Minganti, FabrizioArkhipov, Ievgen I.Miranowicz, AdamNori, Franco2022-01-012022-01-012022-01-012021-12-0110.1088/1367-2630/ac3db8https://infoscience.epfl.ch/handle/20.500.14299/184247WOS:000733018000001The paradigm of second-order phase transitions (PTs) induced by spontaneous symmetry breaking (SSB) in thermal and quantum systems is a pillar of modern physics that has been fruitfully applied to out-of-equilibrium open quantum systems. Dissipative phase transitions (DPTs) of second order are often connected with SSB, in close analogy with well-known thermal second-order PTs in closed quantum and classical systems. That is, a second-order DPT should disappear by preventing the occurrence of SSB. Here, we prove this statement to be wrong, showing that, surprisingly, SSB is not a necessary condition for the occurrence of second-order DPTs in out-of-equilibrium open quantum systems. We analytically prove this result using the Liouvillian theory of DPTs, and demonstrate this anomalous transition in a paradigmatic laser model, where we can arbitrarily remove SSB while retaining criticality, and on a Z (2)-symmetric model of a two-photon Kerr resonator. This new type of PT cannot be interpreted as a 'semiclassical' bifurcation, because, after the DPT, the system steady state remains unique.Physics, MultidisciplinaryPhysicsphase transitionsscully-lamb laseropen system symmetriesquantumdriventext::journal::journal article::research article