Spectral distribution of widely nondegenerate parametric down-conversion and generation of heralded Fock statesEntangled photon pairs of wide nondegeneration are attracting more interest recently, which gives rise to an urgent demand for their spectral distribution. Analyzing deeply the widely nondegenerate parametric down-conversion (WND-PDC), we put forward simplified expressions to describe the absolute spectral distribution of entangled photons, which have been experimentally demonstrated by the 532-nm pumped WND-PDC in BBO crystal. Our measured spectrum curves of both signal and idler photons agree well with the theoretical results in the whole spectrum range over 100 nm even 400 nm and more. We further reveal new features of spectral distinguishability and bandwidth broadening which show that, as signal wavelength increases, firstly the entangled photons are spectral distinguishable with ascertainable bandwidths, then partly indistinguishable without any measurable bandwidth, finally they are completely indistinguishable with the same maximum bandwidth ~500 nm. Increasing signal wavelength, the idler bandwidth goes broader as well as the signal's although its wavelength gets shorter. Utilizing photon-number-resolving single photon detectors, we observe the average photon number goes down much slower than the spectral intensity as wavelength moving away from the peak, and the photon numbers obey the Poisson and multi-thermal distributions well for wavelengths around the peak, but near spectrum boundaries the best one is thermal distribution. Finally, we use the signal photons as the trigger to generate heralded Fock states up to 10-photons in near-infrared range, which are available for quantum key distribution in practical optical fiber network.
