Superfluid Cavity Electromechanics with Microfluidic Helmholtz Resonators*

Superfluid helium-4 is a promising mechanical element for cavity optomechanical and electromechanical experiments, with a large bandgap, low dielectric loss, and ultra-low acoustic loss at millikelvin temperatures. We demonstrate the first microwave cavity electromechanical implementation of a superfluid Helmholtz device, continuing development of superfluid micromechanical resonators within the Davis Lab. This experiment advances recent kHz cavity electromechanics-like devices [1] into the GHz microwave regime, greatly increasing sensitivity, while avoiding the photothermal effect associated with optics. We use a two probe electromechanically induced transparency / amplification scheme [2] to measure the more fundamental properties of the coupling scheme, incorporating additional phase-locked demodulation to remove the probe tones, leaving only coherently driven effects. The electromechanical coupling scheme of this work can now be used to create powerful tools for observing the properties of superfluid helium, at low temperatures and within confined geometries, potentially leading to studies of non-equilibrium thermodynamics connected to the superfluid state.

[1] E. Varga and J. P. Davis, New J. Phys. 23, 113041 (2021).

[2] A. D. Kashkanova et al., J Opt. 19, 034001 (2017).