Estimating the phase modulator location in Sagnac interferometers for quantum key distribution systems

Subject of study . A phase modulation device for encoding quantum states based on an electro-optical phase modulator asymmetrically located in a Sagnac interferometer is studied. Aim of study. An easily implemented and repeatable method for measuring the path difference to a phase modulator inside a Sagnac interferometer in quantum key distribution systems is developed. Method. A phase difference between the clockwise and counterclockwise continuous propagating light is introduced in the beamsplitter of a Sagnac interferometer when an electrical pulse is transmitted to a phase modulator. Moreover, the asymmetric location of the phase modulator means that this single electrical pulse introduces twice the optical phase difference. Main results. A technique for measuring the difference in propagation time for light traveling in opposite directions to reach the phase modulator in a Sagnac interferometer was proposed and experimentally tested; this technique can be used for simultaneous modulation of phase and intensity in quantum key distribution systems. Practical significance. Our results regarding this highly accurate and easily implementable technique for measuring the difference in propagation time for light traveling in opposite directions to reach the asymmetrically positioned phase modulator in a Sagnac interferometer are relevant for the production of optoelectronic modules used in quantum key distribution systems, where this time difference directly affects the system quality and overall performance.