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Physica Scripta

Adaptive optics based on all-optical machine learning enhances the performance of satellite-based quantum key distribution

Guoqi Huang, Mingyuan Sun, Rongzhen Jiao

Published Mar 20, 2025• 0 citations
Publisher

Abstract

Abstract Satellite-based quantum key distribution (QKD) faces the challenge of wavefront aberration in free-space links. The aberration causes a mismatch between the mode field of the received beam and the single mode fiber (SMF), thereby reducing the coupling efficiency of the SMF and affecting the system’s performance. Here, adaptive optics (AO) based on diffractive deep neural network (D2NN) is applied to satellite-based QKD system to achieve rapid and effective wavefront correction. Simulation results show that the coupling efficiency has been improved under different turbulence intensities, demonstrating and large dynamic range of D2NN. Especially under strong turbulence, coupling efficiency has led to an order-of-magnitude increase in the average key rate of satellite-based QKD. Compared to AO based on adaptive stochastic parallel gradient descent (ASPGD) algorithm, D2NN AO has the advantages of light-speed real-time correction and stability. Furthermore, using reflective D2NN compensates for low transmittance of the network layers. D2NN AO provids a highly promising solution for the future development of satellite quantum communication.

DOI
10.1088/1402-4896/adc349

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Abstract Satellite-based quantum key distribution (QKD) faces the challenge of wavefront aberration in free-space links. The aberration causes a mismatch between the mode field of the received beam and the single mode fiber (SMF), thereby reducing the coupling efficiency of the SMF and affecting the system’s performance. Here, adaptive optics (AO) based on diffractive deep neural network (D2NN) is applied to satellite-based QKD system to achieve rapid and effective wavefront correction. Simulation results show that the coupling efficiency has been improved under different turbulence intensities, demonstrating and large dynamic range of D2NN. Especially under strong turbulence, coupling efficiency has led to an order-of-magnitude increase in the average key rate of satellite-based QKD. Compared to AO based on adaptive stochastic parallel gradient descent (ASPGD) algorithm, D2NN AO has the advantages of light-speed real-time correction and stability. Furthermore, using reflective D2NN compensates for low transmittance of the network layers. D2NN AO provids a highly promising solution for the future development of satellite quantum communication.
Satellite-based quantum communication is a promising approach for establishing global-scale quantum networks. In free-space quantum channels, single-mode-fiber coupling plays a crucial role in increasing the signal-to-noise ratio of daylight quantum key distribution (QKD) and ensuring compatibility with standard fiber-based QKD protocols. However, consistently achieving high efficiency and stable single-mode-fiber coupling under strong atmospheric turbulence remains an ongoing experimental challenge. In this study, we experimentally demonstrate an adaptive method based on convolutional neural networks capable of directly estimating phase information from a single defocused image. We developed a convolutional neural network to establish the relationship between intensity distribution and phase information of turbulent distortions. We demonstrate the real-time performance of our deep-learning adaptive method in increasing single-mode-fiber coupling efficiency across various turbulence scales and quantify turbulence frequencies. Notably, the method proved highly effective in strong-turbulence scenarios, with frequencies reaching up to 200 Hz, leading to a significant increase in single-mode-fiber coupling efficiency. We demonstrate the corrective capability of our adaptive method for strong turbulence, enabled by the generalization of the convolutional neural network. Our results offer an efficient solution for daytime free-space QKD applications.
Satellite-based quantum communication is a promising approach for realizing global-scale quantum networks. For free-space quantum channel, single-mode fiber coupling is particularly important for improving the signal-to-noise ratio of daylight quantum key distribution (QKD) and compatibility with standard fiber-based QKD. However, achieving a highly efficient and stable single-mode coupling efficiency under strong atmospheric turbulence remains experimentally challenging. Here, we develop a single-mode receiver with an adaptive optics (AO) system based on a modal version of the stochastic parallel gradient descent (M-SPGD) algorithm and test its performance over an 8 km urban terrestrial free-space channel. Under strong atmospheric turbulence, the M-SPGD AO system obtains an improvement of about 3.7 dB in the single-mode fiber coupling efficiency and a significant suppression of fluctuation, which can find its applications in free-space long-range quantum communications.
Quantum Key Distribution (QKD) promises a provably secure method to transmit information from one party to another. Free-space QKD allows for this information to be sent over great distances and in places where fibre-based communications cannot be implemented, such as ground-satellite. The primary limiting factor for free-space links is the effect of atmospheric turbulence, which can result in significant error rates and increased losses in QKD channels. Here, we employ the use of a high-speed Adaptive Optics (AO) system to make real-time corrections to the wavefront distortions on spatial modes that are used for high-dimensional QKD in our turbulent channel. First, we demonstrate the effectiveness of the AO system in improving the coupling efficiency of a Gaussian mode that has propagated through turbulence. Through process tomography, we show that our system is capable of significantly reducing the crosstalk of spatial modes in the channel. Finally, we show that employing AO reduces the quantum dit error rate for a high-dimensional orbital angular momentum-based QKD protocol, allowing for secure communication in a channel where it would otherwise be impossible. These results are promising for establishing long-distance free-space QKD systems. High-dimensional quantum key distribution will allow for higher information density and greater error tolerance in future quantum networks. This work experimentally demonstrates how implementing an adaptive optics system in a spatial-mode free-space optical link can allow for quantum communications where it would otherwise be impossible.
Abstract Future quantum communication infrastructures will rely on both terrestrial and space-based links integrating high-performance optical systems engineered for this purpose. In space-based downlinks in particular, the loss budget and the variations in the signal propagation due to atmospheric turbulence effects impose a careful optimization of the coupling of light in single-mode fibers required for interfacing with the receiving stations and the ground networks. In this work, we perform a comprehensive study of the role of adaptive optics (AO) in this optimization, focusing on realistic baseline configurations of prepare-and-measure quantum key distribution, with both discrete and continuous-variable encoding, and including finite-size effects. Our analysis uses existing experimental turbulence datasets at both day and night time to model the coupled signal statistics following a wavefront distortion correction with AO, and allows us to estimate the secret key rate for a range of critical parameters, such as turbulence strength, satellite altitude and ground telescope diameter. The results we derive illustrate the interest of adopting advanced AO techniques in several practical configurations.
This article offers a solution to the fragmentation problem through a unified model and simulation showing how Adaptive Optics (AO) improves both classical and quantum communications. Free-space optical links can carry Quantum Key Distribution (QKD) over distances that cannot be reached with fiber; however, atmospheric turbulence corrupts the wavefront. For a single-mode quantum receiver, this drives the quantum bit error rate above the security threshold before detector noise can even be considered. AO addresses this by restoring the wavefront and recovering the coupling loss. The theory, mitigation techniques, and quantum security analysis needed to understand this are spread across three research communities that rarely cite one another: atmospheric turbulence and AO theory, classical modulation over fading channels, and BB84 quantum key distribution. We consolidate and develops all three topics in order to trace the chain from wavefront error to bit error rate and secret key rate. We survey six families of turbulence mitigation and provide an assessment of their suitability for single-mode quantum links. In addition, we survey the free-space QKD experimental record, from terrestrial hops to satellite downlinks. A wave optics case study of a representative 500-km Low Earth Orbit (LEO) downlink, corrected by a closed-loop Shack–Hartmann and deformable-mirror receiver at 1.55 μm ties the review together. Across a turbulence sweep, the correction stabilizes single-mode coupling near its physical ceiling, while the uncorrected channel varies by more than an order of magnitude. When carried into the downlink budget, this lifts the quantum link across the 11% security bound at 12 of 15 turbulence conditions where the uncorrected link yields no key, thereby restoring a secure quantum bit error rate in a narrow 4.2–4.9% band throughout and a secret key rate of 7.6–11.2 kbit/s while simultaneously widening the margin of the co-propagating classical channel above its forward error correction threshold.
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.