Quantum Key Distribution in the Iberian Peninsula

A promising use of quantum networking is quantum key distribution (QKD), which can provide information-theoretic security unattainable by classical means. While optical fiber-based QKD networks suffer from exponential loss, satellite-assisted quantum communication offers a scalable solution for long-distance secure key exchange. In this work, we propose and evaluate a satellite-based QKD setup covering the Iberian Peninsula, linking Madrid with Barcelona, Bilbao, and Lisbon. Our proposed setup uses a Low-Earth-Orbit (LEO) state-of-the-art satellite equipped with a spontaneous parametric down-conversion (SPDC) source to distribute entangled photon pairs to ground stations. Considering vibrations in the satellite, we optimize the beam waist to enhance the transmission probability and improve the secret key rate (SKR). Our results show that key rates sufficient for real-world applications, such as secure communication between hospitals, using hybrid classical-quantum protocols are feasible with existing protocols. Our results highlight the viability of near-term satellite-based QKD networks for national-scale secure communications.

💡 Research Summary

This paper presents a comprehensive design and performance analysis of a satellite‑based quantum key distribution (QKD) network that spans the Iberian Peninsula, linking Madrid with Barcelona, Bilbao, and Lisbon. The authors propose a low‑Earth‑orbit (LEO) satellite equipped with a state‑of‑the‑art spontaneous parametric down‑conversion (SPDC) source that generates polarization‑entangled photon pairs at a rate of approximately 5.9 × 10⁶ pairs s⁻¹. The satellite operates at an altitude of about 400 km on a medium‑inclination orbit that repeatedly passes over the target region.

A central technical contribution is the optimization of the transmitted beam waist (w₀) in the presence of satellite pointing jitter (≈0.47 µrad). By modeling the free‑space propagation, atmospheric absorption, scattering, and the truncation of the Gaussian beam by the transmitter aperture (15 cm), the authors identify an optimal w₀ (≈2.5 cm) that maximizes the average photon capture probability η at the ground stations (60 cm apertures). This optimization balances two opposing effects: a larger waist reduces diffraction‑induced spreading, while an excessively large waist incurs loss due to aperture clipping. The resulting η improvement reduces link loss by roughly 3 dB and boosts the secret key rate (SKR) by an order of magnitude.

The atmospheric channel is modeled using real‑time meteorological data (cloud cover, visibility, solar irradiance) following the framework of Ref.