Phonon coupling between a nanomechanical resonator and a quantum fluid

TL;DR

This paper demonstrates efficient coupling between a nanomechanical resonator and phonons in superfluid helium-4, enabling new quantum fluid studies at nano-scale and low excitation levels with high phonon exchange efficiency.

Contribution

It introduces a novel method for coupling nanomechanical resonators to superfluid phonons, achieving high efficiency and strong coupling, expanding quantum fluid research capabilities.

Findings

Achieved >92% phonon exchange efficiency

Demonstrated minimum excitation rate of 0.25 phonons per oscillation

Achieved cooperativity up to 880 in coupling superfluid phonons

Abstract

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.