Quantum Valley Hall Effect-Based Coupler with Continuously Tunable Transmission for Topological Information Communication.

Topological insulators have taken classical wave systems to the next level by their unique characteristics such as highly efficient, robust, and unidirectional wave propagation. As a step further, tunability in fundamental building blocks such as couplers is desired toward topological integrated circuits and communication systems. However, most of existing topologically protected couplers exhibit a constant transmission coefficient within the bandgap, and achieving continuously tunable transmission coefficients remains challenging. Here it is experimentally showed that a four-port coupler consisting of quantum valley Hall effect-based waveguides can exhibit continuously tunable transmission coefficients from one input port to each of the two output ports over the bandgap, while the rest port is consistently isolated by the topological protection. Further topologically protected digital signal transfer is demonstrated with frequency shift keying modulation in the coupler, revealing the data-transfer capability close to the theoretical limit established in the digital communication scheme. These results pave the way for topological data transfer and signal processing.