High-dimensional orbital-angular-momentum quantum key distribution (OAM-QKD) in free space is jointly limited by aperture-induced diffraction and turbulence-induced modal crosstalk. We develop a numerical framework that first selects an encoding for finite-aperture propagation and then addresses the breakdown of conventional adaptive optics (AO) in branch-point-rich turbulence. Comparing Laguerre–Gaussian, perfect Laguerre–Gaussian, and minimum-diffraction-loss (MDL) modes identifies MDL modes as the most robust of the tested encodings against diffraction-induced quantum dit error rate (QDER). Using this baseline, we introduce branch-cut-length-minimized AO: branch points are localized from beacon-wavefront slopes, equivalent hidden-phase representations are ranked by intensity-weighted branch-cut length, and the selected reconstruction is fitted to a finite-actuator deformable mirror. Under moderate-to-strong turbulence, conventional AO provides useful recovery only while the wavefront remains predominantly smooth, whereas branch-cut-length minimization reduces QDER once branch points become prevalent. For a five-dimensional MDL-encoded QKD link at r 0 = 0.01 m, the mean QDER over 500 turbulence realizations decreases from 0.5550 for traditional AO to 0.5302 after branch-cut-length minimization, compared with 0.4957 for the Ideal AO reference. Residual-error analysis further shows that the remaining QDER is most strongly associated with residual phase mismatch and out-of-subspace leakage. At the same time, receiver-plane amplitude scintillation remains inaccessible to phase-only AO. These results establish branch-cut-length minimization as a physically implementable route to extend beacon-based AO beyond smooth-wavefront conditions, while defining the residual limits imposed by the finite measurement subspace and amplitude distortion.
High-dimensional orbital-angular-momentum quantum key distribution (OAM-QKD) in free space is jointly limited by aperture-induced diffraction and turbulence-induced modal crosstalk. We develop a numerical framework that first selects an encoding for finite-aperture propagation and then addresses the breakdown of conventional adaptive optics (AO) in branch-point-rich turbulence. Comparing Laguerre–Gaussian, perfect Laguerre–Gaussian, and minimum-diffraction-loss (MDL) modes identifies MDL modes as the most robust of the tested encodings against diffraction-induced quantum dit error rate (QDER). Using this baseline, we introduce branch-cut-length-minimized AO: branch points are localized from beacon-wavefront slopes, equivalent hidden-phase representations are ranked by intensity-weighted branch-cut length, and the selected reconstruction is fitted to a finite-actuator deformable mirror. Under moderate-to-strong turbulence, conventional AO provides useful recovery only while the wavefront remains predominantly smooth, whereas branch-cut-length minimization reduces QDER once branch points become prevalent. For a five-dimensional MDL-encoded QKD link at r 0 = 0.01 m, the mean QDER over 500 turbulence realizations decreases from 0.5550 for traditional AO to 0.5302 after branch-cut-length minimization, compared with 0.4957 for the Ideal AO reference. Residual-error analysis further shows that the remaining QDER is most strongly associated with residual phase mismatch and out-of-subspace leakage. At the same time, receiver-plane amplitude scintillation remains inaccessible to phase-only AO. These results establish branch-cut-length minimization as a physically implementable route to extend beacon-based AO beyond smooth-wavefront conditions, while defining the residual limits imposed by the finite measurement subspace and amplitude distortion.
The orbital angular momentum (OAM) of photons is a promising degree of freedom for high-dimensional quantum key distribution (QKD). However, effectively mitigating the adverse effects of atmospheric turbulence is a persistent challenge in OAM QKD systems operating over free-space communication channels. In contrast to previous works focusing on correcting static simulated turbulence, we investigate the performance of OAM QKD in real atmospheric turbulence with real-time adaptive optics (AO) correction. We show that even though our AO system provides a limited correction, it is possible to mitigate the errors induced by weak turbulence and establish a secure channel. The crosstalk induced by turbulence and the performance of AO systems is investigated in two configurations: a lab-scale link with controllable turbulence, and a 340 m long cross-campus link with dynamic atmospheric turbulence. Our experimental results suggest that an advanced AO system with fine beam tracking, reliable beam stabilization, precise wavefront sensing, and accurate wavefront correction is necessary to adequately correct turbulence-induced error. We also propose and demonstrate different solutions to improve the performance of OAM QKD with turbulence, which could enable the possibility of OAM encoding in strong turbulence.
Quantum key distribution (QKD) employed orbital angular momentum (OAM) for high-dimensional encoding enhances the system security and information capacity between two communication parties. However, such advantagesare significantly degraded because of the fragility of OAM states in atmospheric turbulence. Unlike previous researches, we first investigate the performance degradation of OAM-based QKD by infinitely long phase screen (ILPS), which offers a feasible way to study how adaptive optics (AO) dynamically corrects the turbulence-induced aberrations in real time. Secondly, considering the failure of AO while encountering phase cuts, we evaluate the quality enhancement of OAM-based QKD under a moderate turbulence strength by AO after implementing the wrapped cuts elimination. Finally, we simulate that, with more realistic considerations; real-time AO can still mitigate the impact of atmospheric turbulence on OAM-based QKD even in the large wind velocity regime.
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) 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.
The orbital angular momentum (OAM) of photons is a promising degree of freedom for high-dimensional quantum key distribution (QKD). Due to the greater flexibility in applications and the lower loss, QAM QKD over the free-space channel is still significant. However, effectively mitigating the adverse effects of atmospheric turbulence is a persistent challenge. In contrast to previous works focusing on correcting static simulated turbulence, we investigate the performance of OAM QKD in real atmospheric turbulence with real-time adaptive optics (AO) correction. We show that, it is possible to mitigate the errors induced by weak turbulence and establish a secure channel under some conditions. The cross-talk induced by turbulence and the performance of AO systems are investigated in a lab-scale link with controllable turbulence. The relations between the crosstalk and AO specifications is also studied. Our experimental results suggest that an advanced AO system with fine beam tracking, reliable beam stabilization, precise wavefront sensing and accurate wavefront correction is necessary to adequately correct turbulence-induced error.
Quantum key distribution (QKD) allows secure key exchange based on the principles of quantum mechanics, with higher-dimensional photonic states offering enhanced channel capacity and resilience to noise. Free-space QKD is crucial for global networks where fibres are impractical, but atmospheric turbulence introduces severe states’ distortions, particularly for spatial modes. Adaptive optics (AO) provides a pathway to correct these errors, though its effectiveness depends on the encoding basis. Here, we experimentally evaluate a high-speed AO system for orbital angular momentum (OAM) modes, mutually unbiased bases (MUB), and symmetric, informationally complete, positive operator-valued measures (SIC-POVM) up to dimension d = 8 in a turbulent free-space channel. While OAM states are strongly distorted, their cylindrical symmetry makes them optimally corrected by AO, yielding error rates below QKD security thresholds. MUB and SIC-POVM exhibit greater intrinsic robustness to turbulence but are less precisely corrected; however their performance remains within protocol tolerances. These results establish AO as a key enabler of secure, high-dimensional QKD and highlight the role of basis choice in optimizing resilience and correction.