  High-dimensional quantum states, or qudits, represent a promising resource in
the quantum information field. Here we present the experimental generation of
four-dimensional quantum states, or ququarts, encoded in the polarization and
orbital angular momentum of a single photon. Our novel technique, based on the
q-plate device, allows to prepare and measure the ququart in all five mutually
unbiased bases. We report the reconstruction of the four dimensional density
matrix through the tomographic procedure for different ququart states.
