  Quantum information processing with multi-level systems (qudits) provides
additional features and applications than the two-level systems. However,
qudits are more prone to dephasing and dynamical decoupling for qudits has
never been experimentally demonstrated. Here, as a proof-of-principle
demonstration, we experimentally apply dynamical decoupling to protect
superpositions with three levels of a trapped $^9\rm{Be}^+$ ion from ambient
noisy magnetic field, prolonging coherence by up to approximately an order of
magnitude. Our demonstration, straightforwardly scalable to more levels, may
open up a path toward long coherence quantum memory, metrology and information
processing with qudits.
