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arXiv:2505.05127 · Physical Review Applied

Quantum acoustics with superconducting qubits in the multimode transition-coupling regime

Li Li, Xinhui Ruan, Si-Lu Zhao, Bing-Jie Chen, Gui-Han Liang, Yu Liu, Cheng-Lin Deng, Wei-Ping Yuan, Jia-Cheng Song, Zheng-He Liu, Tian-Ming Li, Yun-Hao Shi, He Zhang, Ming Han, Jin-Ming Guo, Xue-Yi Guo, Qianchuan Zhao, Jing Zhang, Pengtao Song, Xiaohui Song, Kai Xu, Heng Fan, Yu-Xi Liu, Zhihui Peng, Zhongcheng Xiang, Dongning Zheng

Published Dec 5, 2025• 0 citations
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Abstract

Hybrid mechanical-superconducting systems for quantum information processing have attracted significant attention due to their potential applications. In such systems, the weak coupling regime, dominated by dissipation, has been extensively studied. The strong coupling regime, where coherent energy exchange exceeds losses, has also been widely explored. However, the transition-coupling regime, which lies between the above two and exhibits rich, unique physics, remains underexplored. In this study, we fabricate a tunable coupling device to investigate the coupling of a superconducting transmon qubit to a seven-mode surface acoustic-wave resonator (SAWR), with a particular focus on the transition-coupling regime. Through a series of phonon oscillation experiments and studies in the dispersive regime, we systematically characterize the performance of the SAWR. We then explore the complex dynamics of energy exchange between the qubit and the mechanical modes, highlighting the interplay between dissipation and coherence. Finally, we propose a protocol for qubit readout and fast reset with a multimode mechanical cavity using one mode for readout and another mode for reset. We have demonstrated in simulation that the qubit achieves both fast reset and high coherence performance when the qubit is coupled to the reset mode in the transition-coupling regime.

DOI
10.1103/hxvq-vvnv

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Hybrid mechanical-superconducting systems for quantum information processing have attracted significant attention due to their potential applications. In such systems, the weak coupling regime, dominated by dissipation, has been extensively studied. The strong coupling regime, where coherent energy exchange exceeds losses, has also been widely explored. However, the transition-coupling regime, which lies between the above two and exhibits rich, unique physics, remains underexplored. In this study, we fabricate a tunable coupling device to investigate the coupling of a superconducting transmon qubit to a seven-mode surface acoustic-wave resonator (SAWR), with a particular focus on the transition-coupling regime. Through a series of phonon oscillation experiments and studies in the dispersive regime, we systematically characterize the performance of the SAWR. We then explore the complex dynamics of energy exchange between the qubit and the mechanical modes, highlighting the interplay between dissipation and coherence. Finally, we propose a protocol for qubit readout and fast reset with a multimode mechanical cavity using one mode for readout and another mode for reset. We have demonstrated in simulation that the qubit achieves both fast reset and high coherence performance when the qubit is coupled to the reset mode in the transition-coupling regime.
Precise control of mechanical modes in the quantum regime is a key resource for quantum technologies, offering promising pathways for quantum sensing with macroscopic systems and scalable architectures for quantum simulation. In this work, we realize a multimode mechanical cavity coupled to a superconducting Kerr resonator, which induces nonlinearity in the mechanical modes. The Kerr mode is realized by a flux-tunable superconducting quantum interference device (SQUID) array resonator, while the mechanical modes are implemented by a surface acoustic wave cavity. Both mechanical and electromagnetic modes are individually addressable via dedicated measurement lines, enabling full spectroscopic characterization. We introduce a straightforward protocol to measure the SQUID array resonator’s participation ratio in the hybrid acoustic modes, quantifying the degree of hybridization. The participation ratio reveals that our device operates at the onset of the multimode coupling regime, where multiple acoustic modes simultaneously interact with the nonlinear superconducting element. Furthermore, this platform allows controllable Kerr-type nonlinearities in multiple acoustic modes, with the participation ratio serving as the key parameter determining both the dissipation rates and nonlinear strengths of these hybridized modes. Close to the resonant regime, we measure a cross-Kerr interaction between seven pairs of mechanical modes, which is controllable via the SQUID array resonator detuning. Finally, we apply a two-photon parametric drive to the SQUID array resonator and observe the resulting parametric down-conversion and metastable state switching in a mechanical mode. These results establish a platform for engineering nonlinear multimode mechanical interactions, offering potential for future integration with superconducting qubits and implementation of multiple mechanical qubits.
We demonstrate an acoustical analog of a circuit quantum electrodynamics system that leverages acoustic properties to enable strong multimode coupling in the dispersive regime while suppressing spontaneous emission to unconfined modes. Specifically, we fabricate and characterize a device that comprises a flux tunable transmon coupled to a 300 μm long surface acoustic wave resonator. For some modes, the qubit-cavity coupling reaches 6.5 MHz, exceeding the cavity loss rate (200 kHz), qubit linewidth (1.1 MHz), and the cavity free spectral range (4.8 MHz), placing the device in both the strong coupling and strong multimode regimes. With the qubit detuned from the confined modes of the cavity, we observe that the qubit linewidth strongly depends on its frequency, as expected for spontaneous emission of phonons, and we identify operating frequencies where this emission rate is suppressed.
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.
Phononic resonators are becoming increasingly important in quantum information science, both for applications in quantum computing, communication, and sensing as well as in experiments investigating fundamental physics. Here, we study the decoherence of phonons confined in a surface acoustic wave (SAW) resonator strongly coupled $(g/2\ensuremath{\pi}\ensuremath{\approx}9\phantom{\rule{0.16em}{0ex}}\mathrm{MHz})$ to a superconducting transmon qubit. By comparing experimental data with numerical solutions to the Markovian master equation, we report a surface phononic energy decay rate of ${\ensuremath{\kappa}}_{1}/2\ensuremath{\pi}=480\phantom{\rule{0.16em}{0ex}}\mathrm{kHz}$ and a pure dephasing rate of ${\ensuremath{\kappa}}_{\ensuremath{\phi}}/2\ensuremath{\pi}=180\phantom{\rule{0.16em}{0ex}}\mathrm{kHz}$. These rates are in good agreement with the level of decoherence we extract from qubit-assisted spectroscopic measurements of the SAW resonator. We additionally find that the timescales over which coherent driven dynamics and decoherence occur are comparable, highlighting the need to model the composite device as an open quantum system. We discuss possible sources of decoherence in SAW-based quantum acoustic devices and the application of these devices in future quantum acoustic dissipation engineering experiments. The decoherence characterization techniques we employ are broadly applicable for investigating and benchmarking the effects of loss and dephasing on mechanical resonators in the quantum regime.
Achieving sufficiently low residual excited-state populations remains a key challenge in superconducting quantum circuits, particularly for protocols operating close to noise limits or requiring repeated qubit initialization. Existing protocols primarily address this challenge through sophisticated control, engineered dissipation, or feedback mechanisms. Here, we demonstrate an alternative approach in which a superconducting qubit is reset using a physically distinct, intrinsically colder phononic bath. Specifically, we interface a transmon with a high-overtone bulk acoustic resonator (HBAR), enabling cooling of the qubit into GHz-frequency modes. Using this approach, we achieve a residual excited-state population of the qubit below 10−410^{-4}10−4, representing an improvement of one to two orders of magnitude compared to existing reset schemes. These results highlight the potential of phononic baths as a resource for high-fidelity qubit initialization in superconducting circuits.
High-frequency mechanical resonators subjected to low thermal phonon occupancy are easier to be prepared to the ground state by direct cryogenic cooling. Their extreme stiffness, however, poses a significant challenge for external interrogations. Here we demonstrate a superconducting cavity piezoelectromechanical system in which multiple modes of a bulk acoustic resonator oscillating at 10 GHz are coupled to a planar microwave superconducting resonator with a cooperativity exceeding 2×10^{3}, deep in the strong coupling regime. By implementation of the noncontact coupling scheme to reduce mechanical dissipation, the system exhibits excellent coherence characterized by a frequency-quality-factor product of 7.5×10^{15} Hz. Interesting dynamics of classical temporal oscillations of the microwave energy is observed, implying the coherent conversion between phonons and photons. The demonstrated high-frequency cavity piezoelectromechanics is compatible with superconducting qubits, representing an important step towards hybrid quantum systems.