Quantum Key Distribution (QKD) systems will play an important role in future networks for secure data communication. In order to provide a global coverage of a future QKD service, satellites are needed to bridge large distances. These satellite systems must be cost efficient to facilitate deployment since many network nodes will be needed. The CubeSat standard is frequently used for New Space projects as a versatile platform for satellite payloads. It is also chosen as a baseline for the construction of the system described in this paper. The DLR Institute of Communications and Navigation develops optical free space communication systems for scientific research in classical and quantum communications. A previous development of a 1/3 U CubeSat communication terminal for up to 100 Mbit/s downlinks is adapted to be used for quantum communications tests with multiple transmitters. Since the original laser terminal was designed for C-band wavelengths, a redesign of the optical system is needed to achieve polychromatic performance for the three sources. The optical system consists of a fiber collimator, a beam steering system and an afocal telescope. Most important requirement of the latter is a similar magnification for all wavelengths to ensure concentric beams pointing to the OGS. Scenarios in which the system is optimized either for lowest divergences or a similar magnification of the telescope are demonstrated in Figure 1. As the afocal telescope is used bidirectionally, it also needs to be optimized for the incident wave-front of the beacon from the OGS with respect to the beam shape at the four quadrant detector. These parameters are important for a correct pointing control. A fiber-based wavelength division multiplexer (WDM) is used for combining signals from three sources into one output fiber. As shown in the block diagram in Figure 2, it is based on cascaded thin film interference filters which are coupled to the fibers. Especially the propagation of 850 nm and 1550 nm signals in one single mode fiber is critical. Therefore the types of optical fibers were selected with respect to the bend loss, single mode propagation behavior, polarization integrity and optimal cladding diameter for the production.
Quantum Key Distribution (QKD) systems will play an important role in future networks for secure data communication. In order to provide a global coverage of a future QKD service, satellites are needed to bridge large distances. These satellite systems must be cost efficient to facilitate deployment since many network nodes will be needed. The CubeSat standard is frequently used for New Space projects as a versatile platform for satellite payloads. It is also chosen as a baseline for the construction of the system described in this paper. The DLR Institute of Communications and Navigation develops optical free space communication systems for scientific research in classical and quantum communications. A previous development of a 1/3 U CubeSat communication terminal for up to 100 Mbit/s downlinks is adapted to be used for quantum communications tests with multiple transmitters. Since the original laser terminal was designed for C-band wavelengths, a redesign of the optical system is needed to achieve polychromatic performance for the three sources. The optical system consists of a fiber collimator, a beam steering system and an afocal telescope. Most important requirement of the latter is a similar magnification for all wavelengths to ensure concentric beams pointing to the OGS. Scenarios in which the system is optimized either for lowest divergences or a similar magnification of the telescope are demonstrated in Figure 1. As the afocal telescope is used bidirectionally, it also needs to be optimized for the incident wave-front of the beacon from the OGS with respect to the beam shape at the four quadrant detector. These parameters are important for a correct pointing control. A fiber-based wavelength division multiplexer (WDM) is used for combining signals from three sources into one output fiber. As shown in the block diagram in Figure 2, it is based on cascaded thin film interference filters which are coupled to the fibers. Especially the propagation of 850 nm and 1550 nm signals in one single mode fiber is critical. Therefore the types of optical fibers were selected with respect to the bend loss, single mode propagation behavior, polarization integrity and optimal cladding diameter for the production.
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.
Quantum Key Distribution (QKD) is a promising method to guarantee future-proof, information theoretic security. Since optical fibers have an exponential loss with distance, satellite-based QKD solutions are being developed in order to realize long-distance links. Therefore, Optical Ground Stations for QKD (QKD-OGS) need to be designed to enable quantum communication with satellites. Different link configurations will result in different integration options of the QKD-OGS in the terrestrial fiber network and therefore impact its performance. Applicable integration options must be identified and trade-off analysis conducted at the architecture, system and sub-system level. The reference scenario is a Low Earth Orbit (LEO) downlink configuration employing a decoy-state BB84 protocol with polarization encoded qubits at 1550 nm wavelength. The satellite acts as trusted node for implementation of the key distribution between two parties on ground. Furthermore, only direct connections from the satellite to the end-users are considered, i.e. the QKD-OGS does not serve as a trusted-node relay. This results in the selection of three main integration concepts: first, a free-space, end-user QKD-OGS which is located directly at the end-user where the quantum signal is measured directly behind the receiver telescope in free-space; second, a fiber-coupled, end-user QKD-OGS where the quantum signal is coupled into a single-mode fiber and guided to a co-located server room hosting the QKD Receiver system; third, a fiber-coupled, provider QKD-OGS, which reaches several end-users by using a local fiber network. In order to evaluate these three concepts, the comparison parameter is defined to be an estimated factor of suppliable end-users. The estimated comparison factor shows that the fiber-coupled, enduser QKD-OGS can supply 6.7 times more end users than the free-space, end-user QKD-OGS. The fiber-coupled, end-user QKD-OGS can supply 129.7 times more end-users than the free-space, end-user QKD-OGS.
A worldwide growing interest in fast and secure data communications pushes technology development along two lines. While fast communications can be realized using laser communications in fiber and free-space, inherently secure communications can be achieved using quantum key distribution (QKD). By combining both technologies in a single device, many synergies can be exploited, therefore reducing size, weight and power of future systems. In recent experiments we demonstrated quantum communications over large distances as well as between an aircraft and a ground station which proved the feasibility of QKD between moving partners. Satellites thus may be used as trusted nodes in combination with QKD receiver stations on ground, thereby enabling fast and secure communications on a global scale. We discuss the previous experiment with emphasis on necessary developments to be done and corresponding ongoing research work of German Aerospace Center (DLR) and Ludwig Maximilians University Munich (LMU). DLR is performing research on satellite and ground terminals for the high-rate laser communication component, which are enabling technologies for the QKD link. We describe the concept and hardware of three generations of OSIRIS (Optical High Speed Infrared Link System) laser communication terminals for low Earth orbiting satellites. The first type applies laser beam pointing solely based on classical satellite control, the second uses an optical feedback to the satellite bus and the third, currently being in design phase, comprises of a special coarse pointing assembly to control beam direction independent of satellite orientation. Ongoing work also targets optical terminals for CubeSats. A further increase of beam pointing accuracy can be achieved with a fine pointing assembly. Two ground stations will be available for future testing, an advanced stationary ground station and a transportable ground station. In parallel the LMU QKD source size will be reduced by more than an order of magnitude thereby simplifying its integration into future free-space optical communication links with CubeSats.
The low-cost small aperture commercial optical ground stations (OGSs) will play an important role in the realization of a global scale satellite-to-ground quantum key distribution (QKD) networks. OGSs with < 60 cm diameter telescope and a small mount, can have stringent size and weight limits on the payload that can be accommodated. In order to ensure easy integration with an OGS, we have designed and produced a 40×30×10 cm receiver module which comprises polarization detection optics, fine point and track (PAT) subsystem and coarse wavelength division multiplexing optics for receiving optical signals over the visible, near-infrared and infrared spectrum. The tracking capability of the module to track multiple beams of different wavelength is evaluated in an emulated beam-wander induced free-space link. The receiver module is characterized for transmission loss and polarization extinction ratio (PER) by sending an 844 nm collimated beam to the module in a 3 m free-space link and measuring the power coupled into the four single mode fibers (SMF) for H, V, D and A polarized components. Furthermore, we have used the receiver module in our satellite-to-ground QKD emulator and have measured the secure key that can be generated within different maximum elevation satellite overpasses when the quantum signal is coupled into AR-coated SMF. Such modules can be incorporated into optical ground stations (OGS) to mitigate atmospheric beam-wandering and coarse misalignment while allowing wavelength division multiplexing of multiple visible, near-infrared and infrared beams to enable co-propagation of classical and quantum communication beams for real-time QKD.
Recent advances in pointing and tracking capabilities of small satellite platforms have enabled adoption of capabilities such as high-resolution Earth Observation (EO), inter-satellite laser communications and, more recently, quantum communications. Quantum communications requires unusually narrow optical beams and tight pointing performance (on the order of ten microradians) to close an inherently brightness-limited quantum link. This limit is due to quantum communication protocols such as quantum key distribution and teleportation requiring individual quantum states to be transmitted with photon number restrictions. We examine an opportunity to combine quantum communications with laser communications in sharing an optical link. We discuss a combined quantum and laser communication terminal capable of performing space-to-ground entanglement-distribution and high data rate communications on a 12U CubeSat with a 95mm beam expander and an 60 cm aperture optical ground telescope. Photon pairs produced by the quantum terminal are entangled in polarization so the polarization must be maintained throughout the optical link. We discuss active and passive compensation methods in space and polarization reference frame correction using a polarized reference beacon at the ground station. The combined quantum and laser communication terminal approach enables secure communications over an optical channel with rates of 100 Mbps and sub-nanosecond time transfer.
This study investigates the design and performance of a transmitter optical system optimized for Low-Earth-Orbit (LEO) satellite-based Free-Space Quantum Key Distribution(FS-QKD). The architecture integrates a 120 mm aperture telescope, a beam expander, and an optical-axis alignment module, complemented by a pointing control unit to maintain link stability. Performance was evaluated across 660 nm, 810 nm, and 1,550 nm wavelengths. Link budget analysis for a 10 km channel with a 200 mm receiver aperture demonstrated that a 660 nm beacon laser achieves a diffraction loss of 4.71 dB and a received power of 15.5 dB, with the beam expanding to approximately 440 mm. Notably, the integration of a Fast Steering Mirror(FSM) enhanced the average Signal-to-Noise Ratio(SNR) by nearly ninefold. These results validate the system's efficacy in mitigating pointing errors and optimizing optical coupling for robust inter-satellite quantum communication.