  Phase transitions of thermal systems and the laser threshold were first
connected more than forty years ago. Despite the nonequilibrium nature of the
laser, the Landau theory of thermal phase transitions, applied directly to the
Scully-Lamb laser model (SLLM), indicates that the laser threshold is a
second-order phase transition, associated with a $U(1)$ spontaneous symmetry
breaking (SSB). To capture the genuine nonequilibrium phase transition of the
SLLM (i.e., a single-mode laser without a saturable absorber), here we employ a
quantum theory of dissipative phase transitions. Our results confirm that the
$U(1)$ SSB can occur at the lasing threshold but, in contrast to the Landau
theory and semiclassical approximation, they signal that the SLLM "fundamental"
transition is a different phenomenon, which we call Liouvillian spectral
collapse; that is, the emergence of diabolic points of infinite degeneracy. By
considering a generalized SLLM with additional dephasing, we witness a
second-order phase transition, with a Liouvillian spectral collapse, but in the
absence of symmetry breaking. Most surprisingly, the phase transition
corresponds to the emergence of dynamical multistability even without SSB.
Normally, bistability is suppressed by quantum fluctuations, while in this
case, the very presence of quantum fluctuations enables bistability. This
rather anomalous bistability, characterizing the truly dissipative and quantum
origin of lasing, can be an experimental signature of our predictions, and we
show that it is associated with an emergent dynamical hysteresis.
