Release-free phononic crystal with strong microwave coupling

• URL: http://arxiv.org/abs/2607.29666v1
• Date: Fri, 31 Jul 2026 17:50:46 GMT
• Title: Release-free phononic crystal with strong microwave coupling
• Abstract summary: phononic crystal cavities confine gigahertz sound to micron-scale volumes well matched to near-infrared light.<n>We show release-free phononic crystal cavity strongly coupled to a high-impedance microwave resonator.<n>Our results establish release-free phononic crystals as compact, scalable interfaces between microwaves and gigahertz sound.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• 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 $g_\mathrm{em}/(2π) \approx 30\,\text{MHz}$ 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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