  Phase transitions occur in a wide range of physical systems and are
characterized by the abrupt change of a physical observable in response to the
variation of an external control parameter. Phase transitions are not
restricted to equilibrium situations but can also be found in nonequilibrium
settings, both for classical and quantum mechanical systems. Here, we
investigate a nonequilibrium phase transition in a single-electron micromaser
consisting of a microwave cavity that is driven by the electron transport in a
double quantum dot. For weak electron-photon couplings, only a tiny fraction of
the transferred electrons lead to the emission of photons into the cavity,
which essentially remains empty. However, as the coupling is increased, many
photons are suddenly emitted into the cavity. Employing ideas and concepts from
full counting statistics and Lee-Yang theory, we analyze this nonequilibrium
phase transition based on the dynamical zeros of the factorial moment
generating function of the electronic charge transport, and we find that the
phase transition can be predicted from short-time measurements of the
higher-order factorial cumulants. These results pave the way for further
investigations of critical behavior in open quantum systems.
