Ultrasound waves in membrane resonators with engineered phononic modes can maintain quantum coherence as long as 100 milliseconds. Optical monitoring of their motion allows approaching and beating the standard quantum limit (SQL) for displacement and force measurements, and to verify a past quantum state of the mechanical system.

Using radiation pressure, electrostatic or magnetic forces couple to optical, microwave and spin degrees of freedom. This allows quantum control of the phononic degrees of freedom, and enables new applications in quantum sensing and information processing, such as spin microscopy and mechanical memories for light.

There has also been growing interest in using traveling phonons to transport, mediate, and process quantum signals by routing them in waveguides. We have developed a new topological phononic waveguide that combines the concept of a valley-Hall topological insulator with the soft clamping technique in thin SiN membranes.

We demonstrate that it achieves dissipation-induced propagation loss as low as 3 dB/km at megahertz frequencies – four orders of magnitude lower than existing chip-scale phononic systems at room temperature – and very low (~0.01%) backscattering at sharp bends. Such low-loss, low-backscattering phononic waveguides are a promising platform for interconnecting hybrid quantum systems when cooled to cryogenic temperatures.