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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Polarization-Encoded BB84 Quantum Key Distribution for the Nano-Satellite Mission QUBE-II

Michael Steinberger, Michael Auer, Adomas Baliuka, Moritz Birkhold, Harald Weinfurter, Lukas Knips

Published Jun 23, 2025• 1 citation
Publisher

Abstract

Quantum Key Distribution (QKD) is a method to securely share secret keys between two parties via a quantum channel. Our QUBE mission [1], launched in summer 2024, is a pathfinder mission to test the suitability of components for economic global-scale QKD based on CubeSats. QUBE and its successor QUBE-II [1]–[3] are joint research projects by the German Aerospace Center Institute of Communications and Navigation (DLR-IKN), the Friedrich Alexander University Erlangen-Nuremberg (FAU), the Max Planck Institute for the Science of Light (MPL), Zentrum fur Telematik (ZfT), OHB System AG, and the Ludwig Maximilian University Munich (LMU).

DOI
10.1109/cleo/europe-eqec65582.2025.11109420

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Quantum Key Distribution (QKD) is a method to securely share secret keys between two parties via a quantum channel. Our QUBE mission [1], launched in summer 2024, is a pathfinder mission to test the suitability of components for economic global-scale QKD based on CubeSats. QUBE and its successor QUBE-II [1]–[3] are joint research projects by the German Aerospace Center Institute of Communications and Navigation (DLR-IKN), the Friedrich Alexander University Erlangen-Nuremberg (FAU), the Max Planck Institute for the Science of Light (MPL), Zentrum fur Telematik (ZfT), OHB System AG, and the Ludwig Maximilian University Munich (LMU).
Quantum Key Distribution (QKD) enables the secure distribution of secret keys for cryptographic purposes between two trusted parties over a quantum channel. For reaching global-scale quantum key distribution, satellite-based approaches are most promising as already demonstrated by the Chinese MICIUS mission. Here, we summarize our efforts in building a very compact QKD sender unit featuring polarization-encoded BB84 with weak coherent pulses. The unit is suitable for deployment in cube satellites such as our missions QUBE and QUBE-II which aim to demonstrate that affordable and scalable key distribution is possible in the near future.
In a world of global satellite communication networks, it is crucial to ensure the security of these data links. QUBE is a project that will develop and launch a CubeSat for the downlink of strongly attenuated light pulses, with encoded quantum information, which can be used for the exchange of encryption keys. This 3U Pico-Satellite will be built using the UNISEC-Europe standard, which has been proven to provide a robust framework for increased reliability for CubeSat missions. In addition to advanced reaction wheels for precision pointing, the satellite will be carrying the DLR-OSIRIS optical downlink system as well as dedicated payloads for testing components required for quantum key distribution. A miniaturized quantum random number generator (QRNG) will create a sequence of numbers, which can be used to set the quantum states of the light. The light pulses will then be downlinked to the optical ground station at DLR in Oberpfaffenhofen, Germany, which is equipped with the corresponding components for receiving the quantum states. Additionally, the random numbers will partially be made available via an RF downlink. This will allow evaluating the link loss as well as the noise and errors in the transmission of quantum signals. In QKD, due to the underlying quantum mechanics, any attempt of reading the quantum states will alter them, which makes interceptions easily detectable. The quantum communication experiments will evaluate whether secure communication links are possible even on a CubeSat scale. A major challenge for building the required CubeSat is the attitude determination and control system that will provide precise pointing. This work will outline detailed mission requirements as well as the chosen subsystems for tackling these challenges in order to deliver a successful mission.
The QUBE-I mission is a technology demonstrator mission for novel building-blocks for satellite-based quantum-key-distribution (QKD). The mission aims to test three highly integrated optical payloads, a quantum random number generator and two transmitters for polarization and phase encoded quantum states as a precursor mission for the first QKD demonstration from a CubeSat. The QKD sources are connected to, and are integrated together with the Optical Space Infrared Downlink System (OSIRIS) for QUBE (O4Q) in a 3U CubeSat. One of the mission's primary challenges is achieving a precise and accurate pointing of the O4Q with an error margin of less than 1° towards the optical ground station (OGS). Additionally, conducting the experiments presents several operational challenges, such as the need for precise orbital elements to track the satellite with the OGS and safety mechanisms to protect the Sunsensitive components of the satellite. Following the launch on 16th August 2024, we conducted an extensive commissioning phase to test all subsystems, payload components, and operational procedures. This paper provides insights and results from QUBE-I's first year in orbit, highlighting lessons learned from integrating multiple complex payload systems into a 3U CubeSat platform. Starting with communication establishment, we present the ground segment and operations capabilities. We elaborate on the use of flexible on-board scripting and recording mechanisms provided by the satellite's flight software stack for operations. An effective review, test, and deployment procedure was established to balance risk mitigation and agile operation planning. We present results of extensive tests conducted for the crucial attitude determination and control system (ADCS), including actuator verification, spin-up and detumbling tests, in-orbit sensor calibration procedures, and the evaluation of various pointing modes, such as the fine pointing mode required for the experiments. Besides that, we give insights in the actual performance of the experimental and scientific payloads after their commissioning. A big milestone marked the successful test of the operational procedure for the QKD experiments. This includes the timely provision of precise orbit determination to the OGS for tracking. In addition, the ADCS needs to prepare the fine pointing mode accordingly and the experiment needs to be started autonomously before actual contact. Flexibly selectable on-board recordings can be downloaded after the experiments for evaluation. As expected, the technical complexity of the QUBE-I mission resulted in a challenging first year of operation. With this paper, we aim to share our experiences and contribute our insights to the space community.
Satellite-based Quantum Key Distribution (QKD) can be used to deliver information-theoretically secure keys for cryptographic applications, no matter where end-users are located. Several space missions using this concept have shown feasibility of systems and component technologies. The QUBE-II mission, with a planned launch in Spring 2026, uses an economic CubeSat platform to show the possibility of a global secure communication by simple means. This paper presents the optical system design of the QUBE-II laser communication terminal (LCT). It is based on the previous developments for the CubeL mission (launched in 2021), for CubeISL, and for the precursor mission QUBE-I (launched in 2024). The 2.5U LCT is integrated into an 8U CubeSat and serves as transmit system for two Alice modules operating at 850 nm and in optical L-band1, 2 respectively. At the same time, bi-directional classical links are used for link acquisition, tracking, and data transmission. With its 85 mm aperture and an 800 mm receiver telescope on the ground station, the antenna gain is sufficiently high enough to perform QKD links. The LCT’s optical system is designed based on a commercially available reflective telescope in combination with custom-designed lenses. It integrates several fiber-optical and electrical components for wavelength combination, wavelength splitting, tracking, and uplink data reception into a compact design. The Alice payloads are connected to the LCT by optical fibres allowing a flexible placement of the modules within the satellite bus. Collimators couple the light into free-space beams which are co-aligned and interfaced to the transmit telescope. In addition to the optical system design, this paper provides further information about the integration of the optical into the mechanical system, and the performed functional and qualification tests.