Patterning-effect-free intensity modulator for secure decoy-state quantum key distribution

TL;DR

This paper presents a Sagnac-based intensity modulator that eliminates patterning effects and enhances the security and stability of practical decoy-state quantum key distribution systems.

Contribution

It introduces a tunable extinction ratio Sagnac interferometer that prevents patterning effects and improves stability in high-rate QKD implementations.

Findings

No patterning effects in decoy state patterns

Enhanced stability of the intensity modulator over time

Immunity to DC drifts in the Sagnac interferometer

Abstract

Quantum key distribution (QKD) is a technology that allows two users to exchange keys securely. The decoy state technique enhances the technology, ensuring keys can be shared at high bit rates over long distances with information theoretic security. However, imperfections in the implementation, known as side-channels, threaten the perfect security of practical QKD protocols. Intensity modulators are required for high-rate decoy state QKD systems, although these are unstable and can display a side channel where the intensity of a pulse is dependent on the previous pulse. Here we demonstrate the superior practicality of a tunable extinction ratio Sagnac-based intensity modulator (IM) for practical QKD systems. The ability to select low extinction ratios, alongside the immunity of Sagnac interferometers to DC drifts, means that random decoy state QKD patterns can be faithfully reproduced with no patterning effects. The inherent stability of Sagnac interferometers also ensures that the modulator output does not wander over time.