Aitor Villar

Future quantum networks depend entirely on the ability to distribute quantum resources \cite{kimble2008quantum, wehner2018quantum}. Although terrestrial optical fibers provide the foundation for such networks, fundamental attenuation limits their utility over intercontinental distances, making satellite-based links essential for global connectivity. A critical prerequisite for this infrastructure is the reliable distribution of entanglement, which enables information-theoretic secure communication via Quantum Key Distribution (QKD) and facilitates advanced protocols, such as entanglement routing via quantum repeaters. Historically, entanglement-based QKD has relied on bespoke, ad hoc experimental configurations tailored for proof-of-concept laboratory demonstrations \cite{rozenman2026free}. However, realizing an operational space-based quantum network necessitates transitioning from these specialized setups to robust, space-qualified hardware \cite{vergoossen2020spooqy}. Bridging the gap between terrestrial prototypes and orbital deployments requires the rigorous characterization of flight-representative payloads and automated optical ground stations under representative field conditions. Such a validation is vital to ensure the scalable distribution of quantum resources necessary for a global quantum internet. We deployed a horizontal free-space QKD experiment spanning a 1.8 km link in a semi-urban desert environment, effectively mimicking the architecture planned for future space-to-ground networks. The transmitter node integrated the space-qualified engineering model of the quantum light source and receiver designed for the upcoming SpeQtre satellite mission, functioning as a highly stable source of polarization-entangled photon pairs. At the receiver node, the experiment utilized the Abu Dhabi Quantum Optical Ground Station (ADQOGS) \cite{amairi2024versatile}, an automated facility engineered for free-space optical and quantum communications. The ADQOGS employs an 80 cm Ritchey–Chrétien telescope equipped with a precision acquisition and tracking system, single-photon detectors, and stringent spatial and spectral filtering stages optimized for background noise suppression. Together, this combination of a flight-ready transmitter and a highly specialized ground station creates a rigorous, real-world testbed for evaluating the performance of operational quantum hardware outside the laboratory. We report nighttime operations under stable atmospheric conditions. The integrated system demonstrated sustained operational stability, yielding a sifted coincidence rate of approximately 24,000 photon pairs per second. We recorded a mean quantum bit error rate (QBER) of 4.78\% and extracted a finite-size secure key rate of 7565 bps, validating the efficacy of the entanglement-based link. Furthermore, extrapolating these metrics to a realistic Low Earth Orbit (LEO) scenario indicates that the hardware can successfully overcome the substantial diffraction and atmospheric losses expected during a LEO satellite downlink. Ultimately, the successful field integration of a flight-representative payload with an automated ground station verifies the operational readiness of the hardware. These results mark a decisive step toward the execution of the SpeQtre mission and provide critical empirical foundations for the deployment of large-scale, satellite-based quantum communication networks.