Spectral correlations of photon-pairs: a comparison of SPDC-based sources

The generation of spectrally uncorrelated single photons and spectrally correlated photon pairs is crucial for catering to various applications in quantum technology. While spectrally correlated biphotons improve versatility, measurement accuracy and security in quantum communication and networking systems, spectrally pure photons can be used for quantum key distribution systems and linear optical quantum computational networks. The joint spectral intensity (JSI) measurements of spontaneous parametric downconversion (SPDC) based photon sources give a holistic view of correlations present in the generated photon pairs. In this work, we compare JSI measurements of three sources which include type-0 fiber coupled Zn indiffused MgO:ppLN ridge waveguide (4 cm), type II MgO:ppLN ridge waveguide (1.69 cm) and free-space ppKTP crystal (3 cm). We observed the JSI plots of two sources, broadband (~46 nm) in type-0 interaction and narrowband (~2.4 nm) in type II interaction, are negatively inclined revealing strong spectral correlations. The JSI plot of type II SPDC based ppKTP crystal also depicts spectral correlations in an even narrower band (~0.62 nm) owing to longer length of the crystal. However, careful selection of aperture size placed after the crystal can help in the generation of spectrally pure single photons as well. The results show that the width of observed JSI depends on the inherent emission bandwidth of photon sources which further depends on the dispersion properties of the material, length of the crystal/waveguide, and filter bandwidths used in the experiment.