Dynamical phase coexistence in a micromaser. A beam of excited two-level atoms (green arrow) pumps a resonant cavity. The resulting random sequence of photon emissions defines a quantum trajectory. The gray line depicts the first-derivative of the large deviation function, which corresponds to the mean number of atoms having emitted a photon, as a function of the conjugate field s (which in this case classifies the trajectories according to their activity). At the onset of the micromaser bistability, the discontinuity or first-order transition occurs in the physical space (at s=0) and separates a high-activity phase (red) from a low-activity phase (blue). The micromaser then operates at the phase coexistence between these two dynamical phases.