🔬 applied physics

Release-free phononic crystal with strong microwave coupling

This paper demonstrates that release-free phononic crystal cavities fabricated on lithium niobate can achieve strong electromechanical coupling with high-impedance microwave resonators and high quality factors at millikelvin temperatures, overcoming previous limitations of suspended devices while enabling scalable interfaces for quantum information systems.

Original authors: Joey Frey, Paul Burger, Trond Hjerpekjøn Haug, Johan Kolvik, Raphaël Van Laer

Published 2026-08-03

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CC BY 4.0

Original authors: Joey Frey, Paul Burger, Trond Hjerpekjøn Haug, Johan Kolvik, Raphaël Van Laer

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where information doesn't just travel as electricity or light, but as tiny, invisible vibrations called "phonons." Think of these not as sound you can hear, but as the microscopic shivering of atoms, moving so fast they vibrate billions of times per second. Scientists are excited about these vibrations because they are the perfect middlemen. They can talk to super-fast computer chips (which use microwaves) and to fiber-optic cables (which use light) at the same time. If we can get them to chat effectively, we could build a super-internet that connects quantum computers across the globe, or sensors that can feel things we've never felt before.

However, there's a catch. To make these vibrations work well, scientists usually have to build them on tiny, floating islands of material, suspended in the air like a trampoline. This keeps the vibrations from leaking away into the table they're sitting on. But there's a problem: if you float something, it gets hot. When you try to control these vibrations with light or electricity, they heat up, and because they are floating, that heat has nowhere to go. It's like trying to cool down a hot cup of coffee by holding it in a vacuum; the heat just stays trapped, messing up the delicate quantum information. For a long time, scientists thought they had to choose: either keep the vibrations cold and trapped by floating them, or let them touch the ground to stay cool but risk losing the signal.

This paper introduces a clever new way to break that rule. The researchers built a "phononic crystal," which is essentially a microscopic maze carved into a thin film of lithium niobate. Usually, these mazes are carved into floating films, but this team decided to leave their maze sitting flat on the table (the substrate). They figured out how to trap the vibrations inside the maze using clever geometry and the laws of physics, even though the floor is right there. It's like building a soundproof room inside a house without needing to hang the walls from the ceiling; the walls just bounce the sound back in so perfectly that it never escapes, even though the floor is solid.

The team successfully built this "release-free" device and connected it to a microwave circuit. They found that the vibrations and the microwave signals could talk to each other incredibly fast—so fast that they could swap energy back and forth before either one had a chance to lose it. They measured a coupling rate of about 30 MHz, which is a very high speed for this kind of interaction. This means the device is now "strongly coupled," a fancy way of saying the two systems are dancing together so tightly that they act like a single unit.

Even better, they tested this on two different types of floors: silicon and sapphire. In both cases, when they cooled the device down to near absolute zero (millikelvin temperatures), the vibrations became very efficient, with quality factors (a measure of how long the vibration lasts) reaching above 10,000. This proves that you don't need to suspend the device to get good performance; you can just let it sit on the chip and still get a strong, clean signal.

The researchers also discovered that while the device works great, the way they built it added a little bit of extra "noise" or resistance to the microwave part of the system. They suspect this is because the metal parts they used to connect everything were a bit messy or because there were extra, unwanted vibrations nearby. They suggest that if they clean up the design and trim the metal connections, the device could become even better, potentially reaching speeds over 100 MHz in the future.

In short, this paper shows that we can build these quantum bridges without the tricky, heat-trapping floating parts. By letting the device sit flat on the chip, we can keep it cool and stable, opening the door to building smaller, more reliable quantum computers and sensors that can talk to light and electricity without getting overheated. It's a step toward making the future of quantum technology not just possible, but practical and cool.

Technical Summary: Release-free Phononic Crystal with Strong Microwave Coupling

Problem Statement
Phonons are promising carriers for quantum information, particularly for bridging superconducting microwave qubits and near-infrared light in hybrid quantum systems. Current state-of-the-art phononic crystal cavities typically rely on suspended thin films to confine mechanical modes and suppress radiation loss into the substrate. However, this suspension severs the primary thermal anchoring path. Under the optical pumping required for electro-optomechanical transduction, parasitic absorption heats the mechanical mode, and the lack of thermal contact leads to heat accumulation. This forces slower duty cycles or reduced pump powers, creating a bottleneck for high-fidelity quantum transduction. While "release-free" (non-suspended) phononic crystals have emerged as a solution to improve thermal anchoring, they had not yet been demonstrated to support the strong electromechanical interactions required for quantum operation, as confining modes without a full phononic band gap or suspension is challenging.

Methodology
The authors designed and fabricated a release-free electromechanical crystal (EMC) cavity in a 150-nm-thick X-cut lithium niobate film on silicon and sapphire substrates.

Design: The device utilizes a one-dimensional phononic crystal patterned with a central defect cell surrounded by mirror cells. The defect mode is engineered near the X-point of the band structure, below the substrate sound cone (defined by surface and bulk acoustic waves). Confinement is achieved through total internal reflection and geometric softening, rather than suspension.
Coupling: An interdigital transducer (IDT) patterned on the waveguide couples the mechanical mode to microwave fields. Two architectures were tested:
1. Bare EMC: The IDT connects to a 50 $\Omega$ microwave feedline for reflection measurements to characterize mechanical quality factors ( $Q_i$ ) and coupling rates ( $\gamma_c$ ).
2. Integrated System: The EMC is integrated with a high-impedance NbTiN microwave resonator (characteristic impedance $Z_{\mu,tot} \approx 2200\,\Omega$ ) to achieve strong coupling.
Characterization: Devices were measured at room temperature and at millikelvin temperatures (10 mK) in a dilution refrigerator. Microwave reflection ( $S_{11}$ ) and transmission ( $S_{21}$ ) spectra were analyzed to extract resonance frequencies, loss rates, and coupling strengths. Finite-element method simulations guided the optimization of defect and mirror geometries.

Key Results

Strong Coupling Regime: When integrated with the high-impedance microwave resonator, the release-free EMC achieved an electromechanical coupling rate of $g_{em}/(2\pi) \approx 30$ MHz. This rate exceeds both the mechanical loss rate ( $\gamma_m$ ) and the microwave loss rate ( $\kappa$ ), placing the system firmly in the strong-coupling regime.
Cooperativity: The device demonstrated a cooperativity of $C \approx 180$ on resonance, a prerequisite for coherent, high-fidelity quantum electromechanical operation.
Quality Factors: The release-free phononic crystals achieved internal mechanical quality factors ( $Q_i$ ) above $10^4$ at millikelvin temperatures on both silicon and sapphire substrates. At room temperature, $Q_i$ values ranged from approximately 380 to 810, consistent with material loss limits.
Thermal Anchoring: By maintaining full contact with the substrate, the design addresses the thermal accumulation issue inherent in suspended devices, offering a direct thermal path to the chip.
Substrate Compatibility: The design, optimized for silicon, transferred successfully to sapphire substrates, demonstrating comparable coupling rates and quality factors, indicating scalability across different material platforms.

Significance and Claims
The paper establishes release-free phononic crystals as a compact and scalable interface between microwaves and gigahertz sound. The primary significance lies in demonstrating that removing the suspension (to improve thermal anchoring) does not preclude reaching the strong-coupling regime necessary for quantum applications.

The authors claim this work completes the electromechanical component of a release-free piezo-optomechanical transducer architecture. By combining the demonstrated strong electromechanical coupling with the improved thermal properties of the release-free design, the platform opens a path toward low-noise microwave-to-optical quantum transduction. The results suggest that future improvements in quality factors and reductions in parasitic capacitance could further enhance performance, making these devices viable building blocks for heterogeneous integration in sensing, communication, and computing systems.

Technical Summary: Release-free Phononic Crystal with Strong Microwave Coupling

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