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Applied Physics Letters

On-chip integration and strong coupling between ScAlN thin-film surface acoustic wave resonators and superconducting qubits

Junfeng Chen, Xiaoliang He, Wanpeng Gao, Xiaoyu Liu, Zhengqi Niu, Kuang Liu, Wei Peng, Zhen Wang, Zhirong Lin

Published Apr 1, 2025• 5 citations
Publisher

Abstract

Thin-film surface acoustic wave (SAW) devices exhibit high quality factors and are promising candidates for quantum transducers, with significant potential for on-chip integration with high-coherence superconducting quantum circuits and optical resonators. However, the experimental demonstration of strong coupling between thin-film SAWs and superconducting qubits in the quantum regime remains a considerable challenge. In this study, we demonstrate strong coupling between a ScAlN thin-film SAW resonator and a high-coherence qubit on the same substrate, where the enhanced qubit coherence was achieved through selective ScAlN etching. Distinct multimode anticrossings are observed in both the qubit and SAW resonator frequency spectra, with extracted coupling strengths closely matching theoretical predictions. Additionally, we investigate qubit decay and the dissipation properties of the SAW resonator in the SAW-qubit hybrid system. Our results confirm the feasibility of integrating thin-film SAW devices with high-coherence superconducting circuits and offer valuable insights for optimizing the performance of such hybrid systems.

DOI
10.1063/5.0258397

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Thin-film surface acoustic wave (SAW) devices exhibit high quality factors and are promising candidates for quantum transducers, with significant potential for on-chip integration with high-coherence superconducting quantum circuits and optical resonators. However, the experimental demonstration of strong coupling between thin-film SAWs and superconducting qubits in the quantum regime remains a considerable challenge. In this study, we demonstrate strong coupling between a ScAlN thin-film SAW resonator and a high-coherence qubit on the same substrate, where the enhanced qubit coherence was achieved through selective ScAlN etching. Distinct multimode anticrossings are observed in both the qubit and SAW resonator frequency spectra, with extracted coupling strengths closely matching theoretical predictions. Additionally, we investigate qubit decay and the dissipation properties of the SAW resonator in the SAW-qubit hybrid system. Our results confirm the feasibility of integrating thin-film SAW devices with high-coherence superconducting circuits and offer valuable insights for optimizing the performance of such hybrid systems.
Quantum excitations of macroscopic surface acoustic waves (SAWs) have been tailored to control, communicate, and transduce stationary and flying quantum states. However, the limited lifetime of these hybrid quantum systems remains critical obstacles to extend their applications in quantum information processing. Here, we present potentials of thin film aluminum nitride to on-chip integrated phonons with superconducting qubits over previous bulk piezoelectric substrates. We have reported high-quality thin film GHz-SAW resonators with the highest internal quality factor Qi of 4.92×104 in the quantum regime. The internal losses of SAW resonators are systematically investigated by tuning the parameters of sample layout, power, and temperature. Our results manifest that SAWs on piezoelectric films are readily integrated with standard fabrication of Josephson junction quantum circuits and offer excellent acoustic platforms for high-coherence quantum acoustodynamics architectures.
The past 20 years has seen rapid developments in circuit quantum electrodynamics, where superconducting qubits and resonators are used to control and study quantum light-matter interaction at a fundamental level. The development of this field is strongly influenced by quantum information science and the prospect of realizing quantum computation, but also opens up opportunities for combinations of different physical systems and research areas. Superconducting circuits in the microwave domain offer a versatile platform for interfacing with other quantum systems thanks to strong nonlinearities and zero-point fluctuations, as well as flexibility in design and fabrication. Hybrid quantum systems based on circuit quantum electrodynamics could enable novel functionalities by exploiting the strengths of the individual components.This thesis covers experiments coupling superconducting circuits to surface acoustic waves (SAWs), mechanical waves propagating along the surface of a solid. Strong coupling can be engineered using the piezoelectric properties of GaAs substrates, and our experiments exploit this to investigate phenomena in quantum field-matter interaction. A key property of surface acoustic waves is the slow propagation speed, typically five orders of magnitude slower than light in vacuum, and the associated short wavelength. This enables the giant atom regime where the artificial atom in the form of a superconducting circuit is large compared to the wavelength of interacting SAW radiation, a condition which is difficult to realize in other systems. Experiments described in this thesis use these properties to demonstrate electromagnetically induced transparency for a mechanical mode, as well as non-Markovian interactions between an artificial giant atom and the SAW field. When the SAW field is confined to a resonant cavity, the short wavelength allows multimode spectra suitable for interacting with a frequency comb. We use a multimode SAW resonator to characterize the ensemble of microscopic two-level system defects with a two-tone spectroscopy approach. Finally, we introduce a hybrid superconducting-SAW resonator with applications in quantum information processing in mind. Experiments with this device demonstrate entanglement of SAW modes, and show promising results on the way to engineer cluster states for quantum computation in continuous variables.
Hybrid systems that couple superconducting qubits to mechanical oscillators facilitate quantum control of individual phonons and have the potential to enable compact, highly coherent quantum memories and processors. To date, however, the utility of hybrid quantum acoustic devices has been limited by poor qubit and mechanical coherence times that are far shorter than those exhibited by their unhybridized counterparts. A primary challenge lies in the intrinsic nature of elastic waves in solids: the slow speed of sound results in a dense spectrum of acoustic modes, making it difficult to prevent spontaneous emission of the qubit into phononic degrees of freedom. In this thesis, I will present a systematic effort to suppress loss mechanisms in superconducting qubits and mechanical resonators in the context of circuit quantum acoustics. I begin by demonstrating the use of crystallographic orientation optimization to passively suppress diffraction losses in narrow-aperture quartz surface acoustic wave (SAW) resonators. We then study the coupling of SAW resonators to a quasi-continuum of bulk acoustic modes; this motivates us to abandon bulk piezoelectric media in favor of bandgap-protected thin-film devices. By fabricating suspended phononic crystal resonators from thin-film quartz, we demonstrate an order-of-magnitude improvement in the coherence of this class of devices. I conclude by presenting preliminary measurements of a new flip-chip architecture for circuit quantum acoustics -- the `acoustic trampoline' -- that is designed to maximize qubit-mechanical coupling while simultaneously suppressing spontaneous acoustic emission.
Abstract The experimental investigation of quantum devices incorporating mechanical resonators has opened up new frontiers in the study of quantum mechanics at a macroscopic level. It has recently been shown that surface acoustic waves (SAWs) can be piezoelectrically coupled to superconducting qubits, and confined in high-quality Fabry–Perot cavities in the quantum regime. Here we present measurements of a device in which a superconducting qubit is coupled to a SAW cavity, realising a surface acoustic version of cavity quantum electrodynamics. We use measurements of the AC Stark shift between the two systems to determine the coupling strength, which is in agreement with a theoretical model. This quantum acoustodynamics architecture may be used to develop new quantum acoustic devices in which quantum information is stored in trapped on-chip acoustic wavepackets, and manipulated in ways that are impossible with purely electromagnetic signals, due to the 10 5 times slower mechanical waves.
Circuit quantum acoustodynamics (cQAD) devices have a wide range of applications in quantum science, all of which depend crucially on the quantum coherence of the mechanical subsystem. In this context, high-overtone bulk acoustic-wave resonators (HBARs) are particularly promising, since they have shown very high quality factors with negligible dephasing. However, the introduction of piezoelectric films, which are necessary for coupling to a superconducting circuit, can lead to additional loss channels, such as surface scattering and two-level systems (TLS). Here, we study the acoustic dissipation of HBAR resonators in cQAD systems and find that the defect density of the piezoelectric material and its interface with the bulk are limiting factors for the coherence. We measure acoustic modes with phonon lifetimes up to 400 μμμs and lifetime-limited coherence times approaching one millisecond in the quantum regime. When coupled to a superconducting qubit, this leads to a hybrid system with a large quantum coherence cooperativity of CT2=1.1×105C_{T_2}=1.1\times10^5CT2​​=1.1×105. These results represent a new milestone for the performance of cQAD devices and offer concrete paths forward for further improvements.
Circuit quantum acoustodynamics (cQAD) devices have a wide range of applications in quantum science, all of which depend crucially on the quantum coherence of the mechanical subsystem. In this context, high-overtone bulk acoustic-wave resonators (HBARs) are particularly promising, since they have shown very high quality factors with negligible dephasing. However, the introduction of piezoelectric films, which are necessary for coupling to a superconducting circuit, can lead to additional loss channels, such as surface scattering and two-level systems. Here, we study the acoustic dissipation of HBAR resonators in cQAD systems and find that the defect density of the piezoelectric material and its interface with the bulk are limiting factors for the coherence. We measure acoustic modes with phonon lifetimes up to 400 mu s and lifetime-limited coherence times approaching 1 ms in the quantum regime. When coupled to a superconducting qubit, this leads to a hybrid system with a large quantum coherence cooperativity of CT2 1/4 1.1 & times; 105. These results represent a new milestone for the performance of cQAD devices and offer concrete paths forward for further improvements.