  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.
