Environmental Effects on Light Propagation and Adaptive Systems
Site characterization for QKD ground stations: a requirements perspective and data of the Optical Ground Station Oberpfaffenhofen
Florian Moll, Alessia Scriminich, Christoph Gohle, Stefanie Häusler, Bettina Heim, Davide Orsucci, Johannes Prell, Javier García Olmedo, Douglas J. Laidlaw
Quantum key distribution (QKD) enables secure communication even in presence of quantum computers. Satellite based systems enable worldwide key distribution and connection of terrestrial quantum networks. In future, QKD satellite constellations can be operated to service ground users that are connected using an optical ground station. These QKD ground stations are assumed to be deployed in two scenarios. One is the option as end-user ground station, in which the ground user is directly connected to the satellite via the ground station. The other option is the use as a provider ground station, in which the ground user is connected via an intermediary fiber line. The performance of the satellite to ground QKD link depends on, amongst other, the environmental and infrastructural impacts of the site where it is located. Unlike in astronomical sites, the QKD Optical Ground Station (OGS) are not positioned in selected optimal environment, especially considering atmospheric turbulence and extinction. The QKD OGS mainly has to cope with the given environment instead. Knowledge of the site’s meteorological and infrastructural environment is important to assess performance of a QKD OGS when deployed. This paper discusses the most relevant aspects of a QKD OGS site characterization. These are atmospheric turbulence, atmospheric extinction, and background light. As a concrete example, the DLR Optical Ground Station Oberpfaffenhofen nearby Munich is used. Existing data is summarized, reviewed and needs for further data acquisition defined.
Quantum key distribution (QKD) enables secure communication even in presence of quantum computers. Satellite based systems enable worldwide key distribution and connection of terrestrial quantum networks. In future, QKD satellite constellations can be operated to service ground users that are connected using an optical ground station. These QKD ground stations are assumed to be deployed in two scenarios. One is the option as end-user ground station, in which the ground user is directly connected to the satellite via the ground station. The other option is the use as a provider ground station, in which the ground user is connected via an intermediary fiber line. The performance of the satellite to ground QKD link depends on, amongst other, the environmental and infrastructural impacts of the site where it is located. Unlike in astronomical sites, the QKD Optical Ground Station (OGS) are not positioned in selected optimal environment, especially considering atmospheric turbulence and extinction. The QKD OGS mainly has to cope with the given environment instead. Knowledge of the site’s meteorological and infrastructural environment is important to assess performance of a QKD OGS when deployed. This paper discusses the most relevant aspects of a QKD OGS site characterization. These are atmospheric turbulence, atmospheric extinction, and background light. As a concrete example, the DLR Optical Ground Station Oberpfaffenhofen nearby Munich is used. Existing data is summarized, reviewed and needs for further data acquisition defined.
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.
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.
As Quantum Key Distribution (QKD) seems to turn towards satellite communication infrastructure to interconnect distant nodes aiming to a global quantum secured network, the number of assisting satellites is expected to increase. In this work, a feasibility analysis of a large-scale Low Earth Orbit (LEO) satellite constellation supporting LEO to ground QKD links is presented. A software tool to simulate the physical properties of a large-scale satellite constellation and a link budget calculator tailored for LEO satellite to ground QKD links have been developed. A large-scale LEO satellite constellation with 100 satellites orbiting at 550km altitude with 53deg inclination angle in ten different orbital planes (10 satellites per orbital plane) has been simulated and subsatellite points for 1 year have been obtained. Assuming, an optical ground station network consisting of nine different optical ground stations located in observatories across Europe, the performance of this large-scale constellation is evaluated in terms of availability and Secure Key Rate (SKR) employing the prepare and measure Decoy-State BB84 QKD protocol, during nighttime, under different atmospheric conditions including cloud coverage and turbulence effects among others, while two different wavelengths (i.e., 800nm and 1550nm) are examined. For each Optical Ground Station (OGS) location, Gbits of distilled keys per year per ground station and key rates up to Kbps are reported.
Quantum key distribution (QKD) has gained a lot of attention over the past few years, but the implementation of quantum security applications is still challenging to accomplish with the current technology. Towards a global-scale quantum-secured network, satellite communications seem to be a promising candidate to successfully support the quantum communication infrastructure (QCI) by delivering quantum keys to optical ground terminals. In this research, we examined the feasibility of satellite-to-ground QKD under daylight and nighttime conditions using the decoy-state BB84 QKD protocol. We evaluated its performance on a hypothetical constellation with 10 satellites in sun-synchronous Low Earth Orbit (LEO) that are assumed to communicate over a period of one year with three optical ground stations (OGSs) located in Greece. By taking into account the atmospheric effects of turbulence as well as the background solar radiance, we showed that positive normalized secure key rates (SKRs) up to 3.9×10−4 (bps/pulse) can be obtained, which implies that satellite-to-ground QKD can be feasible for various conditions, under realistic assumptions in an existing infrastructure.
Quantum key distribution (QKD) enables private communication with information theoretic security. Free-space optical communication allows one to implement QKD without the limitations imposed by fiber networks such as the exponential scaling of transmission losses in optical fibers. Therefore, free-space QKD via satellite links is a promising technology to provide long-distance quantum communication connections. In free-space QKD systems, background light is the main source of noise, which has to be suppressed by means of spectral, spatial, and temporal filtering to reach a sufficiently low quantum bit error rate (QBER). Only then a quantum key can be exchanged successfully. To be able to define the requirements for a free-space QKD system, the background light must be examined more closely. Current considerations concentrate on cloud-free skies and rural environments. Free-space QKD will also take place when the sky is partly clouded and most likely also in urban environments. Here, an overview of physical causes of background light for downlink scenarios is given. Furthermore, the relation between QBER and background light is derived for a decoy-state BB84 protocol with polarization-encoded qubits to give an example of the dependency. Moreover, a setup to experimentally investigate the background light is shown. Measurement data were taken with this setup in Oberpfaffenhofen near Munich (Germany) in C-band. The measurement data are used to verify a background light simulation tool. The outcome underlines that simulation tools are sufficient for clear sky scenarios.
Space quantum communications is a potential means for establishing global secure communications and quantum networking. Despite pioneering demonstrations of satellite quantum key distribution, considerable challenges remain for wide deployment such as the local effects of the atmosphere on the transmission of single-photon level quantum signals. As part of Ireland's efforts to establish quantum links with the rest of Europe and further afield, we present a preliminary study of the feasibility of satellite quantum key distribution taking into account geographic and weather effects on the space-Earth channel. Weather data over 5 years covering 4 locations across Ireland were used to assess performance and the prospects of optical ground station (OGS) geographic diversity to improve service availability. Despite significant cloud cover that may reduce the performance of a single OGS location, the use of a 4-OGS network can provide up to 45% improvement for a single satellite exploiting anti-correlation in cloud cover, though most gains are achieved with 2 or 3 OGSs.