PROBLEM TO BE SOLVED: To efficiently generate a polarization-entangled photon couple of 1,550 nm in wavelength.
SOLUTION: A polarization beam splitter 2 demultiplexes a 45°-polarized femtosecond pulse 1 from a femtosecond pulse 1. A Sagnac interferometer is composed of a polarization maintaining optical fiber 5 connected to the polarization beam splitter 2. In the Sagnac interferometer, two waveguide type PPLNs are incorporated which are developed as wavelength converting elements for optical communication. The 1st waveguide type PPLN3 and 2nd waveguide type PPLN4 are pumped from both sides with light from the femtosecond pulse laser to generate a polarization-entangled photon couple of 1,550 nm in wavelength. Thus, the polarization entangled photon couple with high purity can be highly efficiently generated.