Fig. 1
QFC as a frequency-domain linear optics with the two polarization modes. a Concept of a non-polarizing frequency-domain half BS for TH(V) = RH(V) = 1/2. b A frequency-domain PBS for TH = RV = 1 and TV = RH = 0. c Energy diagram related to the QFC used in our experiment. The difference frequency generation of a photon from 780 nm (upper frequency) to 1522 nm (lower frequency) is performed by using a strong pump light at 1600 nm. d The experimental setup of dual-polarization-mode QFC. Type-0 quasi-phase-matched PPLN crystal as a nonlinear optical medium which acts on only the V-polarized photons is installed in the Sagnac type interferometer. The detailed explanation of the QFC is in the main text. DM dichroic mirror, HWP half wave plate, PBS polarizing beamsplitter. e Conversion efficiency from the front to the end of the QFC. It is measured by using classical laser light and a power meter. The error of the power meter was about 1% and they are smaller than the markers of data. Solid curves are fitted to data for CW and ACW directions separately by using functions \(\eta _{{\mathrm{max}}}{\kern 1pt} {\mathrm{sin}}^2\left( {\sqrt {gP} } \right)\), where P is the pump power. Parameters ηmax and g for CW(ACW) directions are estimated as 0.39 (0.33) and 5.8/W (10.7/W), respectively. A broken curve is fitted to two data set together, resulting in ηmax = 0.34 and g = 8.5/W
