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ACS Photonics

Turbulence-Invariant Polarization Readout via Spin-Dependent Angular Mapping

Chaoxu Chen, Yuan Wei, Fang Dong, Haoyu Zhang, Ouhan Huang, Ziwei Li, Fang Wei, Wei Chu, Haiwen Cai, Nan Chi, Junwen Zhang, Jianyang Shi

Published Oct 14, 2026• 0 citations
Publisher

Abstract

Abstract Atmospheric turbulence disrupts spatial coherence in free-space optical (FSO) systems, fundamentally limiting the reliable extraction of optical information encoded in intensity, phase, or spatial degrees of freedom. While polarization is intrinsically immune to scalar turbulence, its robust detection remains challenging due to the lack of turbulence-invariant observables. Here, we establish a general spin-dependent phase-difference-to-angle (PDA) mapping mechanism that converts polarization states into global angular intensity modulations, forming a class of turbulence-invariant observables accessible via intensity-only detection. This mapping, enabled by spin–orbit coupling in a vortex waveplate, eliminates the need for wavefront reconstruction and preserves information integrity under strong turbulence. We experimentally validate turbulence-invariant polarization readout over an equivalent long-distance FSO link under both weak and strong turbulence, achieving error rates below 2%. Leveraging this invariant readout, we further demonstrate a 100-Gbps-class orbital angular momentum polarization shift keying (OAMPolSK) communication scheme, with a 12.43-dB reduction in the required optical power compared with PAM-6 at a reference SER of 0.1 under the implemented experimental configurations. Our results demonstrate a turbulence-resilient optical information extraction strategy, in which polarization information is mapped onto a robust angular observable, offering a potential approach for optical communication and sensing in complex environments.

DOI
10.1021/acsphotonics.6c02024

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Abstract Atmospheric turbulence disrupts spatial coherence in free-space optical (FSO) systems, fundamentally limiting the reliable extraction of optical information encoded in intensity, phase, or spatial degrees of freedom. While polarization is intrinsically immune to scalar turbulence, its robust detection remains challenging due to the lack of turbulence-invariant observables. Here, we establish a general spin-dependent phase-difference-to-angle (PDA) mapping mechanism that converts polarization states into global angular intensity modulations, forming a class of turbulence-invariant observables accessible via intensity-only detection. This mapping, enabled by spin–orbit coupling in a vortex waveplate, eliminates the need for wavefront reconstruction and preserves information integrity under strong turbulence. We experimentally validate turbulence-invariant polarization readout over an equivalent long-distance FSO link under both weak and strong turbulence, achieving error rates below 2%. Leveraging this invariant readout, we further demonstrate a 100-Gbps-class orbital angular momentum polarization shift keying (OAMPolSK) communication scheme, with a 12.43-dB reduction in the required optical power compared with PAM-6 at a reference SER of 0.1 under the implemented experimental configurations. Our results demonstrate a turbulence-resilient optical information extraction strategy, in which polarization information is mapped onto a robust angular observable, offering a potential approach for optical communication and sensing in complex environments.
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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.
Free-space optical communication based on orbital angular momentum (OAM) offers advantages such as high security, large information capacity, and high-speed transmission. However, turbulence can induce random perturbations to the wavefront, thereby affecting communication performance. Hence, accurately measuring turbulence intensity is crucial. We delve into the quantum Fisher information (QFI) of the spatial coherence length, a pivotal parameter in atmospheric turbulence. Within the framework of free-space optical channels, we conduct a comprehensive analysis of the QFI in models encompassing both single and biphoton systems. As photons propagate through free space, the QFI regarding turbulence parameter initially increases and then decreases, due to the coupling and decoherence between the photons and turbulence. Increasing the number of OAM modes can significantly enhance the QFI regarding turbulence parameters. We considered situations closer to practical preparation, our results indicate that the impact of entanglement on estimation precision depends on the purity of the prepared entangled state. Moreover, we discuss and compare the variations in the QFI of various parameters upon considering backward scattering (BS). We believe our work is the first theoretical attempt towards the exploration of estimating the turbulence parameter via QFI.
The well defined spin and orbital angular momentum states of photons offers a practical realization of quantum digits and a means of secure single-photon optical communication. The orbital angular momentum is associated with the spatial distribution of the wavefunction and the number of orbital angular momentum eigenstates is unlimited, giving the possibility of arbitrary base-N digits. In this paper, free-space optical communications using angular momentum states of single photons is investigated and in particular the effect of atmospheric turbulence on the angular momentum of the photons is modelled. The refractive index fluctuations in the atmosphere perturb the complex amplitude of a propagating beam so that the photons that were launched in an eigenstate of orbital angular momentum are no longer guaranteed to be in the original eigenstate after propagation. By considering the resulting wave as a superposition of angular momentum states, the probability of obtaining correct or incorrect measurements of the transmitted digit is calculated. The effect on a free-space optical communication using orbital angular momentum and the use of adaptive optics is discussed. The information capacity per photon is quantified and compared to that using polarization states for binary digits.
Free-space quantum communications offers a promising route for securely transmitting information over long distances. However, a major challenge for these systems is atmospheric turbulence, which distorts the spatial structure of light and can severely limit the amount of information that can be reliably transmitted. In this work, we propose and demonstrate a turbulence-resilient scheme for free-space quantum communication. By leveraging the phase conjugation property of stimulated parametric down-conversion, our scheme enables all-optical dynamic correction of spatial-mode distortion induced by atmospheric turbulence, thereby enhancing the secure key rate in high-dimensional quantum key distribution. We develop a theoretical model that provides detailed guidelines for selecting the optimal basis and spatial properties needed to maximize the efficiency of the proposed scheme. Both numerical simulations and experimental results show that, even under strong turbulence, our scheme can reduce the quantum error rates well below the security threshold. These results highlight the potential of nonlinear optical approaches as powerful tools for robust quantum communication in realistic free-space environments.