Alice Drozdov

Structured light offers a promising solution for the increasing data demands of modern optical networks, opening up 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-meter 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 polarization consequently decays, the topology remains intact. Using topology, we show that 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. This work idemonstrates 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.

Atmospheric turbulence typically impairs free-space optical communication systems, particularly those using mode-division multiplexing, by causing detrimental crosstalk among higher-order modes. This interaction can also be harnessed through mode averaging at the receiver and transmitter to passively counteract turbulence effects. Mode averaging involves the simultaneous transmission and detection of multiple, strategically selected modes. Prior research on mode averaging—often referred to as modal diversity—primarily selected modes by using the spread of crosstalk among modes. In this study, we show that it is crucial to include the impact of mode-dependent loss. This is demonstrated by identifying optimal two-mode combinations that maximize received power while efficiently utilizing mode space. The optimal combinations assessed include pairs of orbital angular momentum (OAM), Laguerre-Gaussian (LG), and Hermite-Gaussian (HG) modes, as well as mixed pairs of LG and HG modes. Interestingly, it is shown that the optimal combinations are not always the neighboring modes.

Vectorial structured light, where the polarization is inhomogeneously distributed in space, has found a myriad of applications in both 2D and 3D optical fields. Here, we present an experimental study of the invariance and distortion of vectorial light through a real-world medium of atmospheric turbulence. We show that the amplitude and polarization structure are both severely distorted by the turbulent medium, yet the non-separability of these two degrees of freedom remains invariant. We monitor this invariance under a range of beam types and atmospheric conditions, over extended time periods, revealing the unitary nature of atmospheric turbulence in our experiment. Our results provide conclusive evidence that invariance and distortion are not mutually exclusive and that the degree of classical entanglement remains unaltered through such channels, and will be of interest to the large community interested in classical and quantum communication in free space.