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Position determination of the phase modulator in the Sagnac interferometer used in quantum key distribution systems

Published Jan 1, 2025• 0 citations
Publisher
DOI
10.17586/1023-5086-2025-92-09-55-61

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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.

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An intrinsically stable quantum key distribution system (QKD) with six polarization states encoded by phase modulation is introduced. The encoder and decoder are in the same structures that consist of two polarizing Sagnac interferometers connected in tandem. The six polarization states are determined and distinguished by different sets of phase shifts induced by two respective electrically-driven integrated phase modulators. A mean visibility of interference fringes is kept stable at 97.58% for an hour's performance. Theoretical and experimental analyses show that the proposed QKD system features intrinsically stability immune from environment fluctuation.

Chinese Physics Letters•Oct 30, 2008•6 citations
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.
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%.
An intrinsically stable quantum key distribution system (QKD) with six polarization states encoded by phase modulation is introduced. The encoder and decoder are in the same structures that consist of two polarizing Sagnac interferometers connected in tandem. The six polarization states are determined and distinguished by different sets of phase shifts induced by two respective electrically-driven integrated phase modulators. A mean visibility of interference fringes is kept stable at 97.58% for an hour's performance. Theoretical and experimental analyses show that the proposed QKD system features intrinsically stability immune from environment fluctuation.