Mathematical Physics Seminar (November 11th) – Marzena H Szymańska (UCL) – Universal phase dynamics in fluids of light and living matter

Fluids of light offer a setting for exploring how coherence and collective order emerge away from thermal equilibrium. Continuous pumping and particle loss can produce behaviour beyond the familiar equilibrium picture of Bose condensation and superfluidity. I will discuss the phase dynamics of exciton–polariton and photon condensates, focusing on the interplay between Berezinskii–Kosterlitz–Thouless (BKT) physics and the Kardar–Parisi–Zhang (KPZ) universality class, familiar from fluctuating interfaces. A central feature is the compact nature of the phase, which allows vortices whose interactions and dynamics can change qualitatively away from equilibrium. I will examine how driving, anisotropy and coupling between polarisation components shape this behaviour.

I will then present recent results for photon condensates in dye-filled microcavities, showing how dissipation can support vortex binding and the characteristic features of an equilibrium BKT transition in finite systems, even without Hamiltonian interactions. Changing the photon–molecule detuning connects this regime to non-equilibrium behaviour, with distinct consequences for coherence and superfluid response. Finally, I will discuss KPZ phase dynamics and the Kibble–Zurek mechanism for spiral defect formation in cell populations, linking fluids of light and living matter through their shared driven-dissipative nature.