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) 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) 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.
Abstract Quantum Key Distribution (QKD) provides information-theoretic security for future communication networks by exploiting the fundamental principles of quantum mechanics. However, satellite-to-ground QKD links suffer significant performance degradation due to atmospheric turbulence, absorption, scattering, and satellite-induced pointing errors. This paper investigates the BB84 QKD protocol over a Free-Space Optical (FSO) channel modeled using the Gamma–Gamma distribution, which accurately captures both small-scale and large-scale atmospheric irradiance fluctuations. Closed-form and numerical results for outage probability and Quantum Bit Error Rate (QBER) are presented under weak, moderate, and strong turbulence regimes. The Secure Key Rate (SKR) is analyzed as a function of link distance under clear and foggy weather conditions. Additionally, the impact of satellite vibration is evaluated using a jitter-dependent pointing error model, demonstrating that misalignment significantly reduces link reliability. Simulation results show that strong turbulence conditions introduce more than 15 dB SNR penalty compared to clear-sky operation, while haze conditions limit QKD range to under 8–10 km. These findings provide practical performance benchmarks for Low-Earth Orbit (LEO) quantum downlinks in realistic atmospheric environments. The results provide practical insights for electronic receiver design and system implementation of satellite quantum communication links.
Atmospheric conditions play a pivotal role in determining the performance of satellite-to-ground quantum key distribution (QKD). In this research, we meticulously investigate the influence of various factors, including background light, dark counts, and aperture coupling efficiency, on the performance of satellite-to-ground QKD under different levels of turbulence intensity associated with diverse atmospheric stratifications. Based on the decoy state BB84 protocol, we refine the calculation formula for the quantum bit error rate (QBER) of the satellite-to-ground QKD system. Subsequently, we conduct simulations to analyze how the quantum BER varies under different conditions, such as varying intensities of background light, different wavelengths, and different zenith angles. The simulation results reveal that, when taking all the above-mentioned factors into comprehensive consideration, employing signals with shorter wavelengths can lead to a more favorable QBER. The findings of this study offer valuable insights and a solid reference for the construction of a satellite-to-ground QKD system, facilitating the development of more reliable and efficient quantum communication links between space and the ground.
To enable global scale quantum key distribution1-3 (QKD), satellite based systems 4,5 are the most promising approach. So far, free-space QKD has already been demonstrated on communication channels with attenuation comparable to satellite downlinks,6 and classical laser communications with satellites and aircrafts is heavily explored.7-10 Here, combining both these challenges, we demonstrate an aircraft to ground QKD transmission obtaining a sifted key rate of 145 bit/s and a QBER, larglely dominated by background events and stray light, of 4:8 %.
Satellite-based free-space quantum key distribution (QKD) provides a practical framework for achieving secure global communication beyond the limitations of optical fibers. In this work, the quantum bit error rate (QBER) and secure key rate of four representative protocols-BB84, B92, BBM92, and E91 are investigated over low earth orbit (LEO) links in both uplink and downlink configurations. The optical link is modeled using a Gaussian beam formalism, incorporating the effects of diffraction, pointing errors, atmospheric turbulence, and background noise contributions. The protocols are examined under day and night-time operating conditions, and their dependence on the zenith angle is analyzed. The findings show that downlink links generally exhibit lower QBER and higher secure key rates than uplinks, and among prepare-and-measure schemes, BB84 consistently outperforms B92, while in entanglement-based approaches, BBM92 achieves higher key rates than E91.