The importance of strongly interacting superfluids ranges from string theory to astrophysics; including dark matter, the quark-gluon plasma in the early universe, and the dense cores of neutron stars. However, progress is impeded across this full range of areas by the lack of a tractable underpinning microscopic theory. Superfluid helium is one of the only strongly interacting quantum fluids accessible on Earth, and therefore provides one of our few means for laboratory-based experiments towards extending our understanding. Exhibiting a macroscopic wavefunction, it also offers prospects for precision quantum sensing and computing technologies.
Two-dimensional films of superfluid helium exhibit complex turbulent dynamics, both due to hydrodynamic effect and because of the presence of quantized vortices. The details of these dynamics are a matter of wide debate. For instance, proposed values for the effective mass of a vortex range from zero to infinity, while soliton wave dynamics predicted may years ago have defied experimental observation.[1] In this seminar I will provide an overview of recent work from my lab to develop new capabilities to address this debate.
We use optomechanical interactions in silicon-chip-based photonic devices to control and probe the hydrodynamics of thin superfluid helium films. This has allowed us to laser cool sound modes,[2] generate and engineer gain processes that lead to sound lasing,[3,4] observe the coherent dynamics of quantized vortices,[5] and more recently study hydrodynamics at extreme length-scales.[6]
This seminar will provide an overview of our work and then focus on recent work that confines superfluid helium-4 to a nanoscale photonic crystal cavity to create a miniature hydrodynamic wavetank.[6] This provides both strong light-matter interactions and hydrodynamic nonlinearities that are orders-of-magnitude larger than has been achieved in any previous wavetank. It allows us to observe long-predicted phenomena, including backwards wave steepening and solitons.
The ability to observe both vortex dynamics and hydrodynamic nonlinearities in superfluid helium films opens a new path to piece together accurate dynamic models and ultimately to build liquid quantum technologies. It also offers the possibility to study hydrodynamics in a regime that can only be accessible due to the zero-viscosity of superfluid helium – the viscosity of other liquids strongly suppresses wave dynamics in nanoscale films.

[1] Huberman, Phys. Rev. Lett. 41 1389 (1978). Ellis & Luo, Third Sound: Where are the solitons? J. Low Temp. Phys. 89 115 (1992).
[2] Harris et al, Nature Physics 12, 788-793 (2016); D. L. McAuslan, PRX 6, 021012 (2016).
[3] He et al, Nature Physics 16 417 (2020).
[4] Sawadsky et al, Science Advances 9 DOI:10.1126/sciadv.ade3591 (2023).
[5] Sachkou et al. Science 366 1480 (2019).
[6] Reeves et al. Science 390 371 (2025).