Polarization-Multiplexed Phase Modulation of Single Photons in a Sagnac Interferometer

Polarization-multiplexed phase modulation was demonstrated in a Sagnac interferometer for polarization-auto-compensating quantum key distribution. The Sagnac interferometer was composed of a polarization beam splitter to split the signal pulses into clockwise and counter-clockwise beams of different polarizations, a Faraday rotator to rotate the polarizations of the split clockwise and counter-clockwise beams, and two phase-modulators asymmetrically set in the Sagnac ring. With this structure, the delays of the electronic triggering signals for the two phase-modulators could be set separately, reducing the difficulty in the setup and achieving more precise phase modulation on different polarized beams. Such polarization-multiplexed phase modulation was appropriately gated in sequential time gates to ensure the long-term stability of automatic polarization compensation in the long-distance quantum key distribution system. Polarization-multiplexed phase modulation was implemented in a protocol quantum key distribution system of 25-km communicating fiber length, generating 20-kHz sifted key rate with a qubit error rate lower than 5%.