Real-Time Polarization Control for Satellite QKD with Liquid-Crystal Beacon Stabilization

TL;DR

This paper presents a real-time polarization control method using liquid-crystal retarders and a beacon signal to improve satellite QKD by compensating atmospheric and motion-induced polarization distortions.

Contribution

The work introduces a compact, fast LC-based polarization compensation system with a polarimeter for real-time tracking, enhancing satellite QKD performance.

Findings

Accurate polarization estimation achieved with limited measurements.

Liquid-crystal switching dynamics significantly affect real-time performance.

Moderate increase in quantum-bit error rate with the proposed method.

Abstract

Polarization instability is a critical challenge for polarization-entangled satellite quantum key distribution (QKD), where atmospheric effects and platform motion continuously distort photon polarization. To maintain entanglement fidelity, these transformations must be accurately identified and compensated prior to detection. In this work, a compact and fast polarization-compensation approach based on liquid-crystal (LC) variable retarders is presented, using a co-propagating classical reference signal (beacon) for real-time polarization tracking. An LC-based polarimeter is implemented, and its performance is evaluated using both direct and Fourier-based Stokes parameter reconstruction. Experimental results indicate that accurate polarization estimation can be achieved with a limited number of measurements, enabling a favorable trade-off between speed and precision. The impact of liquid-crystal switching dynamics is also analyzed, highlighting the importance of selecting appropriate operating conditions for real-time applications. In addition, the effect of polarimetric inaccuracies on QKD performance is assessed through simulations of an entanglement-based protocol. The results show that only a moderate increase in quantum-bit error rate is introduced, while remaining compatible with secure key distribution. These findings demonstrate that LC-based polarization control represents an efficient and practical solution for real-time compensation in satellite QKD systems.