Phonon Coupling between a Nanomechanical Resonator and a Quantum Fluid

摘要

Owing to their extraordinary sensitivity to external forces, nanomechanical systems have become an important tool for studying mesoscopic physics and realizing hybrid quantum systems. While nanomechanics has been widely applied in solid-state systems, its use in liquid receives less attention. There it finds unique applications such as biosensing, rheological sensing, and studying both classical and quantum fluid dynamics in unexplored regimes. In this work, we demonstrate efficient coupling of a nanooptomechanical resonator to a bosonic quantum fluid, superfluid He-4, through ultrahigh-frequency phonons (i.e., sound waves) approaching gigahertz frequencies. A high phonon exchange efficiency >92% and minimum excitation rate of 0.25 phonons per oscillations period, or equivalently k(B)T/hf(m)Q(m) = 0.044 1, are achieved. Based on our experimental results, we further predict that strong coupling between a nanomechanical resonator and superfluid cavity phonons with cooperativity up to 880 can be achieved. Our study opens new opportunities in controlling and manipulating superfluid at the nanoscale and lowexcitation level.