Superfluid Helium-4 as an Ultra-low Loss Optomechanical Element

We investigate the low loss acoustic motion of superfluid He-4 parametrically coupled to a high Q, superconducting niobium TE$_{011}$ microwave resonator, forming a gram-scale, sideband resolved, optomechanical system. We demonstrate the detection of a series of acoustic modes with quality factors as high $3\cdot10^7$. The lowest dissipation modes are limited by an intrinsic three phonon process at higher temperatures, which leads to a $T^4$ dependent attenuation. In isotopically purified samples at temperatures below 10 mK, acoustic quality factors over $10^{10}$ may be possible. A system of this type may be utilized to study macroscopic quantized motion and as a freqency tunable, ultra-sensitive sensor of extremely weak displacements and forces, such as continuous gravity wave sources.