Researchers at the University of the Witwatersrand (Wits) in Johannesburg, South Africa, have demonstrated that the topological properties of light can be used to transfer information robustly through the atmosphere, a finding that could lead to more reliable long-distance optical communications, including space-based systems, and help connect remote communities worldwide.
The work, detailed in two separate studies on classical and quantum optics, shows that topological information encoded in light remains remarkably stable even when the light is severely distorted by atmospheric turbulence. Instead of relying solely on sophisticated hardware or computational algorithms to correct distortions, the new approach offers a fundamentally different way to achieve robust optical communication in challenging environments such as air or water.
Both studies are based on creating an optical topological structure known as a skyrmion in a light field and testing its resilience as it passes through a distorting medium. Skyrmions are quasiparticles with a two-dimensional, knot-like structure, first observed in magnetic materials and more recently in electromagnetic fields and the electric field of light waves. In light, these swirling structures are formed by twisting the polarization or spin vectors so that each polarization appears exactly once, twice, or three times, corresponding to the skyrmion number.
«Skyrmions are topologically stable, which means that external perturbations do not affect them in any way,» explains Andrew Forbes, head of the Structured Light Lab at the Wits School of Physics.
In the first study, Forbes and colleagues used laser light at 532 nm, shaped into vectorial beams with a spatial light modulator and a modified Mach–Zehnder interferometer to generate optical skyrmions with skyrmion numbers of 1, 2, and 3. They then transmitted the light through a 270-meter free-space optical link on the Wits Braamfontein campus in central Johannesburg. Using Stokes polarimetry, they measured the topology and found that the skyrmion number remained robust across a wide range of atmospheric conditions, from calm morning air to the intense distortions of midday heat.
«This result holds true even when the vectorial polarization of the underlying laser beam has been highly scrambled by atmospheric turbulence effects – and was therefore unrecognizable,» Forbes says.
In the second study, the team created topology through in-built correlations between two photons entangled in their optical angular momentum (OAM). Although OAM quantum states are not stable in distorting environments, the researchers found that a skyrmion with a skyrmion number of 1 maintained topological numbers close to 1 in a controlled laboratory setting with varying turbulence levels. This suggests that the topological number is fundamentally decoupled from modal crosstalk and the spreading of an individual photon’s OAM.
Despite the different physical systems, both experiments reveal a similar outcome: conventional properties of optical fields can be strongly degraded by distortions, while the topological information remains remarkably robust. Together, the works demonstrate that topology can be harnessed in both classical and quantum optical systems as a reliable carrier of information in diverse communication and information-processing scenarios.
Topology is a concept that appears throughout physics, from particle physics to magnetism and condensed matter. In many systems, topology comes with a physical protection mechanism, such as an energy barrier, that explains its resistance to disturbances. In optics, however, such protection is not guaranteed. «We therefore wanted to address a fundamental question: how robust is optical topology when light encounters a real-world, highly distorting environment?» Forbes says. «The atmosphere provides a particularly interesting testbed because it is highly relevant to optical sensing and communications and represents an extreme case where the distortions vary rapidly both in space and time.»
One of the major challenges was conducting precision optical experiments outdoors. Forbes notes that they had to contend with unpredictable changes in sunlight, temperature, wind, rain, and even thermal expansion of buildings, which can affect optical alignment and make stable measurements difficult over long periods.
The researchers believe their approach could advance long-distance optical communications, including space-based links, and help connect remote and underserved communities. By exploiting the robustness of optical topology, future systems may require less complex hardware or computational correction, offering a more efficient path to reliable communication through turbulent environments.
