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Exploring the Decoherence of Light in Cavities

Final Report Summary - DECLIC (Exploring the Decoherence of Light in Cavities)

The DECLIC project has demonstrated the versatility of Cavity QED to manipulate, control and measure non-classical fields made of a few photons, using Rydberg atoms as probes. It has also shown that the Rydberg atoms themselves, due to their very large number of states, contain huge quantum resources, which can be exploited to generate novel quantum states useful for possible applications. One of the highlights of the project has been the first demonstration of quantum feedback on a quantum field undergoing decoherence. Monitoring closely the field evolution without destroying photons and recognizing the moments when field quanta are gained or lost, the feedback mechanism has allowed us to maintain a highly non-classical photon number state containing up to 7 photons for an indefinite time in the cavity. Such a control procedure might have applications for quantum information processing. Exploiting Quantum Zeno Dynamics (QZD) to tailor non-classical states of a quantum system has also been an important result of the project. Our theoretical studies of QZD were first motivated by the possibility they opened to manipulate quantum fields. We soon realized that they could be applied as well to any kind of multilevel system. We have experimentally demonstrated that QZD could be used to prepare a highly non-classical state of a Rydberg atom, superposition of two states with large angular momenta pointing in two different directions. This kind of state presents coherent features, which are very sensitive to external electric and magnetic fields, leading to promising applications in quantum metrology. We have experimentally shown that a Rydberg atom in such a state superposition is indeed an extremely sensitive electrometer able to detect tiny electric fields, of the order of the one produced by a single electron at a few hundreds of micrometers. This kind of electrometer could be useful for testing nano-electronic devices. Finally, DECLIC has also contributed to elucidate important blockade mechanisms occurring in small samples of cold quantum gases excited into Rydberg states. The understanding of these processes is crucial to design novel deterministic sources of atoms for Cavity QED as well as for the development of quantum simulators using ensembles of Rydberg atoms.