Experimental verification of minimally-diffracting quartz for quantum acoustodynamics*

Acoustic systems offer quantum technologies a favorable combination of long on-chip delays, competitive coherence times, and the ability to connect disparate quantum systems. In particular, surface acoustic wave (SAW) resonators can be integrated with superconducting qubits to investigate quantum acoustodynamics in the multi-mode, strong dispersive regime. These experiments have required a compromise between the anharmonicity of the qubit and the acoustic aperture of the SAW device, resulting in significant diffraction loss as the aperture is reduced in typical flat-flat SAW cavity designs. Although previous work has identified an orientation of quartz which minimizes SAW diffraction loss at room temperature [1], this orientation is unsuitable for ultra-low temperature applications. Through finite-element simulations we find an orientation of quartz which minimizes SAW diffraction at millikelvin temperatures while maintaining a near-zero beam-steering angle. Using this orientation of quartz we demonstrate 500MHz highly-coherent (Q > 3x10⁵ ) flat-flat SAW resonators with narrow apertures (10λ).

[1] B. P. Abbott and L. Solie, "A minimal diffraction cut of quartz for high performance SAW filters," 2000 IEEE Ultrasonics Symposium. Proceedings. An International Symposium (Cat. No.00CH37121), 2000, pp. 235-240 vol.1, doi: 10.1109/ULTSYM.2000.922547.