Light’s hidden properties save quantum information from the chaos of bad weather

By sending quantum information through a storm, researchers at The University of the Witwatersrand in Johannesburg, South Africa, have shown that the information in light can be kept completely intact, even though the light itself was completely warped beyond recognition. 

This breakthrough offers a revolutionary way to keep data safe from environmental interference, paving the way for unbreakable global communication networks and ultra-resilient quantum computers.

For years, researchers have tried to harness the “twist” of light to transmit data. This property describes how light spirals as it travels forward, and because it can be moulded into virtually infinite different twists, it provides a massive, promising alphabet for high-capacity communication. 

This twisted light has proven to be notoriously fragile in real-world environments like bad weather, atmospheric turbulence, and water. Once it passes through these chaotic media, the twisted pattern becomes completely unrecognisable, a major historical barrier that has stalled the use of this large alphabet for global communications.

The Wits team shifted their focus from trying to protect the fragile physical structure of the quantum light to exploring a deeper mathematical property embedded within it, known as its topology. Topology is a mathematical property that allows a structure to be severely squished and deformed without losing its core information. 

Until now, scientists always thought topology had to be built out of robust properties of light to survive. The Wits researchers asked what would happen if they used the fragile twist of light instead, driven by one critical advantage: these twisting structures are naturally and freely present when creating quantum light. Because they require zero extra effort to engineer, the topology is essentially available for free.

Reporting today in Physical Review Letters, the team showed that these hidden topological invariants remained completely intact as they passed through strong atmospheric turbulence, even while the fragile twisting structures upon which they were built decayed heavily.

“We watched the physical patterns warp under extreme turbulence, and the traditional quantum connection hit a point of severe deterioration. Yet, because the topology is inherently embedded in the entanglement itself, it remained completely unbroken,” says lead author Tatjana Kleine. “The physical building blocks themselves are fragile, but the topology built upon them is indestructible.”

Even at a point where the light is distorted beyond recognition by bad weather and the entanglement fades, the topological number emerges unscathed. 

“We now have access to this huge alphabet of spatial modes once again, as long as we look at the topology rather than the state itself,” says Professor Andrew Forbes, head of the Structured Light Laboratory at the Wits School of Physics.

The team believes that this discovery will reopen long-dormant avenues for communicating securely using a high-dimensional property of light that is freely available to us. Better yet, these states can easily be scaled up to higher dimensions for even greater communication security under realistic, real-world conditions.