Quantum Non-Demolition measurement of light and tomography of photonic Schroedinger cats in Cavity QED Rydberg atoms crossing one by one a high-Q cavity extract information from the field stored in it, without absorbing the photons. The procedure amounts to an ideal quantum-non demolition (QND) measurement of light. Initially prepared in a coherent state, the field quickly collapses into a Fock state of well-defined photon number, then undergoes successive jumps towards vacuum due to cavity relaxation. We have checked Planck's law and the predictions of quantum field theory by performing a statistical analysis of thousands of individual quantum trajectories recorded in this way. As the poton number gets pinned down to a single value by the QND procedure, the field phase gets blurred. This is a manifestation of complementarity. The first stage of this blurring process, induced by a single atom, prepares a photonic Schroedinger cat in the cavity, i.e. a coherent superposition of two field states with different phases. By displacing this cat state in phase space and performing a QND measurement of the translated field, we have reconstructed its Wigner function. It exhibits two classical components and, between them, an interference feature presenting negative parts. which is a signature of the cat state quantum coherence. This interference component vanishes much faster than the decay of the field intensity. This tomographic procedure opens the way to a direct investigation of the decoherence process on cat states containing up to a few tens of photons. 10 Minute Talk: High-sensitivity diamond magnetometer with nanoscale resolution