Time-bin polarization format exchange technique for entangled optical source

A quantum key distribution system comprising:

• a photon generator for generating a first photon and a second photon which has shorter wavelength than the wavelength of the first photon;
• an asymmetric Mach-Zehnder interferometer into which the first photon generated by the photon generator enters;
• and
• a polarization and time delay adder that gives polarization and time delay to the second photon generated by the photon generator;
• wherein the polarization and time delay adder comprises:
• a polarization controller into which the second photon enters;
• a first beam splitter which separate photon from the polarization controller based on its plane of polarization;
• a second beam splitter into which one part of photon separated by the first beam splitter and remaining photon separated by the first beam splitter enter;
• and
• a first time delay adder that can provide time delay with the remaining photon separated by the first beam splitter compared to the one part of photon separated by the first beam splitter.