Engineering strong interactions between superconducting microwave circuits and acoustic modes in piezoelectric LiNbO3

Abel Hugot

Published Jan 8, 2026• 0 citations

Abstract

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.

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Engineering strong interactions between superconducting microwave circuits and acoustic modes in piezoelectric LiNbO3

Abel Hugot

Conception d'interactions fortes entre circuits micro-ondes supraconducteurs et modes acoustiques dans le LiNbO3 piézoélectrique Les ondes acoustiques constituent des excitations polyvalentes, capables de s’hybrider avec une grande variété de systèmes physiques. Elles interagissent avec les champs optiques en optomécanique, avec les défauts de spin par l’intermédiaire de la déformation mécanique, et avec les circuits supraconducteurs via des mécanismes capacitifs ou piézoélectriques. Cette versatilité a motivé leur intégration dans des systèmes quantiques hybrides, notamment pour la transduction quantique, le contrôle de qubits et la métrologie de précision. Ce travail se concentre sur l’optimisation du couplage micro-ondes-acoustique entre des circuits supraconducteurs et les modes de Lamb dans des films minces suspendus de niobate de lithium (LiNbO₃), en exploitant la forte piézoélectricité de ce matériau pour concevoir des interactions robustes.La première partie aborde la transduction linéaire. Les transducteurs à ondes interdigitées (IDT) nécessitent une adaptation d’impédance pour fonctionner efficacement, ce qui implique généralement un compromis entre efficacité et bande passante. Bien qu’une conception optimale puisse être identifiée théoriquement, sa réalisation reste délicate avec les techniques classiques de l’ingénierie micro-ondes. Nous proposons ici une solution fondée sur des réseaux de SQUID à haute impédance, agissant comme des inductances linéaires et accordables, pour réaliser la transformation d’impédance requise. Cette approche permet de concevoir des transducteurs opérant près de la limite efficacité-bande passante imposée par le couplage piézoélectrique du LiNbO₃. La stratégie est appliquée à différents dispositifs : des IDT bidirectionnels classiques, des IDT unidirectionnels où des miroirs acoustiques récupèrent les ondes émises vers l’arrière, et des IDT monomodes dont le confinement transverse additionnel induit une propagation quasi unidimensionnelle. Parmi ceux-ci, les IDT unidirectionnels atteignent un couplage sans précédent, avec des produits efficacité-bande passante de l’ordre de 440 MHz et des rendements crêtes de 62 %. L’accordabilité par flux des SQUID permet en outre de réaliser des coupleurs reconfigurables entre circuits micro-ondes et résonateurs à ondes de Lamb.La seconde partie explore le régime non linéaire, où des qubits transmon sont couplés à des résonateurs acoustiques. Contrairement aux résonateurs à haut facteur de qualité mais faiblement couplés souvent étudiés dans la littérature, le LiNbO₃ offre des interactions piézoélectriques suffisamment fortes pour accéder à un régime de couplage ultra-fort, et ce sans nécessiter des facteurs de qualité extrêmes.Dans son ensemble, cette thèse établit des transducteurs linéaires efficaces et étudie des interactions non linéaires marquées, faisant progresser les interfaces supraconducteur-acoustique à base de LiNbO₃. Ces résultats enrichissent la boîte à outils des circuits supraconducteurs avec des éléments acoustiques sensibles, et ouvrent des perspectives pour des applications hybrides telles que la transduction micro-ondes-optique, la spectroscopie acoustique ou des expériences fondamentales explorant l’interface entre mécanique quantique et gravitation.

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