Penn-led researchers have shown for the first time that multiple, information-carrying light signals can be safely guided through chip-based, reconfigurable networks using topology, the esoteric branch of mathematics that says donuts and mugs are identical.
Because topological properties remain stable even when objects are deformed — hence the field equating mugs and donuts, since both have one opening — the advance could help make light-based technologies for computing and communications more powerful and reliable.
"We already knew how to guide light using topology," says Liang Feng , Professor in Materials Science and Engineering (MSE) with a secondary appointment in Electrical and Systems Engineering (ESE) within Penn Engineering and senior author of a study in Nature Physics describing the result. "But we had never been able to guide multiple, concurrent signals before."
That opens the door to building networks of chips that communicate using light while taking advantage of the robustness topology provides. "Signals guided by these principles can be extremely reliable," says Feng. "It's like building a highway for light where even large potholes have no effect on traffic — it's as if the defects simply aren't there."
The Power of Topology
From a topological perspective, a donut and a coffee mug are identical: Because they each have one opening, their topology remains the same even if you flatten the donut or warp the mug.
"It's an unusual way of looking at the world," says Li Ge , Professor in Physics and Astronomy at the City University of New York and a co-author of the study. "But these mathematical principles can result in incredibly robust designs for information systems."
That robustness is particularly valuable for photonic technologies, which use light to carry information. In conventional optical systems, light is guided through carefully engineered structures that confine beams to specific paths. But even tiny imperfections in those structures can scatter light and disrupt signals.
Topological photonic systems offer a different approach. Instead of relying solely on precise fabrication, they guide light along special pathways defined by the system's topology — routes that remain stable even when defects are present.
"Those defects don't change the topology of the system," says Ge. "So the signal can keep moving along its path even if the structure isn't perfect."
The Limitations of a Single Mode
Despite their remarkable stability, topological photonic systems have long faced a key limitation: Each protected pathway could carry only a single stream, or "mode," of light. In effect, the system functioned like a single-lane road, restricting how much data could travel at once.
In 2019, Feng's lab published a study in Science demonstrating a topological photonic system in which a beam of light could travel through a lattice of tiny rings known as "microring resonators," while remaining protected from defects even as the researchers changed the light's path.
That work showed that topological principles could guide light across an entire photonic network, turning corners and navigating complex routes without scattering. But the system still carried only a single protected mode of light at a time.
Pseudo-spins, Interfaces and Boundaries
The new breakthrough came from a theoretical insight into how different "pseudo-spin" states of light interact within the system. By carefully designing the way those states couple at the boundary between regions of the lattice, the researchers realized they could create conditions where several protected channels would appear at the same time, in the same location.
"In conventional topological systems, each interface usually supports only one protected mode in a propagation direction," says Tianwei Wu , a postdoctoral fellow in ESE and co-first author of the study. "We found that by engineering the coupling between these states, it's possible to create multiple topological channels along the same pathway."
Turning One Lane Into Many
Turning that idea into a working device required precise control over the network of microring resonators that guide light through the system. The team had to carefully fabricate the lattice that allows different states of light to interact in just the right way to produce the multi-channel behavior their mathematical work predicted.
"The coupling between the resonators had to be engineered very carefully," says Xilin Feng , a doctoral student in ESE and co-first author of the study. "But once everything was in place, we were able to observe multiple protected channels propagating along the same interface, even when defects were introduced."
The result transforms what was once a single-lane road for light into a multi-lane highway capable of carrying several streams of information at once — a crucial step toward scalable topological photonic networks.
Future Directions
While the new system is still a laboratory demonstration, the researchers say it represents an important step toward building networks capable of carrying information in the form of light with built-in resilience to the defects that frequently arise during manufacturing and deployment.
Future work may focus on expanding the number of protected channels, integrating the design into larger circuits and exploring how similar principles could be used to route light in complex communication and computing systems.
"Light is already the backbone of modern communication networks," says Liang Feng. "Now, we can bring the robustness of topology into systems that carry many signals at once. If we can scale these designs further, they could become a powerful platform for future photonic technologies."
This study was conducted at the University of Pennsylvania School of Engineering and Applied Science and supported by the Army Research Office (Grant No. W911NF-21-1-0148), the Office of Naval Research (Grant No. N00014-23-1-2882) and the National Science Foundation (Grant Nos. ECCS-2023780, ECCS-2425529, DMR-2326699 and DMR-2326698).
