Prototype superfluid gravitational wave detector

We study a cross-shaped cavity filled with superfluid $^4$He as prototype resonant-mass gravitational wave detector. Using membrane and re-entrant microwave sensitive optomechanical transducer, we were able to observe the thermally excited high-$Q$ acoustic modes of helium at 20 mK temperature achieved strain sensitivity $8 \times 10^{-19}$ Hz$^{-1/2}$ waves. To facilitate broadband detection continuous waves, tune kilohertz-scale mechanical resonance frequencies up 173 Hz/bar by pressurizing helium. With reasonable improvements, this architecture will enable search for GWs in 1-30 kHz range, relevant number astrophysical sources both within beyond Standard Model.