Folded multistability and hidden critical point in microwave-driven Rydberg atoms.

The interactions between Rydberg atoms and microwave fields provide a robust platform for exploring non-equilibrium dynamics, exotic phases, and critical phenomena. In this work, we observe a phase transition from bistability to multistability in strongly interacting Rydberg atoms. By tuning the microwave field intensity, we drive the system through a transition from Z2- to Z3-symmetry breaking. This process features a hidden critical point where multistable states are intrinsically difficult to measure. By manipulating the initial state of the system, we successfully uncover a hidden multistable state and its dynamical trajectory, allowing us to track the hidden critical point. Furthermore, we observe multiple spectral phase transitions, indicating higher-order multistability induced by split Rydberg sublevels. These findings advance the manipulation of multistability in microwave-driven Rydberg systems and open new avenues for applications in non-equilibrium collective atomic physics.