Curator's Take
AI Commentary
This article demonstrates for the first time that a release‑free phononic crystal cavity can reach strong electromechanical coupling with a high‑impedance microwave resonator, achieving g ≈ 30 MHz and cooperativity near 180 while maintaining Q > 10⁴ at millikelvin temperatures. By eliminating the need for suspended structures, the platform offers robust thermal anchoring and straightforward integration on silicon or sapphire, addressing a key bottleneck that has limited the scalability of mechanical quantum interconnects. The result paves the way for compact microwave‑to‑phonon interfaces useful in quantum transduction, sensing, and modular superconducting processors, although further work will be needed to push coherence times into the regime required for error‑corrected operations.
— Mark Eatherly
Summary
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.