Confined quantum Zeno dynamics of a watched atomic arrow

In a quantum world, a watched arrow never moves. This is the quantum Zeno effect(1). Repeatedly asking a quantum system 'are you still in your initial state?' blocks its coherent evolution through measurement back-action. Quantum Zeno dynamics (QZD; refs 2,3) gives more freedom to the system. Instead of pinning it to a single state, it sets a border in its evolution space. Repeatedly asking the system 'are you beyond the border?' makes this limit impenetrable. As the border can be designed by choosing the measured observable, QZD allows one to dynamically tailor the system's Hilbert space. Recent proposals, particularly in the cavity quantum electrodynamics context(4,5), highlight the interest of QZD for quantum state engineering tasks(6-11), which are the key to quantum-enabled technologies and quantum information processing. We report the observation of QZD in the 51-dimensional Hilbert space of a large angular momentum J = 25. Continuous selective interrogation limits the evolution of this angular momentum to an adjustable multi-dimensional subspace. This confined dynamics leads to the production of non-classical 'Schrodinger cat' states(12,13), quantum superpositions of angular momenta pointing in different directions. These states are promising for sensitive metrology of electric and magnetic fields. This QZD approach could also be generalized to cavity and circuit quantum electrodynamics experiments(4,5,13) by replacing the angular momentum with a photonic harmonic oscillator.