Engineering strong interactions between superconducting microwave circuits and acoustic modes in piezoelectric LiNbO3
Abel Hugot
Acoustic waves are versatile excitations that couple to many different physical systems. They interact with optical fields in optomechanics, with spin defects through strain, and with superconducting circuits via capacitive or piezoelectric mechanisms. This versatility has motivated their use in hybrid quantum systems for quantum transduction, qubit control, and precision sensing, among other applications. In this thesis, the focus is on the optimization of microwave-to-acoustic coupling between superconducting circuits and Lamb modes in suspend thin-film lithium niobate (LiNbO₃). We leverage the strong piezoelectricity of this material to engineer strong interactions.The first part of the work addresses linear transduction. IDTs require impedance matching to operate efficiently, but this generally imposes a trade-off between efficiency and bandwidth. An optimal design can be identified in theory but is difficult to reach with standard microwave engineering. Here, high-impedance SQUID arrays are employed as linear, tunable inductors to realize the necessary impedance transformation. This approach enables transducers that operate close to the efficiency-bandwidth limit imposed by the piezoelectric coupling strength of LiNbO₃. The design strategy is applied to different types of devices: standard bi-directional IDTs, unidirectional IDTs where acoustic mirrors recover backward-emitted waves, and single-mode IDTs where additional transverse confinement produces quasi-one-dimensional propagation. Among these, unidirectional IDTs achieve unprecedented coupling, with efficiency–bandwidth products of ≈440 MHz and peak efficiencies of 62%. By leveraging flux tunability of SQUIDs we get tunable couplers between microwave circuits and Lamb-wave resonators.The second part investigates the nonlinear regime, where transmon qubits are coupled to Lamb-wave resonators. Unlike the high-Q but weakly coupled resonators often used in the literature, LiNbO₃ offers strong piezoelectric interactions that enable access to very strong coupling even without extremely high quality factors.Overall, this thesis establishes efficient linear transducers and explores strong nonlinear interactions, advancing superconducting-acoustic interfaces based on LiNbO₃. These results extend the superconducting circuit toolbox with sensitive acoustic elements and open pathways toward hybrid applications such as microwave-optical transduction, acoustic spectroscopy, and fundamental experiments exploring the interplay of quantum mechanics and gravity.
•Jan 8, 2026•0 citations