Nonequilibrium Quantum Criticality from Fermionic Absorbing State Transitions

Many-body systems with absorbing states generically exhibit continuous transitions between absorbing phases with only pure absorbing stationary states and active phases in which the absorbing state becomes dynamically unstable and additional mixed stationary states appear. Critical fluctuations near absorbing state transitions cannot be captured by equilibrium universality classes, and thus provide examples of nonequilibrium universality. Modern examples of absorbing state transitions in quantum systems are provided by dissipative state preparation protocols, in which there often exist parametric thresholds beyond which a target dark state is never reached, with the system instead being scrambled into a mixed state at late times. I will present an effective field theory of dark state phase transitions that occur in certain fermionic state preparation protocols, which consists of dissipative critical fermions coupled to a bosonic order parameter that measures the distance to the dark state. The fermionic nature of the critical points in these theories render them distinct from those observed in classical absorbing state transitions and thus define new universality classes that are both nonequilibrium and truly quantum. I will also discuss some example microscopic models from Lindbladian dynamics of open quantum systems that exhibit such phase transitions.