The EAGLE-1 mission aims to deliver Europe’s first autonomous space based quantum key distribution (QKD) system launching the EAGLE-1 satellite in 2026. A key element of this demonstration mission is the development and space qualification of a QKD transmitter, essential for enabling secure quantum communications. This paper presents the subsystem-level design, component qualification, system calibration, and functional testing of the QKD transmitter developed by the Institute of Communications and Navigation (KN) of the German Aerospace Center (DLR). The QKD transmitter is responsible for converting quantum key information into an optical signal for transmission to ground. The quantum key and QKD protocol is based on a quantum random number generator (QRNG) and the key bits are encoded in single photons. The transmitter hardware consists of various electrical and optical components, including modulators, attenuators, amplifiers, and passive elements. We discuss component selection criteria, as well as vibration and shock testing to verify launch resilience and thermal vacuum testing to assess operational stability under space conditions. One of the key challenges is the time and bias drift of critical components, which occurs after system boot-up and varies with temperature and time. These drifts directly affect the timing, amplitude, and phase of the optical signal, impacting overall QKD performance. We present the design of the calibration methodologies which ensure autonomous calibration of the transmitter in orbit, which provides the system stability required to adhere to the QKD protocol requirements. Finally, we describe the laboratory tests assessing quantum bit error rates (QBER).
The EAGLE-1 mission aims to deliver Europe’s first autonomous space based quantum key distribution (QKD) system launching the EAGLE-1 satellite in 2026. A key element of this demonstration mission is the development and space qualification of a QKD transmitter, essential for enabling secure quantum communications. This paper presents the subsystem-level design, component qualification, system calibration, and functional testing of the QKD transmitter developed by the Institute of Communications and Navigation (KN) of the German Aerospace Center (DLR). The QKD transmitter is responsible for converting quantum key information into an optical signal for transmission to ground. The quantum key and QKD protocol is based on a quantum random number generator (QRNG) and the key bits are encoded in single photons. The transmitter hardware consists of various electrical and optical components, including modulators, attenuators, amplifiers, and passive elements. We discuss component selection criteria, as well as vibration and shock testing to verify launch resilience and thermal vacuum testing to assess operational stability under space conditions. One of the key challenges is the time and bias drift of critical components, which occurs after system boot-up and varies with temperature and time. These drifts directly affect the timing, amplitude, and phase of the optical signal, impacting overall QKD performance. We present the design of the calibration methodologies which ensure autonomous calibration of the transmitter in orbit, which provides the system stability required to adhere to the QKD protocol requirements. Finally, we describe the laboratory tests assessing quantum bit error rates (QBER).
The EAGLE-1 mission aims to develop Europe's first sovereign, end-to-end space-based quantum key distribution (QKD) system. The mission is led by the European Space Agency (ESA) and SES in collaboration with several European National Space Agencies and private partners. The state-of-the-art QKD system will consist of a payload on board the EAGLE-1 low Earth orbit (LEO) satellite, optical ground stations, quantum operational networks, and key management system. The EAGLE-1 mission represents a major step for next-generation quantum communication infrastructures, delivering valuable technical results and mission data. The Institute of Communications and Navigation (IKN) of the German Aerospace Center (DLR) is a key partner in the EAGLE-1 mission and is involved in the research and development of elements in both space and ground segments. Here we report on the development of the QKD transmitter, a vital part of the QKD payload, and the customization of the Optical Ground Station Oberpfaffenhofen (OGS-OP) to conduct the IOT phase of EAGLE-1. For the space segment, DLR-IKN is in charge of the design of the QKD transmitter, including the development of the software and firmware. This transmitter generates quantum states which are used to implement a QKD protocol based on an optical signal, that will be transmitted to ground. For the ground segment, The OGS-OP will serve as the in-orbit testing ground station for EAGLE-1. Building upon the expertise with a range of satellites for quantum communication, as well as new implementations, OGS-OP will validate the performance of the payload, optical link and QKD system for the first time. We present the main developments of OGS-OP for the mission, which includes the implementation of an upgraded adaptive optics system to correct for atmospheric distortions and optimize the coupling of the incoming light into a single mode optical fiber.
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.
The technological development of QKD (Quantum Key Distribution) is accelerating from a research topic towards commercial deployment. QKD initiatives are increasingly addressing real-world use cases. Besides lots of literature on theory and lab-demos, first high Technology-Readiness Level systems are available. Due to the security guarantees it has to provide, the performance of any QKD implementation is particularly sensitive to any imperfections that may lead to an increase of the QBER (Quantum Bit Error Rate). Achieving high performance, or a low QBER value, constitutes a challenging engineering task, as the operating systems need to be highly optimized in many aspects. In particular, accurate phase modulation is the foundation of high system performance in case of phase-encoding protocols. In most cases, the overall QBER is measured with a QKD receiver, which accumulates errors from transmitter, quantum channel and receiver. This requires an operational transmitter system and additionally prevents the investigation of the pure transmitter output signal’s characteristics. However, for support of transmitter integration and test activities as well as for validation tasks, a more suitable method remains to be developed. In this paper, we propose such a method by making use of a coherent receiver. This device provides extended signal information including the signal amplitude and its current relative phase. Therefore, it is possible to monitor phase-encoding correctness and precision, among other relevant signal properties. We analyze the reasonability of using a coherent receiver, by outlining its capabilites to measure QBER contributors occurring in a given basic reference QKD transmitter system. Our findings indicate that employing a coherent receiver is a simple method to accelerate development and validation of phase-encoding QKD transmitter systems.
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.
Satellite based quantum key distribution (QKD) enables the delivery of keys for quantum secure communications over long distances. Maturity of the technology as well as industrial interest keep increasing. So does the technology readiness of satellite free-space optical communications. A satellite QKD system comprises a quantum communication subsystem and a classical communication subsystem (public channel). Both are implemented with free-space optics. Thus, in satellite QKD system design, there are strong synergies that should be exploited as much as possible and lead to an all-optical satellite QKD system. In this paper, we present a system like this locating all optical channels in ITU DWDM C-band. We focus on the overall conceptual design and the setup of the optical channels for quantum and classical signal transmission. The system description addresses the breadboards of a transmitter laser terminal (Alice terminal), a receiver laser terminal, (Bob terminal), the public channel implementation, the interfaced QKD system and the deployed encryption system. The design basis for the Alice terminal is the laser terminal development OSIRISv3. The design basis for the Bob terminal is the ground station development THRUST. The later contains an adaptive optics correction to enable single mode fiber coupling. This enables the interfacing to almost arbitrary quantum receivers such as the Bob modules used in the described experiment. The public channel is composed of a bi-directional 1 Gbps IM/DD system and a MODEM that implements a proprietary waveform optimized for free-space channels. The system was experimentally analyzed in a field test in the framework of the German initiative QuNET which addresses the use case of quantum secure communication for authorities. The results of the experiment are used to model a feasible LEO satellite-ground link. Performance indicators such as quantum bit error rate and secure key rate of a potential mission are estimated analytically.