  Repeated observations inhibit the coherent evolution of quantum states
through the quantum Zeno effect. In multi-qubit systems this effect provides
new opportunities to control complex quantum states. Here, we experimentally
demonstrate that repeatedly projecting joint observables of multiple spins
creates coherent quantum Zeno subspaces and simultaneously suppresses dephasing
caused by the environment. We encode up to two logical qubits in these
subspaces and show that the enhancement of the dephasing time with increasing
number of projections follows a scaling law that is independent of the number
of spins involved. These results provide new insights into the interplay
between frequent multi-spin measurements and non-Markovian noise and pave the
way for tailoring the dynamics of multi-qubit systems through repeated
projections.
