Atmospheric background radiance is a critical factor in free-space Quantum Key Distribution (QKD) systems since it can significantly increase the Quantum Bit Error Rate (QBER). Accurate assessment of its impact on QKD requires measurements within the narrow spectral bandwidth and with detectors capable of sensing single photons used in QKD scenarios. These measurements serve as a valuable tool for site characterization, helping to asses the impact of background light at potential Quantum Optical Ground Station locations and to develop appropriate mitigation strategies. The single-photon detectors in the background light measurement setup need to be operated both at low detection rates at night and high detection rates by day. This is challenging because the dynamic range of sky background radiance spans roughly seven orders of magnitude. It is therefore crucial to accurately determine the statistical uncertainty of the photon rate measurements for incident background radiance of any intensity. Only then can a full error budget for the measurement results be confidently constructed. This contribution presents analytic results, obtained using Bayesian techniques, for the statistical uncertainty of rate measurements with realistic counting detectors including the effects of a finite detector dead time and a finite dark count rate. The effects of the after-pulsing probability and the detection efficiency are also discussed. In addition, these results give a formula for the measurement duration required to attain a given measurement accuracy, which can be used to optimize the ‘hemisphere scan time’ of a background radiance measurement instrument in real-time. The results are generic and can also be applied to other types of counting detectors, such as Geiger-Müller counters.
Atmospheric background radiance is a critical factor in free-space Quantum Key Distribution (QKD) systems since it can significantly increase the Quantum Bit Error Rate (QBER). Accurate assessment of its impact on QKD requires measurements within the narrow spectral bandwidth and with detectors capable of sensing single photons used in QKD scenarios. These measurements serve as a valuable tool for site characterization, helping to asses the impact of background light at potential Quantum Optical Ground Station locations and to develop appropriate mitigation strategies. The single-photon detectors in the background light measurement setup need to be operated both at low detection rates at night and high detection rates by day. This is challenging because the dynamic range of sky background radiance spans roughly seven orders of magnitude. It is therefore crucial to accurately determine the statistical uncertainty of the photon rate measurements for incident background radiance of any intensity. Only then can a full error budget for the measurement results be confidently constructed. This contribution presents analytic results, obtained using Bayesian techniques, for the statistical uncertainty of rate measurements with realistic counting detectors including the effects of a finite detector dead time and a finite dark count rate. The effects of the after-pulsing probability and the detection efficiency are also discussed. In addition, these results give a formula for the measurement duration required to attain a given measurement accuracy, which can be used to optimize the ‘hemisphere scan time’ of a background radiance measurement instrument in real-time. The results are generic and can also be applied to other types of counting detectors, such as Geiger-Müller counters.
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.
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.
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.
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.
InGaAs/InP single-photon detectors are today the most frequently used detectors for fibre-based Quantum Key Distribution (QKD) [1,2]. The performance of the QKD-systems greatly depends on the optical properties of its detector, such as quantum efficiency, dead time, dark counts, gating rate, etc., which need to be metrologically characterized to fully grantee the QKD network security. Therefore, several European National Metrological Institutes (NMIs) are putting today great efforts in developing novel measurement methods and calibration facilities, which allow to perform the traceable characterisation of single-photon detectors by using reference standards [3, 4]. From the metrological, and specifically radiometric, point of view, the detection efficiency of the single-photon detector is the main parameter that needs to be traceable to the primary standard for optical power (i.e. the cryogenic radiometer), which is maintained by most of the NMIs. Recently, four European NMIs have carried out a pilot comparison on the detection efficiency of a free-running InGaAs/InP single-photon detector, which main purpose has been to provide a snapshot of the measurement capabilities of the four European NMIs in the field of photon counting detection. The comparison was carried out in the framework of the EMPIR project 14IND05 “Optical metrology for quantum-enhanced secure telecommunication (MIQC2)” at the telecom wavelength of 1550 nm by using different reference standards with independently traceability chains. The results of this comparison, including the different experimental setups, the measurement methods, the traceability chain and the uncertainty evaluation of each participant, will be presented in this conference. References: [1] Akihisa Tomita, et al., “High speed quantum key distribution system”, Optical Fiber Technology, 16, Issue 1, 55-62 (2010). [2] Damien Stucki, et al., “Photon counting for quantum key distribution with peltier cooled InGaAs/InP APDs”, Journal of Modern Optics, 48, Issue 13, 1967-1981 (2001). [3] M. López, et al, “Detection efficiency calibration of single-photon silicon avalanche photodiodes traceable using double attenuator technique, Journal of Modern Optics 62, S21 – S27, 2015. [4] G. Porrovecchio, et al., “Comparison at the sub-100 fW optical power level of calibrating a single-photon detector using a high-sensitive, low-noise silicon photodiode and the double attenuator technique”, Metrologia 53, 1115-1122 (2016).
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.