Multimode Quantum Acoustics with Tunable Nonlinearity and Non-Reciprocal Interactions

Surface Acoustic Wave (SAW) resonators can confine a dense spectrum of mechanical modes in compact geometries while maintaining long lifetimes, owing to the intrinsically slow velocity of sound. We harness these properties to demonstrate two key advancements in quantum acoustics. First, we realize the superstrong coupling regime by interfacing a high-impedance SQUID array with a SAW resonator [1]. In this configuration, the SAW–SQUID coupling matches or exceeds the resonator’s free spectral range, transferring the SQUID’s nonlinearity to multiple SAW modes. We introduce the SQUID participation ratio as a quantitative metric for multimode coupling strength. System modeling and cross-Kerr experiments consistently confirm the emergence of multimode acoustic nonlinearity. Second, we demonstrate tunable and non-reciprocal SAW–SAW interactions via parametric flux drives applied to the SQUID array. These interactions create synthetic lattices in the frequency domain [2], enabling directional phonon propagation and controllable circulation [3]. Looking ahead, replacing the moderately nonlinear SQUID array with a transmon element could enable a scalable platform hosting tens of mechanical qubits [4], advancing hybrid quantum information processing and quantum simulations of interacting bosonic systems.

[1] Scigliuzzo*, Peyruchat*, et al., arXiv:2505.24865 (2025)

[2] Hung et al., PRL 127, 100503 (2021)

[3] Slim et al., Nat. Comm. 16, 7471 (2025)

[4] Yang et al., Science 386, 6723 (2024)