Finite frequency criticality of driven-dissipative correlated lattice bosons
Critical points and phase transitions are characterized by diverging susceptibilities, reflecting the tendency of the system toward spontaneous symmetry breaking. Equilibrium statistical mechanics bounds these instabilities to occur at zero frequency, giving rise to static order parameters. In this work we introduce a new class of dynamical transitions in a quantum many body system far from thermal equilibrium, characterized by a susceptibility diverging at a finite non-zero frequency, an emerging scale set by interactions and non-equilibrium effects. In the broken-symmetry phase the corresponding macroscopic order parameter becomes non-stationary and oscillates in time without damping, thus breaking continuous time-translational symmetry. Our results, obtained for a paradigmatic model of bosons interacting on lattice in presence of drive and dissipation, are relevant for the upcoming generation of circuit QED arrays experiments and outline a generic framework to study time-domain instabilities in non-equilibrium quantum systems, including Floquet time crystals and quantum synchronization.