Prototype SuperfluidGravitational WaveDetector

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