  We have frozen the coherent evolution of a field in a cavity by repeated
measurements of its photon number. We use circular Rydberg atoms dispersively
coupled to the cavity mode for an absorption-free photon counting. These
measurements inhibit the growth of a Field injected in the cavity by a
classical source. This manifestation of the Quantum Zeno effect illustrates the
back action of the photon number determination onto the Field phase. The
residual growth of the Field can be seen as a random walk of its amplitude in
the two-dimensional phase space. This experiment sheds light onto the
measurement process and opens perspectives for active quantum feedback.
