  Large spin Mn2+ ions (S=5/2) diluted in a non-magnetic MgO matrix of high
crystalline symmetry are used to realize a six level system that can be
operated by means of multi-photon coherent Rabi oscillations. This spin system
has a very small anisotropy which can be tuned in-situ to reversibly transform
the system between harmonic and non-harmonic level configurations. Decoherence
effects are strongly suppressed as a result of the quasi-isotropic electron
interaction with the crystal field and with the 55Mn nuclear spins. These
results suggest new ways of manipulating, reading and resetting spin quantum
states which can be applied to encode a qubit across several quantum levels.
