Automated Polarization Basis Adjustment and Security Monitoring in Quantum Communication via Coincidence Entropies

TL;DR

This paper introduces a method for birefringence compensation and security monitoring in quantum communication systems using coincidence entropies derived from polarization-entangled photons, enabling all-fiber setups to maintain alignment and detect eavesdropping.

Contribution

It presents a novel all-fiber birefringence compensation technique based on coincidence entropies and a gradient descent algorithm for polarization basis alignment, also enabling security monitoring.

Findings

Effective birefringence compensation using coincidence entropy as a cost function.

Ability to monitor entanglement quality and detect intercept-resend attacks.

Enhanced stability and security in fiber-based quantum communication systems.

Abstract

Polarization-sensitive receivers for single photons are of crucial importance in various applications within the fields of quantum communication and quantum sensing, and are more commonly implemented in free-space optics rather than in optical fibers. This is primarily due to the unpredictable and varying birefringence in single-mode optical fibers. We present a method for birefringence compensation in an all-fiber detection setup that relies solely on coincidence measurements of a polarization-entangled state, or known correlations in a prepare-and-measure scenario. We define coincidence entropies as functions of the measured coincidence counts. These quantify the randomness of measurement outcomes, remain independent of the transmitted Bell state, and serve as indicators of the degree of entanglement. By leveraging coincidence entropy as a cost function in a gradient descent algorithm, we are able to align the polarization bases between two distinct polarization-sensitive receivers. Additionally, coincidence entropies can be employed to monitor the quality of entanglement transmission, thereby enhancing the system's ability to detect potential eavesdropping attempts, such as intercept-resend quantum attacks.