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arXiv:2303.09942 · Optics Express

Adjustable spatial filter for optimal free-space quantum communication round the clock

Andrej Kržič, Nico Döll, Uday Chandrashekara, Christopher Spiess, Fabian Steinlechner

Published Aug 23, 2023• 1 citation
arXivHTMLPDF

Abstract

Free-space quantum communication in daylight relies crucially on spatial filtering. The optimal filter configuration, however, depends on ever-changing link conditions. To this end, we developed an adjustable spatial filter that can be used to change the system field of view on the fly. We demonstrate its use in quantum key distribution over a 1.7-km free-space link. Furthermore, we compare it to filtering with multi-mode fiber coupling. Finally, we extrapolate our results to a broader range of realistic link conditions and show that active field-of-view optimization has the potential to substantially improve the overall secure key output of the system.

DOI
10.1364/oe.489022

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Free-space quantum communication in daylight relies crucially on spatial filtering. The optimal filter configuration, however, depends on ever-changing link conditions. To this end, we developed an adjustable spatial filter that can be used to change the system field of view on the fly. We demonstrate its use in quantum key distribution over a 1.7-km free-space link. Furthermore, we compare it to filtering with multi-mode fiber coupling. Finally, we extrapolate our results to a broader range of realistic link conditions and show that active field-of-view optimization has the potential to substantially improve the overall secure key output of the system.
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.
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.
Optimising the interconnection between free-space and fibre links will be necessary for future quantum communication networks. In daylight free-space quantum communication based on direct detection, the required Field Of View (FoV) of the receiver system needs to be minimised to reduce solar background noise coupling into the detectors. Reducing the FoV requires minimising beam wander effects caused by atmospheric turbulence through active optics. We implement a fine tracking system designed to correct tip and tilt wavefront aberrations, using two feedback loops; each of them consisting of a quadrant detector and a fast steering mirror for stabilising the beam in the whole optical axis of the receiver. We test the performance of the tracking system with different optical fibres in order to evaluate the reduction in the quantum bit error rate (QBER) caused by solar background noise. A reduction of 75% for single mode fibre was obtained, and 45% reduction for a 25 µm core diameter fibre, both cases for strong turbulence (Cn2~10-12 – 10-13 m-2/3) and 100 m propagating channel. These results look promising for enabling free-space Quantum Key Distribution (QKD) in wireless networks for realistic/adverse conditions such as daylight and strong turbulent regimes.
Quantum key distribution (QKD) enables remote users to share cryptographic keys with information theoretic security. Satelliteto-ground QKD offers a route to extend the distances between users that is otherwise limited by losses through optical fibre. However, satellite QKD is typically limited to operation at night due to the background photons from sunlight. A proposed solution to reduce the background counts is tight spatial filtering via single-mode fibre coupling, but this comes at the cost of losses because of imperfect coupling efficiency due to wavefront distortions from atmospheric turbulence. To maximise the number of photons entering the fibre, adaptive optics (AO) can be used to correct for the atmospheric effects. We present results of end-to-end link simulations which show the trade-off between AO complexity and fibre coupling efficiency. These results show that for representative turbulence characteristics, a relatively small deformable mirror is required to obtain the required coupling efficiency for a reasonable key rate. This paves the way for affordable quantum communication in daylight conditions.