Time-resolved purification of photon pairs from ultrasmall sources

Generation of entangled photons through spontaneous parametric down-conversion (SPDC) from ultrasmall sources like thin films, metasurfaces, or nanoantennas, offers unprecedented freedom in quantum state engineering. However, as the source of SPDC gets smaller, the role of photoluminescence increases, which leads to the contamination of two-photon states with thermal background. Here we propose and implement a solution to this problem: by using pulsed SPDC and time distillation, we increase the purity and the heralding efficiency of the photon pairs. In the experiment, we increase the purity of two-photon states generated in a 7 $\mu$m film of lithium niobate from 0.002 to 0.99. With the higher purity, we were able to observe and characterize different polarization states of photon pairs generated simultaneously due to relaxed phase matching. In particular, we showed the presence of orthogonally polarized photons, potentially usable for the generation of polarization entanglement.