describes the statistics near the spectral boundaries. Finally, we use the signal photons as the trigger to generate heralded Fock states up to 10 photons in near-infrared range, which are suitable for quantum simulation and quantum key distribution in optical fiber networks.