Acoustic modes of superfluid helium in a cross geometry*

Superfluid helium is a low-loss optomechanical element, and an acoustic quality factor value up to 10⁸ has been realized experimentally in a macroscopic quantum system using a cylindrical microwave cavity [1]. It has been predicted that three orders higher quality factor may be attained with improvements to the experimental system. With these parameters, superfluid helium is a potential candidate for detecting continuous gravitational waves [2]. Here, we study the acoustic modes of superfluid helium inside a cross geometry using a re-entrant microwave cavity that provides improved detection of the acoustic modes. The cross shaped geometry is also predicted to be more sensitive than a cylinder for detection of gravitational waves.

1. L. A. De Lorenzo and K. C. Schwab, New Journal of Physics, 16, 113020 (2014).
2. S. Singh, L. A. De Lorenzo, I. Pikovski, K. C. Schwab, New Journal of Physics, 19, 073023 (2017).