  It is generally impossible to probe a quantum system without disturbing it.
However, it is possible to exploit the back-action of quantum measurements and
strong couplings to tailor and protect the coherent evolution of a quantum
system. This is a profound and counterintuitive phenomenon known as quantum
Zeno dynamics (QZD). Here we demonstrate QZD with a rubidium Bose-Einstein
condensate in a five-level Hilbert space. We harness measurements and strong
couplings to dynamically disconnect different groups of quantum states and
constrain the atoms to coherently evolve inside a two-level subregion. In
parallel to the foundational importance due to the realization of a dynamical
superselection rule and the theory of quantum measurements, this is an
important step forward in protecting and controlling quantum dynamics and,
broadly speaking, quantum information processing.
