The team has fabricated optical fibers (the flexible glass channels through which information is sent) that can shield the light (the medium through which data is transmitted) using the mathematics of topology. Best of all, these modified fibers are easily scalable, meaning that the structure of each fiber can be maintained over thousands of kilometers.

In its simplest form, optical fiber—which typically has a diameter of 125 µm (similar to that of a thick strand of hair)—consists of a solid glass core surrounded by a cladding. Light passes through the core, where it bounces back as if reflected in a mirror.

However, the path an optical fiber takes through its environment is rarely straight and uninterrupted: twists, loops, and bends are the norm. Distortions in the fiber can degrade the information transmitted between the transmitter and receiver.

"The challenge was to build a network that took robustness into account," explains Nathan Roberts, a PhD student in Physics who led the research.

"When a fiber optic cable is manufactured, small variations in the fiber's physical structure are inevitable. When deployed in a network, the fiber can also twist and bend. One way to counteract these variations and defects is to ensure that the fiber design process includes a real focus on robustness. This is where we find topology concepts useful.".

To design this new fiber, the Bath team used topology, the mathematical study of quantities that remain constant despite continuous distortions of geometry. Its principles are already applied to many areas of physics research. By connecting physical phenomena with invariable numbers, the destructive effects of a disordered environment can be avoided.

The fiber designed by the Bath team applies topological ideas by including several light-conducting cores within a single fiber, wound together in a spiral. Light can jump between these cores but is trapped at the edge thanks to the topological design. These edge states are protected from structural disorder.

In the words of Dr. Anton Souslov, a Bath-based physicist and co-author of the study as the one responsible for the theory: "Using our fiber, light is less influenced by ambient disorder than it would be in an equivalent system lacking topological design."

"By adopting optical fibers with a topological design, researchers will have the necessary tools to avoid and prevent the effects of signal degradation by building intrinsically robust photonic systems.".

Theory meets practice

Dr. Peter Mosley, a Bath physicist and co-author of the study as experimental director, said: "Until now, scientists had applied theory to practice: "Previously, scientists had applied the complex mathematics of topology to light, but here at the University of Bath, we have a lot of experience in the physical fabrication of optical fibers, so we combined the mathematics with our expertise to create the topological fiber."

The team, which also includes PhD student Guido Baardink and Dr. Josh Nunn from the Department of Physics, is now looking for industrial partners to further develop its concept.

"We are very eager to help people build strong communication networks and are ready for the next phase of this work," said Dr. Souslov.

Roberts added: "We have shown that kilometers of topological fiber wound on a spool can be manufactured. We envision a quantum internet in which information will be transmitted robustly across continents using topological principles."

He pointed out that this research has implications that go beyond communications networks. He added, "The development of fiber optics is not only a technological challenge, but also a fascinating scientific field in its own right."

"Understanding how to design optical fiber has led to light sources ranging from the brilliant 'supercontinuum' that spans the entire visible spectrum to quantum light sources that produce individual photons, single particles of light.".

The future is quantum

Quantum networks are expected to play a significant technological role in the coming years. Quantum technologies have the capacity to store and process information more powerfully than current "classical" computers, as well as to securely send messages across global networks without the possibility of eavesdropping.

However, the quantum states of light that transmit information are easily affected by their environment, and finding a way to protect them is a significant challenge. This work may be a step forward in preserving quantum information in optical fibers through topological design.

This work gratefully acknowledges funding from the Engineering and Physical Sciences Research Council (EPSRC) (EP/T000961/1, EP/T001062/1), the Royal Society (RGS/R2/202135) and the Air Force Office of Scientific Research (FA8655-22-1-7028).