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 photons and phonons, 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. In principle, topology protects propagation against backscattering, but not against loss, which has remained limited to the dB cm level for phononic waveguides, whether topological or not. Here we combine advanced dissipation engineering-in particular, the recently introduced method of soft clamping-with the concept of valley-Hall topological insulators for phonons. This enables on-chip phononic waveguides with propagation losses due to dissipation of 3 dB km 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.