Fig. 1: Schematic of the SU(1,1) interferometer for FTIR with undetected photons.

HWP half-wave plate, DM dichroic mirror, M mirror, LPFs long-pass filters, PD photodetector. Either ppKTP or apKTP is used as the nonlinear crystal for the interferometer. The inset shows a signal interferogram generated by the ppKTP crystal without any sample, which is measured with the Si power meter and recorded with the software for the console (PM100D, Thorlabs).