Superfluidity and multicomponent fluids

Light as a flowing quantum medium

Superfluidity is collective transport without ordinary viscous drag. In our paraxial platform, a broad laser beam behaves as a two-dimensional compressible fluid: intensity is its density, phase gradients define its velocity, and the optical nonlinearity provides repulsive interactions. This direct access to both density and phase lets us follow sound, vortices, solitons and other nonlinear excitations as they evolve.

Our first experiments established superfluid behaviour through the suppression of scattering from a fixed obstacle below a critical velocity. We then used the same platform to ask a more dynamical question: what happens when the obstacle is free to respond to the fluid?

A moving object in a superfluid of light

Instead of treating the defect as an immobile probe, we couple it dynamically to the photon fluid and measure their mutual backaction. The resulting “quantum fish” can propel itself and even move upstream in the superfluid regime. This optomechanical signature provides a direct view of momentum exchange between a collective medium and an embedded object, and offers a new route to studying critical velocities, drag and nonlinear excitations.

Binary fluids and collective modes

Polarisation gives light an internal degree of freedom from which we build a two-component quantum fluid. Its excitations separate into density and spin modes, each with its own speed of sound and interaction scale. Controlling these modes gives us access to the miscible–immiscible transition, symmetry breaking and coarsening after a quench.

The same setting supports composite defects, including vortices whose cores are filled by the second component. These “massive” vortices connect topological hydrodynamics to the dynamics of an effective particle and provide a transverse thread across our studies of transport, turbulence and phase ordering.

Associated publications