  Full quantum state tomography is used to characterize the state of an
ensemble based qubit implemented through two hyperfine levels in Pr3+ ions,
doped into a Y2SiO5 crystal. We experimentally verify that single-qubit
rotation errors due to inhomogeneities of the ensemble can be suppressed using
the Roos-Moelmer dark state scheme. Fidelities above >90%, presumably limited
by excited state decoherence, were achieved. Although not explicitly taken care
of in the Roos-Moelmer scheme, it appears that also decoherence due to
inhomogeneous broadening on the hyperfine transition is largely suppressed.
