Nano Lett. (2019)

Nanomechanical resonators are sensitive tools to probe photons, charges or spins in mesoscopic systems, and can operate both in the quantum and classical limit. Fong et al. now disperse a nano-optomechanical resonator in superfluid 4He, itself a mesoscopic quantum object, and show coupling in the low-excitation regime compatible with quantum measurements.

The researchers use a high-quality microdisk resonator and immerse it into 4He. They record the quality factor and resonance frequency as a function of temperature when cooling down through the phase transition from standard to superfluid 4He. Phonon coupling of the resonator to the superfluid dominates the dynamics of the coupled system, which is distinct from coupling to classical fluids, where viscous damping usually dominates. While earlier works investigated the coupling of nano- and micromechanical resonators to superfluid He already, the experiments were operated in different regimes. In the high MHz frequency regime explored in the work by Fong et al., the resonator transduces only 0.25 phonons per oscillation period to the superfluid at an exchange efficiency of >92%. This low-excitation level fulfils a major requirement for quantum operation in such a device. In future, the manipulation of superfluids in the quantum regime and at length scales below the superfluid coherence length seems possible.