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.
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.
Free-space optical communication is an inherently secure and robust method to transfer information over large distances. To increase the level of secure communication even further, it marks the base for quantum key distribution, a tap-proof encryption method based on the principles of quantum mechanics. The ever-increasing number of small spacecrafts and the growing global threats require secure communication channels, robust against jamming, spoofing, and eavesdropping. Classical radio-frequency communication is, due to its physical properties and its wide-angled beacons sensitive for unwanted interceptions. The technology transfer of laser communication to small satellites like CubeSats provides solutions to overcome the limitations of classical communication, but it comes with the necessity of very precise and accurate pointing. To demonstrate quantum key distribution technologies on a CubeSat, the project QUBE has the goal to verify two experimental quantum sources which signals are transmitted over an optical link from a 3U CubeSat. Two quantum sources, developed by the Ludwig-Maximilian University of Munich and the FriedrichAlexander University in Erlangen, are coupled into the laser communication terminal OSIRIS4QUBE, an evolution of OSIRIS4CubeSat developed by the German Aerospace Center. These systems are integrated into the 3U CubeSat QUBE built by the Center for Telematics. Establishing an optical connection between the laser terminal and the optical ground station requires precise and accurate pointing of the satellite. This task is handled by a high-precision attitude determination and control system. This paper presents the first results of the launch and early orbit phase of the QUBE mission. It highlights the achievements of the attitude determination and control system and the commissioning of the OSIRIS4QUBE terminal. The paper focuses on these two topics and explicitly excludes the results of the experimental quantum payloads.
Laser communication technology, especially QKD (Quantum Key Distribution), is coming more and more into practical operations. It is a maturing technology that enables the distribution of cryptographic key between two remote parties [1][3] for secure communication even in presence of quantum computers[3]. To demonstrate this technology DLR is part of the QUBE project to have a precursor mission for QKD from CubeSats in orbit as well as hosting and operating the ground-based counterpart the Optical Ground Station Oberpfaffenhofen (OGSOP).The OGS is equipped with an 80cm primary mirror, a Coudé path with optical bench, adaptive optics system, fiber coupling and a single photon detector. The ground station already performed successful downlinks from low Earth orbit (LEO) (Flying Laptop, CubeL and others) and geostationary platforms (Alphasat, Project BayernQSat), as well as aircrafts (Project QuNET) [4][11][13].In the last year the ground station was upgraded from its basic configuration to adapt for multi-mission support and QKD experiments. Several features were integrated and upgraded at the telescope, in the Coudé path and on the optical bench. The focus of the upgrades was to connect the OGSOP (Optical Ground Station Oberpfaffenhofen) with the satellite QUBE. The target of compatibility could be achieved and improved from OGS side.Knowledge from ground-to-ground-tests were considered to rebuild the receiving setups in an improved and more operational way. The QUBE satellite contains an optical terminal designed and developed by DLR to transmit two experimental quantum signals over the optical link. These sources were developed by the Ludwig-Maximilian University (LMU) of Munich and the Friedrich-Alexander University (FAU) in Erlangen.On May the 2nd 2025 the OGSOP was able to catch the first light from QUBE and establish a link between both sides[2]. This started the optical communication commissioning phase of the satellite, including the classical communication terminal and the quantum light sources. This paper describes the entire setup, from satellite to ground detector, and presents the key achievements so far.
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.
The limited range of quantum key distribution (QKD) in fiber based systems led to several projects aiming for the development of a satellite based QKD infrastructure, like the conduction of quantum limited measurements from a geostationary satellite [1] or the MICIUS mission, which impressively demonstrated QKD on a global scale [2]. However, due to the high costs of satellite launches the profitability of commercial systems would benefit dramatically from further reduction in size, weight and power. Photonic integration [3] addresses these points by allowing the integration of all optical components of a QKD system on a photonic integrated circuit (PIC). In this work, we are going to demonstrate building blocks for quantum communication from space based on photonic integration technology by building a payload for the CubeSat mission QUBE [4] consisting of an integrated sender for weak coherent states as well as an integrated Quantum Random Number Generator (QRNG).
In the absence of technically mature quantum repeaters, losses in optical fibers limit the distance for ground-bound quantum key distribution. One way to overcome these losses is via optical links to satellites, which has been demonstrated in course of the Chinese-Austrian QUESS mission. Though its findings were impressive, such a large-scale project requires massive financial and time resources. We propose a 32x10x10cm³ nanosatellite orders of magnitude cheaper which is able to perform quantum key distribution (QKD) in a trusted-node scenario, using only commercially available components. We have performed a detailed analysis of such a CubeSat mission (“Q³Sat”), finding that cost and complexity can be reduced by sending the photons from ground to satellite, i.e. using an uplink. Calculations have been done for a prepare-and-send protocol (BB84 with decoy pulses) and for a protocol exploiting quantum entanglement (E91), both using polarization as information carrier. We specified the minimum requirements for the sender stations for these two different protocols. Possible orbits have been assessed, regarding both height and ellipticity to maximize link time and minimize losses. Using long-term weather data, we developed a beam model taking into account absorption, turbulence-induced beam divergence and pointing stability of sender and receiver telescope. Using light pollution measurements from space and their spectra, we arrive at maximum expectable noise count rates. We also specify the requirements for clock stability, classical communication speed and computing requirements. Incorporating all these parameters into our model, we arrive at a link budget which we can use to calculate the expected quantum secure key rates. We have also created a preliminary design of such a 3U CubeSat including a detailed size, weight and power budget and a CAD to account for the assembly of the components. Deploying a 10 cm long mirror telescope covering the small surface of the satellite leaves enough space for a polarization analysis module and housekeeping, communication and computing electronics. Polarization analysis can be done via a polarizing beam splitter and single-photon detectors with a cross section small enough to rule out radiation damage. Pointing stability and detumbling is crucial especially for such a small satellite and can be achieved via spinning wheels, achieving a precision in the tilt and yaw axis of 40 mrad. For one such CubeSat, we estimate the quantum secure key to be acquired between two ground stations during one year to be about 13 Mbit when deploying a decoy protocol. A Bell test between ground and satellite would also be feasible. The uplink design allows to keep the more sensitive, computation-intensive and expensive devices on ground. The experiment proposed by us therefore poses a comparably low-threshold quantum space mission. For a two-year lifetime of the satellite, the price per kilobit would amount to about 20 Euro. In large constellations, Q³Sats could be used to establish a global quantum network, which would further lower the cost. Summarizing, our detailed design and feasibility study can be readily used as a template for global-scale quantum communication.