Topological insulators are materials whose structure forces photons and electrons to move only along the material’s boundary and only in one direction. These particles experience very little resis­tance and travel freely past obstacles such as impurities, fabrication defects, a change of signal’s trajectory within a circuit, or objects placed intentionally in the particles’ path. That’s because these particles, instead of being reflected by the obstacle, go around it “like river-water flowing past a rock,” says Romain Fleury, head of EPFL’s Laboratory of Wave Engi­neering.

Until now, these particles’ excep­tional resilience to obstacles applied only to limited pertur­bations in the material, meaning this property couldn’t be exploited widely in photonics-based appli­cations. However, that could soon change thanks to research being conducted by Romain Fleury along with his PhD student Zhe Zhang and Pierre Delplace from the ENS Lyon Physics Labora­tory. Their study intro­duces a topological insulator in which the trans­mission of microwave photons can survive unpre­cedented levels of disorder. “We were able to create a rare topological phase that can be charac­terized as an anomalous topo­logical insulator. This phase arises from the mathe­matical properties of unitary groups and gives the material unique – and unexpected – trans­mission properties,” says Zhang.

This discovery holds great promise for new advances in science and techno­logy. “When engineers design hyper­frequency circuits, they have to be very careful to make sure that waves are not reflected but rather guided along a given path and through a series of components. That’s the first thing I teach my electrical engi­neering students,” Fleury says. “This intrinsic constraint, known as impedance matching, limits our ability to mani­pulate wave signals. However, with our discovery, we can take a completely different approach, by using topo­logy to build circuits and devices without having to worry about impedance matching – a factor that currently restricts the scope of modern techno­logy.”

Fleury’s lab is now working on concrete applications for their new topo­logical insulator. “These types of topo­logical circuits could be extremely useful for developing next-generation communi­cation systems,” he says. “Such systems require circuits that are highly reliable and easily recon­figurable.” His research group is also looking at how the disco­very could be used for developing new kinds of photonic processors and quantum computers. (Source: EPFL)

Reference: Z. Zhang et al.: Superior robustness of anomalous non-reciprocal topological edge states, Nature 598, 293 (2021); DOI: 10.1038/s41586-021-03868-7

Link: Laboratory of Wave Engineering, School of Electrical Engineering, EPFL, Lausanne, Switzerland