Braiding reflectionless states in non-Hermitian magnonics

A thorough understanding of the topological classifications of non-Hermitian energy bands is essential for advancing non-Hermitian band theory and its applications. As evidenced in various disciplines of physics, including optics, electronics and acoustics, the process of braiding plays a crucial role in the classification of non-Hermitian bands that manifest topological characteristics. Here we demonstrate topological braiding of both reflectionless states and resonant states in non-Hermitian magnons, unveiling a reversal in their braiding handedness. Furthermore, we constitute parity–time symmetric reflectionless scattering modes, along with their degenerate exceptional points. Our results not only underscore the importance of braided scattering states, but also establish magnonics as a versatile platform for exploring non-Hermitian band theory and developing magnon-based applications, including topological energy transfer, tunable absorbers and logic circuits. Extending topological braids of complex energy bands to non-Hermitian systems of magnons—the quanta of spin waves—is a crucial step in the development of spin-based topological devices. This has now been experimentally demonstrated.