  We study the tomography of propagators for spin systems in the context of
finite-dimensional Wigner representations, which completely characterize and
visualize operators using shapes assembled from linear combinations of
spherical harmonics. The Wigner representation of a propagator can be
experimentally recovered by measuring expectation values of rotated axial
spherical tensor operators in an augmented system with an additional ancilla
qubit. The methodology is experimentally demonstrated for standard one-qubit
quantum gates using nuclear magnetic resonance spectroscopy. In particular,
this approach provides a direct and compelling visualization of the spinor
property of the propagators corresponding to the rotation of a spin 1/2
particle.
