Reciprocity is a hallmark of thermal equilibrium, but ubiquitously broken in far-from-equilibrium systems on various scales. An underlying nonequilibrium conditions can give rise to effective nonreciprocal interactions at the mesoscale, e.g., between catalytic particles or membrane proteins. Such nonreciprocal coupling between fluid components can cause the emergence of travelling waves through PT symmetry-breaking phase transitions. Using non-Hermitian field theories with noise, we show for a wide class of models that close to such transitions, fluctuations not only inflate, as in equilibrium criticality, but also develop an asymptotically increasing time-reversal asymmetry and associated surging entropy production [1-3]. The formation of dissipative patterns and the emergence of irreversible fluctuations can both be attributed to a mechanism of mode coupling in the vicinity of critical exception points. For a nonreciprocal Cahn-Hilliard model, we show that this manifests itself in actively propelled interfaces whose dynamics can be mapped to the motion of a single microswimmer [1].
[1] Suchanek, Kroy, & SL, Irreversible mesoscale fluctuations herald the emergence of dynamical phases, PRL 131, 258302 (2023)
[2] Suchanek, Kroy, & SL, Time-reversal and parity-time symmetry breaking in non-Hermitian field theories, PRE 108, 064610 (2023)
[3] Suchanek, Kroy, & SL, Entropy production in the nonreciprocal Cahn-Hilliard model, PRE 108, 064123 (2023)