In a quantum world, a watched arrow never moves. This is the Quantum ZenoEffect (QZE). Repeatedly asking a quantum system "are you still in your initialstate?" blocks its coherent evolution through measurement back-action. QuantumZeno Dynamics (QZD) leaves more freedom to the system. Instead of pinning it toa single state, it sets a border in its evolution space. Repeatedly asking thesystem "did you cross the border?" makes it impenetrable. Since the border canbe designed at will by choosing the measured observable, QZD allows one totailor the system's evolution space. Recent proposals, particularly in theCavity Quantum Electrodynamics (CQED) context, highlight the interest of QZDfor quantum state engineering tasks, which are the key to quantumenabledtechnologies and quantum information processing. We report the observation ofQZD in the 51-dimension Hilbert space of a large angular momentum J = 25.Continuous selective interrogation limits the evolution of this angularmomentum to an adjustable multi-dimensional subspace. This confined dynamicsleads to the production of non-classical "Schr\"odinger cat" states, quantumsuperpositions of angular momentums pointing in different directions. Thesestates are promising for sensitive metrology of electric and magnetic fields.This QZD approach could be generalized to other systems, opening novelperspectives for quantum information processing.
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