Minimally-diffracting quartz for ultra-low temperature surface acoustic wave resonators

We simulate and experimentally demonstrate the existence of an orientation of quartz which minimizes diffraction losses in surface acoustic wave (SAW) resonators at ultra-low temperatures. The orientation is optimized for applications to quantum technologies which benefit from high mechanical quality factors, strong electromechanical coupling, and narrow acoustic apertures. We fabricate narrow aperture SAW resonators on this substrate and measure internal quality factors greater than 100,000 at mK temperatures.