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

Abstract

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.

Figures (5)

• Figure 1: Skyrmionic topology through turbulence. Illustration showing how optical skyrmions are formed via a mapping from the transverse plane to the Poincaré sphere, characterised by the skyrmion number $N$. When these states pass through a turbulent channel, the beam's structure is heavily distorted in phase, amplitude and polarisation, but the mapping to the Poincaré sphere is unchanged.
• Figure 2: Generation and detection through a free space link.a. Optical skyrmions were generated using complex amplitude holograms to generate two scaler modes. These modes were vectorially combined with a Mach-Zehnder interferometer and transmitted through a 270 m free space optical link. At the receiver, Stoke polarimetry was performed on the aberrated beam using a 50:50 beam splitter, quarter-wave plate and a polarisation sensitive camera b. Polarisation intensity projections for all 6 polarisation states captures simultaneously for a skyrmion with $N=1$ before the beam had propagated through the free space link. c. Polarisation intensity projections for all 6 polarisation states captures simultaneously for a skyrmion with $N=1$ after the beam had propagated through the free space link. d. State of polarisation, Stokes vector texture and mapping to the spatial sphere for a unaberrated skyrmion beam with $N=1$ captured using the detection scheme e. State of polarisation, Stokes vector texture and mapping to the spatial sphere for a unaberrated skyrmion beam with $N=2$ captured using the detection scheme
• Figure 3: Robustness through free space link.a. Total intensities and the measured state of polarisation for skyrmion with $N=1$ and $N=2$ after propagating indoors and through the free space link in the morning, midday and late afternoon. Experimentally measured Stokes vector textures (main) and corresponding OAM scalar component (inset) for skyrmions transmitted in the b. morning, c. at midday and d. in the late afternoon. The measured skyrmion number for encoded numbers of $N=1$, 2 and 3 and the measured scintillation index $\sigma_I^2$ measured over 120 s in the e. morning, f. at midday and g. in the late afternoon.
• Figure 4: Resilience to depolarisation and decoherence.a. The measured intensity and locally normalised Stokes parameters of a skyrmion beam with $N=1$ measured indoors (top row), after averaging for 2 ms (2nd row), after averaging for 25 ms (3rd row) and after averaging for 120 ms (fourth row). b. Line plot showing the measured skyrmion number for encoded numbers of $N=1$, 2 and 3 against the time over which the measurement was averaged. c. Line plot showing the measured degree of polarisation against the time over which the measurement was averaged.
• Figure 5: Robust information encoding. The measured skyrmion numbers over the measurement period and the reconstructed image of the transmitted image performed a in the morning under mild distortion, b at midday during the most severe distortion conditions, c in the late afternoon under moderate distortion and d under the effects of increasing depolarisation.