Fig. 2: Schematic of the experimental setup.

The 780 nm pump from a narrow-band laser diode is sent to the PPSF through a wavelength division multiplexer (WDM). The polarization of the pump is controlled by a polarization controller (PC1) for type II phase-matched SPDC. The biphotons generated from the forward and backward paths interfere, producing an output state, which is sent to the measurement apparatus. In inset a, the L/C band splitter separates signal (λ > 1564 nm) and idler (λ < 1564 nm) into two spatial modes, where we can use polarization analyzers (PA_1 and PA_2) to characterize the polarization state through quantum state tomography (QST). This setup is also used to obtain frequency-resolved QST by inserting a dispersion compensating module (DCM). In inset b, we use this DCM to temporally disperse the biphotons for spectral measurements³.