Fig. 2: Generation of frequency-tunable entangled photon pairs.

a dashed line - ‘singles’ measurement of overall signal spectrum generated by the ppKTP crystal at 56 °C, measured with a TWINS interferometer in front of the SPAD. The spectral correlations allow to select different signal spectra (solid lines, grouped under the ‘signal’ label) by placing a tunable interference filter on the herald arm (solid lines, collected under the ‘herald’ label); b SPDC spectra generated by the ppKTP crystal for different temperatures, showing tunability from 670 nm to 1100 nm; c g⁽²⁾ measurement of the signal pulses performed with an HBT interferometer; d IRF of the system measured detecting coincidences between signal and idler pulses, with two SPADs from MPD and Excelitas. The FWHM of the response is 260 ps (MPD) and 600 ps (Excelitas) respectively. The correlation times measured by TCSPC are limited by the IRF of the detectors and are significantly longer than the intrinsic bi-photon correlation times that arise from SPDC.