Quantum key distribution (QKD) allows secure data transfer in a post-quantum era by exploiting the laws of quantum mechanics to generate a secure-key for data encryption. Satellite based QKD is the gateway to a global communication network due to its quadratic transmission loss scaling opposed to the exponential losses in fiber-based QKD. However, the free-space nature of satellite QKD imposes a new challenge due to the additional noise source of background light (BGL). BGL heavily influences the feasibility of a secure QKD protocol, and its impact has to be mitigated through temporal, spatial and spectral filtering techniques. To design optical systems for satellite QKD, the contributions of BGL must be characterized, but current observations of BGL in the C-band are limited to daytime measurements with collimator setups. These measurements are only translated to the receiving systems of QKD downlinks, Optical Ground Stations (OGSs), via theoretical models. This work presents an optical setup built and calibrated for BGL measurements in the C-band with single mode fiber (SMF) coupling during the day and night utilizing the Optical Ground Station Oberpfaffenhofen (OGSOP) designed for satellite QKD downlinks. The transmission loss of the optical system and the efficiency of the deployed Superconducting Nanowire Single Photon Detector (SNSPD) are determined and BGL measurement data in the C-band acquired during the day and night. In addition the measured BGL is compared with BGL simulations and prior measurements with collimator setups for verification. During the night, the measured BGL only raises slightly above the noise floor of the detector with larger radiances observed only close to the surface with up to 0.00023 (mW nm-1 m-2 sr-1) and close to the full Moon with up to 0.0043 (mW nm-1 m-2 sr-1). In daytime BGL measurements radiances between 1.09 (mW nm-1 m-2 sr-1) and 4.17 (mW nm-1 m-2 sr-1) were observed, exhibiting an offset from simulation tools. The results highlight the importance of system calibration for Optical Ground Station (OGS) based BGL measurements, while also providing insights into the magnitude of BGL usable for the system design in future satellite QKD missions.
Quantum key distribution (QKD) allows secure data transfer in a post-quantum era by exploiting the laws of quantum mechanics to generate a secure-key for data encryption. Satellite based QKD is the gateway to a global communication network due to its quadratic transmission loss scaling opposed to the exponential losses in fiber-based QKD. However, the free-space nature of satellite QKD imposes a new challenge due to the additional noise source of background light (BGL). BGL heavily influences the feasibility of a secure QKD protocol, and its impact has to be mitigated through temporal, spatial and spectral filtering techniques. To design optical systems for satellite QKD, the contributions of BGL must be characterized, but current observations of BGL in the C-band are limited to daytime measurements with collimator setups. These measurements are only translated to the receiving systems of QKD downlinks, Optical Ground Stations (OGSs), via theoretical models. This work presents an optical setup built and calibrated for BGL measurements in the C-band with single mode fiber (SMF) coupling during the day and night utilizing the Optical Ground Station Oberpfaffenhofen (OGSOP) designed for satellite QKD downlinks. The transmission loss of the optical system and the efficiency of the deployed Superconducting Nanowire Single Photon Detector (SNSPD) are determined and BGL measurement data in the C-band acquired during the day and night. In addition the measured BGL is compared with BGL simulations and prior measurements with collimator setups for verification. During the night, the measured BGL only raises slightly above the noise floor of the detector with larger radiances observed only close to the surface with up to 0.00023 (mW nm-1 m-2 sr-1) and close to the full Moon with up to 0.0043 (mW nm-1 m-2 sr-1). In daytime BGL measurements radiances between 1.09 (mW nm-1 m-2 sr-1) and 4.17 (mW nm-1 m-2 sr-1) were observed, exhibiting an offset from simulation tools. The results highlight the importance of system calibration for Optical Ground Station (OGS) based BGL measurements, while also providing insights into the magnitude of BGL usable for the system design in future satellite QKD missions.
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.
Quantum Key Distribution (QKD) allows sharing encryption keys with information theoretic security. Satellitebased QKD can establish long distance links due to the quadratic transmission loss in free-space instead of the exponential transmission loss in optical fibers. Atmospheric background light plays an important role in the QKD scheme as it may significantly contribute to the system Quantum Bit Error Rate (QBER). Therefore, background light needs to be examined closely. Due to the high variability of atmospheric conditions, direct measurements of the background light under different meteorological conditions are the best option to properly characterize the effect. Current considerations are mainly limited to the analysis of cloud-free scenarios by simulation and by experiment. Links can also take place when the environment differs from this ideal condition. Measurement data was recorded in C-band at the campus of the University of Waterloo, Canada, during the day with clear sky and during sunset with clear sky and partly-clouded sky conditions. The measurement data is shown and compared to simulation results and to the measurement data taken in Oberpfaffenhofen, Germany. The impact of background light is discussed on a chosen reference scenario outlining the importance of detector gating time and end-to-end transmission loss when wanting to realize daylight QKD.
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.
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.
Suppressing background noise is a fundamental challenge in satellite-to-ground Quantum Key Distribution (QKD), where quantum signals traverse free-space optical links rather than isolated dark fibers. As future satellite QKD systems aim to support both nighttime and daylight operations, effective mitigation of solar background radiance becomes essential to maintaining low Quantum Bit Error Rates (QBER) and ensuring stable secure key generation. In this work, we experimentally evaluate background sky radiance and its suppression using two small-aperture (20 cm and 28cm) optical telescopes and the 2.3-meter Aristarchos telescope, applying a unified filtering approach that combines spectral and spatial filtering techniques. The experiments were performed under three distinct observational environments: at an urban location in Athens (~250 m altitude) and Thessaloniki (~60 m altitude) for the small-aperture system, and at the summit of Mount Helmos, Greece, for the large-aperture Aristarchos facility. Single-Photon Avalanche Diodes (SPADs) were used as the primary photon-counting detectors to perform all radiance and noise measurements. These results offer valuable insight into how aperture size, filtering architecture, and observing conditions jointly affect the feasibility and reliability of satellite QKD, particularly during daylight operations where background suppression is most challenging.
This paper presents an in-depth satellite-to-ground Quantum Key Distribution (QKD) feasibility analysis between a Low Earth Orbit (LEO) satellite and a 0.28 m small-sized Optical Ground Stations (OGSs) located in urban areas, over nighttime. Both Prepare & Measure (P&M) and Entanglement (ENT)- based QKD protocols are employed and compared in terms of distilled key bits. The performance of various detection setups and environmental conditions including different wavelengths (1550 nm and 810 nm) as well as different types of detectors (Avalanche and Superconducting Nanowires) is simulated. The results are compared to those obtained when OGSs with much larger receiver apertures are placed in rural environment. Our analysis delivers a framework examining the parameters affecting the QKD satellite downlink, and also provides a thorough theoretical framework for the employment of small-sized OGSs in urban environments.