A soft-clamped topological waveguide for phonons

Topological insulators were originally discovered for electron waves in condensed-matter systems. Recently, this concept has been transferred to bosonic systems such as photons¹ and phonons², which propagate in materials patterned with artificial lattices that emulate spin-Hall physics. This work has been motivated, in part, by the prospect of topologically protected transport along edge channels in on-chip circuits^2,3. In principle, topology protects propagation against backscattering, but not against loss, which has remained limited to the dB cm⁻¹ level for phononic waveguides, whether topological^{4, 5, 6–7} or not^{8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18–19}. Here we combine advanced dissipation engineering²⁰—in particular, the recently introduced method of soft clamping²¹—with the concept of valley-Hall topological insulators for phonons^{22, 23, 24, 25–26}. This enables on-chip phononic waveguides with propagation losses due to dissipation of 3 dB km⁻¹ at room temperature, orders of magnitude below any previous chip-scale devices. The low losses also allow us to accurately quantify backscattering protection in topological phononic waveguides, using high-resolution ultrasound spectroscopy. We infer that phonons follow a sharp, 120° bend with a 99.99% probability instead of being scattered back, and less than one phonon in a million is lost. Our work will inspire new research directions on ultralow-loss phononic waveguides and will provide a clean bosonic system for investigating topological protection and non-Hermitian topological physics.