Superfluid Optomechanics: Coupling of a Superfluid to a Superconducting Condensate

We investigate the low loss acoustic motion of superfluid ^4He parametrically coupled to a very low loss, superconducting Nb, 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 as 7⋅10^6. At higher temperatures, the lowest dissipation modes are limited by an intrinsic three phonon process. Acoustic quality factors approaching 10^(11) may be possible in isotopically purified samples at temperatures below 10 mK. A system of this type may be utilized to study macroscopic quantized motion and as an ultra-sensitive sensor of extremely weak displacements and forces, such as continuous gravity wave sources.