  We present the experimental reconstruction of the Wigner function of an
individual electronic spin qubit associated with a nitrogen-vacancy (NV) center
in diamond at room temperature. This spherical Wigner function contains the
same information as the density matrix for arbitrary spin systems. As an
example, we exactly characterize the quantum state of a single qubit undergoing
a nearly pure dephasing process by Wigner function. The fidelities and purities
during this process are extracted from the experimental reconstructed Wigner
functions, whose dynamics agree with the theoretical prediction. Our method can
be applied to multi-qubit systems for measuring the Wigner function of their
collective spin state.
