  Coupling a system to a nonthermal environment can profoundly affect the phase
diagram of the closed system, giving rise to a special class of
dissipation-induced phase transitions. Such transitions take the system out of
its ground state and stabilize a higher-energy stationary state, rendering it
the sole attractor of the dissipative dynamics. In this work, we present a
unifying methodology, which we use to characterize this ubiquitous
phenomenology and its implications for the open system dynamics. Specifically,
we analyze the closed system's phase diagram, including symmetry-broken phases,
and explore their corresponding excitations' spectra. Opening the system, the
environment can overwhelm the system's symmetry-breaking tendencies, and
changes its order parameter. As a result, isolated distinct phases of similar
order become connected, and new phase-costability regions appear.
Interestingly, the excitations differ in the newly-connected regions through a
change in their symplectic norm, which is robust to the introduction of
dissipation. As a result, by tuning the system from one phase to the other
across the dissipation-stabilized region, the open system fluctuations exhibit
an exceptional point-like scenario, where the fluctuations become overdamped,
only to reappear with an opposite sign in the dynamical response function of
the system. The overdamped region is also associated with squeezing of the
fluctuations. We demonstrate the pervasive nature of such dissipation-induced
phenomena in two prominent examples, namely in parametric resonators and in
light-matter systems. Our work draws a crucial distinction between quantum
phase transitions and their zero-temperature open system counterparts.
