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arXiv:2603.24230

A material-agnostic platform to probe spin-phonon interactions using high-overtone bulk acoustic wave resonators

Q. Greffe, A. Hugot, S. Zhang, J. Jarreau, L. Del-Rey, E. Bonet, F. Balestro, T. Chanelière, J. J. Viennot

cond-mat.mes-hallcond-mat.mtrl-sciquant-phSubmitted Mar 25, 2026• Announced Mar 26, 2026
v1 · Mar 25, 2026v2 · Mar 27, 2026
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Abstract

Spin-phonon interactions have a dual role in emerging spin-based quantum technologies. While they can be a limitation to device performance through decoherence, they also serve as a critical resource for coherent spin control, detection, and the realization of spin-based quantum networks. However, their direct characterization remains a challenge and is usually material-dependent. Here, we introduce a technique to probe spin-phonon coupling at millikelvin temperatures and gigahertz frequencies, using high-overtone bulk acoustic wave resonators (HBARs) integrated with arbitrary crystals via visco-elastic transfer of thin-film lithium niobate transducers. By tuning the Larmor frequency of dilute spin ensembles into resonance with HBAR modes, we extract the anisotropy and strength of spin-phonon interactions from acoustic dispersion and dissipation measurements. We demonstrate this approach in calcium tungstate (CaWO4) and yttrium orthosilicate (Y2SiO5), achieving cooperativities up to 0.5 for erbium dopant ensembles. Our method enables the study of spin-phonon interactions in complex crystalline materials, with minimal fabrication constraints. These results will facilitate the design of hybrid quantum systems and the quest for ion-matrix combination with enhanced spin-phonon coupling.

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21 pages

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High-frequency mechanical oscillators with long coherence times are essential to realizing a variety of high-fidelity quantum sensors, transducers, and memories. However, the unprecedented coherence times needed for quantum applications require exquisitely sensitive new techniques to probe the material origins of phonon decoherence and new strategies to mitigate decoherence in mechanical oscillators. Here, we combine non-invasive laser spectroscopy techniques with materials analysis to identify key sources of phonon decoherence in crystalline media. Using micro-fabricated high-overtone bulk acoustic-wave resonators (μμμHBARs) as an experimental testbed, we identify phonon-surface interactions as the dominant source of phonon decoherence in crystalline quartz; lattice distortion, subsurface damage, and high concentration of elemental impurities near the crystal surface are identified as the likely causes. Removal of this compromised surface layer using an optimized polishing process is seen to greatly enhance coherence times, enabling μμμHBARs with Q-factors of > 240 million at 12 GHz frequencies, corresponding to > 6 ms phonon coherence times and record-level f-Q products. Complementary phonon linewidth and time-domain ringdown measurements, performed using a new Brillouin-based pump-probe spectroscopy technique, reveal negligible dephasing within these oscillators. Building on these results, we identify a path to > 100 ms coherence times as the basis for high-frequency quantum memories. These findings clearly demonstrate that, with enhanced control over surfaces, dissipation and noise can be significantly reduced in a wide range of quantum systems.
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High-frequency mechanical oscillators with long coherence times are essential for realizing a variety of high-fidelity quantum sensors, transducers and memories. However, the coherence times needed for quantum applications require advances in probing and mitigating the origins of phonon decoherence in the materials used for mechanical oscillators. Here we identify key sources of phonon decoherence in crystalline media by combining non-invasive laser spectroscopy with materials analysis. Using micro-fabricated high-overtone bulk acoustic-wave resonators as an experimental test bed, we find that phonon–surface interactions are the dominant source of phonon decoherence in crystalline quartz. The probable causes are lattice distortion, subsurface damage and a high concentration of elemental impurities. We use an optimized polishing process to remove the compromised surface layer and produce resonators with quality factors exceeding 240 million at 12 GHz, corresponding to phonon coherence times of over 6 ms. We verify that these mechanical oscillators have negligible dephasing. Building on these results, we propose a path that could reach coherence times beyond 100 ms as the basis for high-frequency quantum memories. These findings demonstrate that enhanced control over surfaces enables a substantial reduction in dissipation and noise. Phonon decoherence in quantum high-overtone bulk acoustic-wave resonators is dominated by imperfections under the material surface. Removing the damaged subsurface layer substantially increases the performance of these devices.
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Abstract Solid-state quantum acoustodynamic (QAD) systems provide a compact platform for quantum information storage and processing by coupling acoustic phonon sources with superconducting or spin qubits. The multi-mode composite high-overtone bulk acoustic wave resonator (HBAR) is a popular phonon source well suited for QAD. However, scattering from defects, grain boundaries, and interfacial/surface roughness in the composite transducer severely limits the phonon relaxation time in sputter-deposited devices. Here, we grow an epitaxial-HBAR, consisting of a metallic NbN bottom electrode and a piezoelectric GaN film on a SiC substrate. The acoustic impedance-matched epi-HBAR has a power injection efficiency >99% from transducer to phonon cavity. The smooth interfaces and low defect density reduce phonon losses, yielding (f × Q) and phonon lifetimes up to 1.36 × 1017 Hz and 500 µs respectively. The GaN/NbN/SiC epi-HBAR is an electrically actuated, multi-mode phonon source that can be directly interfaced with NbN-based superconducting qubits or SiC-based spin qubits.