Quantum Key Distribution in the Iberian Peninsula

TL;DR

The paper tackles extending quantum key distribution to national-scale secure communications across the Iberian Peninsula using a satellite-based, entanglement-based BBM92 system. It presents an end-to-end model with source, transmission, and measurement components, including beam waist optimization to counter satellite pointing jitter and realistic atmosphere and detector noise. A depolarizing/Werner-channel SKR framework, complemented by a look-up table for fast key-extraction efficiency, enables concrete performance estimates under real weather. Results show hospital-grade secure communication is feasible with current technology and beam optimization, while VPN-scale usage would require much higher entangled-photon rates (around 1 GHz). The work highlights the practical viability of near-term satellite QKD networks for national security and identifies concrete engineering targets to scale up to more demanding applications, potentially leveraging quantum memories and untrusted-ground configurations in future work.

Abstract

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.

Figures (4)

• Figure 1: Experimental setup. (a) Ground stations of the regional quantum key distribution (QKD) network that we want to implement. The central node is Madrid, and the key distribution is always between Madrid and any of the other cities (Barcelona, Bilbao and Lisbon). (b) We consider a downlink scenario, with an entangled photon source firing photons from the satellite to the ground stations. At the ground stations, the photons are randomly measured in the X or Z basis. To perform such a measurement, the photons go through a 50/50 beam splitter (B.S.), followed by a half-wave plate (H.W.P.) in one of the outputs, which changes the basis of the input photons. Finally, before measuring, the photons go through a polarizing beam splitter (P.B.S.) such that we measure different polarizations- horizontal ($D_H$), vertical ($D_V$), diagonal ($D_D$) and antidiagonal ($D_A$)- at the output ports of the B.S. The measurements results are fed into a classical memory from which we can then extract secret keys for the two applications we consider.
• Figure 2: Optimization of the beamwaist for a pointing jitter $\sigma_\textrm{PJ}=0.47\;\mu\textrm{rad}$, satellite's aperture radius of $a=15\;\textrm{cm}$ and ground station's aperture radius of $60\;\textrm{cm}$. Different distances $z$ between the transmitter and the receiver are shown.
• Figure 3: Secret key material between Madrid and Barcelona (left), and Madrid and Bilbao (right). The blue data shows the usable key without carrying out the beam optimization, and the red data shows the results when applying the optimization of the beam. The discontinuous red line shows the average key per pass. In the link Madrid-Barcelona, the successful passes, where key is shared between the satellite and the ground stations, are around 39% for the non-optimized case and 48% for the optimized case. While in the Madrid-Bilbao link, the successful passes for the non-optimized case are around 22% and for the optimized case 24%, leading to a lower average of secret key material.
• Figure 4: Key Extraction Efficiency - Look Up Table The heatmap shows the value of $\eta_{LUT}$ (Eq. \ref{['eq:SKRLUT']}) as a function of the block length and the estimated QBER.