  The experimental observation of quantum phase transitions predicted by the
quantum Rabi model in quantum critical systems is usually challenging due to
the lack of signature experimental observables associated with them. Here, we
describe a method to identify the dynamical critical phenomenon in the quantum
Rabi model consisting of a three-level atom and a cavity at the quantum phase
transition. Such a critical phenomenon manifests itself as a sudden change of
steady-state output photons in the system driven by two classical fields, when
both the atom and the cavity are initially unexcited. The process occurs as the
high-frequency pump field is converted into the low-frequency Stokes field and
multiple cavity photons in the normal phase, while this conversion cannot occur
in the superradiant phase. The sudden change of steady-state output photons is
an experimentally accessible measure to probe quantum phase transitions, as it
does not require preparing the equilibrium state.
