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arXiv:2509.07900

Quartz phononic crystal resonators for hybrid acoustic quantum memories

Yang Hu, Angad Gupta, Jacob Repicky, Michael Hatridge, Thomas Purdy

Published Sep 9, 2025• 0 citations
arXivHTMLPDF

Abstract

Circuit quantum acoustodynamics systems have emerged as a promising platform for quantum information by coupling superconducting qubits to mechanical resonators, with their long-lived mechanical modes serving as quantum memories. We demonstrate suspended quartz phononic crystal resonators at 100 MHz with millisecond lifetimes at 8 K. With a contactless electrode geometry suppressing both two-level system losses and other electrode-induced energy dissipation, we evaluate the piezoelectric coupling rate between the mechanical modes and fluxonium qubits (resonant coupling) and transmon qubits (parametric coupling mediated by a Josephon-junction-based three-wave mixer). We further discuss multi-period defect geometries for enhancing these coupling rates.

DOI
10.48550/arxiv.2509.07900

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Circuit quantum acoustodynamics systems have emerged as a promising platform for quantum information by coupling superconducting qubits to mechanical resonators, with their long-lived mechanical modes serving as quantum memories. We demonstrate suspended quartz phononic crystal resonators at 100 MHz with millisecond lifetimes at 8 K. With a contactless electrode geometry suppressing both two-level system losses and other electrode-induced energy dissipation, we evaluate the piezoelectric coupling rate between the mechanical modes and fluxonium qubits (resonant coupling) and transmon qubits (parametric coupling mediated by a Josephon-junction-based three-wave mixer). We further discuss multi-period defect geometries for enhancing these coupling rates.
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.
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Piezoelectric nanomechanical resonators operating at gigahertz frequencies are a promising platform for quantum information processing and precision sensing, but phonon lifetimes have been limited to the microsecond range, widely attributed to the higher intrinsic loss of piezoelectric materials. Here we show that this limitation is not fundamental. Using a fabrication process informed by recent studies of defect-mediated loss in lithium niobate, we combine chemical restoration of ion-milled surfaces with a shadow-mask metallization process to produce lithium niobate phononic crystal resonators with inferred internal phonon lifetimes of up to 21 milliseconds at 3.6 gigahertz, corresponding to quality factors exceeding 4×1084\times10^84×108 (f⋅Qi>1018f\cdot Q_i > 10^{18}f⋅Qi​>1018) at millikelvin temperatures. By systematically varying the electrode-to-resonator spacing, we demonstrate that the measured lifetime is set by external coupling between the device and the measurement circuit rather than material loss. This establishes that lithium niobate, once fabrication-induced surface damage is removed, achieves an intrinsic quality comparable to the best mechanical materials while remaining an intrinsically piezoelectric platform with strong electromechanical coupling.
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.
Hybrid quantum systems in which acoustic resonators couple to superconducting qubits are promising quantum information platforms. High quality factors and small mode volumes make acoustic modes ideal quantum memories, while the qubit-phonon coupling enables the initialization and manipulation of quantum states. We present a scheme for quantum computing with multimode quantum acoustic systems, and based on this scheme, propose a hardware-efficient implementation of a quantum random access memory (QRAM). Quantum information is stored in high-Q phonon modes, and couplings between modes are engineered by applying off-resonant drives to a transmon qubit. In comparison to existing proposals that involve directly exciting the qubit, this scheme can offer a substantial improvement in gate fidelity for long-lived acoustic modes. We show how these engineered phonon-phonon couplings can be used to access data in superposition according to the state of designated address modes-implementing a QRAM on a single chip.
Mechanical resonators are a promising way for interfacing qubits in order to realize hybrid quantum systems that offer great possibilities for applications. Mechanical systems can have very long energy lifetimes, and they can be further interfaced to other systems. Moreover, integration of a mechanical oscillator with qubits creates a potential platform for the exploration of quantum physics in macroscopic mechanical degrees of freedom. The utilization of high overtone bulk acoustic resonators coupled to superconducting qubits is an intriguing platform towards these goals. These resonators exhibit a combination of high-frequency and high-quality factors. They can reach their quantum ground state at dilution refrigeration temperatures and they can be strongly coupled to superconducting qubits via their piezoelectric effect. In this paper, we demonstrate our system where bulk acoustic phonons of a high overtone resonator are coupled to a transmon qubit in a planar circuit architecture. We show that the bulk acoustic phonons are interacting with the qubit in a simple design architecture at the quantum level, representing further progress towards the quantum control of mechanical motion.