Slow Light by Topological Routing in Synthetic Dimensions

Delay lines are essential components in photonic systems, facilitating data processing, storage, interconnects, and generally enhancing light-matter interactions. They rely on slow light phenomena, which can be realized in integrated photonic platforms such as coupled-resonator optical waveguides, photonic crystals and optical waveguides. In practice, delay lines require robustness, broadband operation and small footprints. Yet, typically, these features need to be traded against each other at the design stage due to constraints stemming from passivity, linearity and time-invariance. Here, we demonstrate that compactness, robustness and bandwidth can be simultaneously achieved by routing light in the synthetic frequency dimension along topological edge states, achieving delay-bandwidth products far beyond conventional limits combined with inherent robustness.