Quantum dynamics of an electromagnetic mode that cannot contain N photons

Electromagnetic modes are instrumental in building quantum machines. In this experiment, we introduce a method to manipulate these modes by effectively controlling their phase space. Preventing access to a single energy level, corresponding to a number of photons N, confined the dynamics of the field to levels 0 to N-1. Under a resonant drive, the level occupation was found to oscillate in time, similarly to an N-level system. Performing a direct Wigner tomography of the field revealed its nonclassical features, including a Schr\"{o}dinger cat-like state at half period in the evolution. This fine control of the field in its phase space may enable applications in quantum information and metrology.