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Scientific Reports

Experimental filtering effect on the daylight operation of a free-space quantum key distribution

Heasin Ko, Kap‐Joong Kim, Joong‐Seon Choe, Byung‐Seok Choi, Jong-Hoi Kim, Yongsoon Baek, Chun Ju Youn

Published Oct 11, 2018• 33 citations
Publisher

Abstract

One of the challenges of implementing free-space quantum key distribution (QKD) systems working in daylight is to remove unwanted background noise photons from sunlight. Elaborate elimination of background photons in the spectral, temporal, and spatial domains is an indispensable requirement to decrease the quantum bit error rate (QBER), which guarantees the security of the systems. However, quantitative effects of different filtering techniques and performance optimization in terms of the secure key rate have not been investigated. In this study, we quantitatively analyze how the performance of the QBER and the key rates changes for different combinations of filtering techniques in a free-space BB84 QKD system in daylight. Moreover, we optimize the conditions of filtering techniques in order to obtain the maximum secure key rate.

DOI
10.1038/s41598-018-33699-y

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One of the challenges of implementing free-space quantum key distribution (QKD) systems working in daylight is to remove unwanted background noise photons from sunlight. Elaborate elimination of background photons in the spectral, temporal, and spatial domains is an indispensable requirement to decrease the quantum bit error rate (QBER), which guarantees the security of the systems. However, quantitative effects of different filtering techniques and performance optimization in terms of the secure key rate have not been investigated. In this study, we quantitatively analyze how the performance of the QBER and the key rates changes for different combinations of filtering techniques in a free-space BB84 QKD system in daylight. Moreover, we optimize the conditions of filtering techniques in order to obtain the maximum secure key rate.
Daylight entanglement-based free-space quantum key distribution (QKD) is limited by accidental coincidences from receiver-admitted background light. We develop and experimentally validate a receiver-level framework anchored to measured singles rates and reference coincidence components, linking receiver bandwidth, accepted temporal width, and background-noise density to Bob singles, sifted-key rate, error rate, and quantum bit error rate (QBER) in telecom-wavelength BBM92 QKD. Indoor sweeps show that the sifted-key rate saturates near the source-matched bandwidth, whereas broader bandwidth or higher background mainly increases accidental contamination. Increasing the accepted temporal width leaves Bob singles nearly unchanged but directly raises QBER by enlarging the random-overlap probability. A two-dimensional design map calculated from the model shows that the temporal-window margin contracts rapidly with increasing background-to-signal ratio, while the bandwidth margin remains comparatively broad near source-matched filtering. A rooftop experiment demonstrates daylight operation over a 10 m free-space link, yielding a mean sifted-key rate of 2,811 cps and a mean QBER of 4.43%. This framework provides a quantitative basis for choosing receiver bandwidth and temporal acceptance to meet a target QBER under specified background conditions.
Abstract Free-space ground-to-ground links will be an integral part of future quantum communication networks. The implementation of free-space and fiber links in daylight inter-modal configurations is however still hard to achieve, due to the impact of atmospheric turbulence, which strongly decreases the coupling efficiency into the fiber. In this work, we present a comprehensive model of the performance of a free-space ground-to-ground quantum key distribution (QKD) system based on the efficient-BB84 protocol with active decoy states. Our model takes into account the atmospheric channel contribution, the transmitter and receiver telescope design constraints, the parameters of the quantum source and detectors, and the finite-key analysis to produce a set of requirements and optimal design choices for a QKD system operating under specific channel conditions. The channel attenuation is calculated considering all effects deriving from the atmospheric propagation (absorption, beam broadening, beam wandering, scintillation, and wavefront distortions), as well as the effect of fiber-coupling in the presence of a partial adaptive optics correction with finite control bandwidth. We find that the channel fluctuation statistics must be considered to correctly estimate the effect of the saturation rate of the single-photon detectors, which may otherwise lead to an overestimation of the secret key rate. We further present strategies to minimize the impact of diffuse atmospheric background in daylight operation by means of spectral and temporal filtering.
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.