Abstract: An experiment realizing a quantum non-demolition measurement of a microwave field trapped in a cavity will be described. While usual photo-detection destroys light quanta, we have developed a new way to count photons without absorbing them, making it possible to measure the same field repeatedly. We use as detectors atoms prepared in a superposition of Rydberg states which cross the cavity one at a time and behave as microscopic clocks whose ticking rate is affected by light. By measuring the clocks' delay, information is extracted without energy absorption and the field progressively collapses into a well-defined photon number state. Quantum jumps between decreasing photon numbers are recorded as the cavity field subsequently relaxes towards vacuum. This new way to "look" at light also generates coherent superpositions of photonic states with different phases called "Schroedinger cats". By monitoring the evolution of these states, we directly observe the process of decohere once in experiments opening new avenues for the exploration of the boundary between the quantum and classical worlds.