Quantum acoustics with a released thickness-mode mechanical resonator, Part 1

The circuit quantum acoustodynamics (cQAD) system combines the strong nonlinearity of a Josephson element with a high-quality mechanical resonator, enabling applications in quantum memories, transducers, and sensors. A primary challenge in this field is achieving strong coupling to the superconducting qubit while maintaining the quality factor of the mechanical resonator. Strong coupling typically requires a small mode volume and often involves metal elements, which increase fabrication complexity and introduce additional dissipation channels; conversely, larger mode volume yields weaker coupling since only the part of the resonator near the qubit can couple effectively. Here, we address the challenge with developing a released thickness-mode mechanical resonator. In the first part of this talk, we focus on the design of our mechanical resonator, its coupling mechanism with the superconducting qubit, and its fabrication process. Our resonator is constructed from an AlN-on-Si wafer, measuring approximately 200 × 200 µm, with the substrate released. Theoretical calculations give coupling strengths exceeding 10 MHz between the qubit's intrinsic electric field and the resonator. The metal-free design simplifies fabrication and facilitates achieving a higher quality factor.