Cavity Optomechanics with Superfluids

Superfluid ⁴He hosts a remarkable variety of mechanical modes — third sound on a thin film, fourth sound in narrow channels and porous media, bulk acoustic resonances in helium-filled cavities — that combine extraordinarily low intrinsic dissipation with rich tunability through geometry and temperature. Coupling these modes to superconducting microwave cavities enables quantum-limited displacement readout and the engineering of nontrivial optomechanical interactions in a regime where the medium is, by definition, naturally cold.

Third sound — the long-wavelength surface mode of a thin superfluid film — has the unusual property that its dispersion can be engineered through the film thickness and the substrate it lives on. We use this to produce mechanical resonators with quality factors comparable to the best solid-state oscillators, while remaining naturally compatible with cryogenic microwave electronics. The same modes form the basis of the analogue-gravity programme.

Fourth sound — pressure oscillations of the superfluid component in a porous medium where the normal component is clamped — can be shaped further by patterning the substrate into a phononic, or sonic, crystal. We have studied fourth-sound resonances in such structures, with a current focus on the non-linear regime: how the modes shift, hybridise, and decay once driven beyond the harmonic limit.

All of these mechanical modes can be read out by coupling them parametrically to superconducting microwave resonators or to three-dimensional re-entrant cavities. The microwave readout chain — quantum-limited amplification, low-loss cabling, careful thermalisation — is the same hardware that enables our work on peripheral superconducting quantum technologies and the gravitational wave detection effort.