  Owing to their extraordinary sensitivity to external forces, nanomechanical
systems have become important tools for studying a variety of mesoscopic
physical systems and realizing hybrid quantum systems. While nanomechanics has
been widely applied in solid-state systems, its use in liquid is scantily
studied. There it finds unique applications such as biosensing, rheological
sensing, and studying fluid dynamics in unexplored regimes. Its use in quantum
fluids offers new opportunities in studying fluids at low excitation levels all
the way down to the quantum limit and in nano-metric scales reaching the fluid
coherence length. Transduction and control of the low-loss excitations also
facilitate long-life quantum information storage. In this work we demonstrate
efficient coupling of a nanomechanical resonator to phonons in a bosonic
quantum fluid -- superfluid $^4$He. By operating an ultra-high frequency
nano-optomechanical microdisk resonator immersed in superfluid $^4$He, we show
that the resonator dynamics is predominately determined by phonon-coupling to
the superfluid. A high phonon exchange efficiency $>92\%$ and minimum
excitation rate of 0.25 phonons per oscillations period are achieved. We
further show that the nanomechanical resonator can strongly couple to
superfluid cavity phonons with cooperativity up to 880. Our study opens up new
opportunities in control and manipulation of superfluids in nano-scale and
low-excitation level.
