The goal of the project is the development and implementation of novel, highly efficient techniques for the quantum non-demolition measurement of photon number states. Cavity-QED experiments using Rydberg atoms to probe microwave fields stored in ultra-high finesse resonators have witnessed enormous progress in the past decades. Optimization of the information extraction rate will enable us to study new types of states which so far have been experimentally inaccessible due to their fast decoherence rates. The scope of the emerging experimental prospects ranges from the stabilization of mesoscopic photon states to the preparation of non-local NOON - states. A thorough understanding of the decoherence dynamics of complex, mesoscopic states is of fundamental interest in quantum physics, while controlling the environment-induced decay is an important prerequisite for virtually all possible future technological implementations of quantum devices.The project goals will be achieved by employing two different advanced QND methods, which will consume significantly fewer resources in terms of probe atoms and measurement time. The first method is based on the real-time evaluation of all information available from preceding QND detections in order to optimize the phase setting of the Ramsey interferometer that is used to perform the QND measurement. In a second step we will further enhance the efficiency of the measurement by simultaneously utilizing different velocity classes to also tune the atom-photon interaction itself. This promises to eventually permit data extraction rates close to the optimum given by information theory. Both methods will initially be evaluated with a single cavity, and then adapted to a dual-cavity setup, which will allow us to operate also with non-local states.