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arXiv · 2607.29666

Release-free phononic crystal with strong microwave coupling

Joey Frey·Paul Burger·Trond Hjerpekjøn Haug·Johan Kolvik·Raphaël Van Laer
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Abstract

Phonons hold promise for storing and transferring quantum information, including in mechanically-mediated quantum interconnects between superconducting qubits and light. Phononic crystal cavities confine gigahertz sound to micron-scale volumes well matched to near-infrared light. So far, these devices have typically been suspended to suppress phononic radiation loss into the substrate, but suspension limits thermal anchoring leading to excess noise. Release-free phononic crystals have emerged as a way to address this challenge -- but had yet to be shown compatible with strong electromechanical interactions. Here, we demonstrate a release-free phononic crystal cavity strongly coupled to a high-impedance microwave resonator, with an electromechanical coupling rate $g_\mathrm{em}/(2\pi) \approx 30\,\text{MHz}$ that exceeds both the mechanical and microwave loss rates, leading to a cooperativity up to $\mathcal{C} \approx 180$ on resonance. In addition, our lithium niobate phononic crystals reach quality factors above $10^4$ at millikelvin temperature on both silicon and sapphire substrates. Our results establish release-free phononic crystals as compact, scalable interfaces between microwaves and gigahertz sound for emerging sensing, communication, and computing systems.

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Joey Frey, Paul Burger, Trond Hjerpekjøn Haug, Johan Kolvik, Raphaël Van Laer. 2026-07-31. Release-free phononic crystal with strong microwave coupling. https://arxiv.org/abs/2607.29666

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