Topological robustness of optical skyrmions through a real-world free-space link

Structured light offers a promising solution for the increasing data demands of modern optical networks, opening up new degrees of freedom that can be leveraged for greater channel capacity and more bits per photon. However, its implementation is hindered by real-world distortions, for example, atmospheric turbulence in free-space, with severe and rapidly evolving phase perturbations that alter the amplitude, phase and vectorial polarization structure of the beam. Here, we demonstrate that optical topologies in the form of skyrmions are highly resilient to the effects of real-world atmospheric turbulence. We create and transmit these particle-like topologies of light through a 270~m free-space optical link, revealing their robustness across a wide variety of conditions and turbulence strengths. While we observe severe distortion in the states’ underlying degrees of freedom, we show that the topological numbers are preserved in all cases. We account for fast changes to the medium, where the channel produces statistically averaged outcomes, by probing the state’s decoherence, showing that while the degree of polarisation consequently decays, the topology remains intact. Using topology, we show information can be transmitted through the channel with almost perfect fidelity (>98%) in most cases, only decreasing to 86% in the most severe conditions tested. Our work is the first to demonstrate the potential for optical topologies as reliable and robust information carriers in a real-world environment and points to the potential for other complex channels too, offering attractive features for classical and quantum communication alike.

💡 Research Summary

This paper investigates the robustness of optical skyrmions—vector beams whose transverse field maps onto the Poincaré sphere with an integer winding number—when transmitted through a real‑world free‑space optical link subject to atmospheric turbulence. The authors generate three skyrmion states (N = 1, 2, 3) by superposing Laguerre‑Gaussian modes with opposite circular polarizations (ℓ₁ = 1, 2, 3; ℓ₂ = 0) using a spatial light modulator and a Mach–Zehnder interferometer. The beams are sent across a 270 m outdoor link on the University of the Witwatersrand campus. At the receiver, a single‑shot Stokes polarimetry setup records all six polarization projections (H, V, D, A, R, L) simultaneously, enabling full reconstruction of the Stokes vector field within the atmospheric coherence time τ₀.

The study examines three typical daily turbulence regimes: calm morning, intense midday, and moderate late‑afternoon conditions. In the morning, intensity profiles remain nearly cylindrical and the polarization texture is only mildly perturbed. Midday turbulence produces severe amplitude speckle, multiple lobes, and rapid spatial polarization fluctuations, yet the skyrmion’s topological winding remains evident. Late‑afternoon conditions lie between these extremes. For each regime the authors compute the winding number N via the integral N = (1/4π)∬ S·(∂ₓS × ∂ᵧS) dx dy, where S is the normalized Stokes vector. Measured values (e.g., N_exp = 0.81 ± 0.01 for N = 1 in the morning, N_exp = 0.97 ± 0.02 for N = 1 under the strongest turbulence) match the encoded integers within experimental uncertainty, confirming topological preservation.

Beyond single‑shot snapshots, the authors perform 120‑second continuous measurements to assess statistical stability. Even as the scintillation index σ_I² rises dramatically, the average winding number remains constant, while the degree of polarization (DoP) can drop by up to 40 %. This demonstrates that turbulence induces a smooth deformation of the field mapping rather than a topological change.

Information fidelity is evaluated by decoding the integer N from the received Stokes field. In most cases fidelity exceeds 98 %; under the most severe turbulence it remains above 86 %. Notably, no adaptive optics, channel probing, or post‑processing corrections are applied, highlighting the intrinsic resilience of topological encoding.

The authors argue that such topological robustness offers a powerful route to increase channel capacity and reliability in free‑space optical communications, both classical and quantum. Skyrmions can be generated and detected with relatively simple hardware, and their integer winding number survives even in highly turbulent environments, making them attractive for high‑speed links, secure line‑of‑sight links, and potentially for quantum key distribution where decoherence is a critical concern.

Future work suggested includes extending the link length, exploring other wavelengths, multiplexing multiple skyrmions, and testing non‑classical entangled states to verify whether topological protection extends to quantum correlations. Overall, the paper provides the first experimental demonstration that topological optical structures can serve as reliable information carriers in realistic atmospheric channels, opening a new avenue for robust, correction‑free free‑space communication.