  Spatial light modulators are versatile devices employed in a vast range of
applications to modify the transverse phase or amplitude profile of an incident
light beam. Most experiments are designed to use a specific polarization which
renders optimal sensitivity for phase or amplitude modulation. Here we take a
different approach and apply the formalism of quantum information to
characterize how a phase modulator affects a general polarization state. In
this context, the spatial modulators can be exploited as a resource to couple
the polarization and the transverse spatial degrees of freedom. Using a
quasi-monochromatic single photon beam obtained from a pair of twin photons
generated by spontaneous parametric down conversion, we performed quantum
process tomography in order to obtain a general analytic model for a quantum
channel that describes the action of the device on the polarization qubits. We
illustrate the application of these concepts by demonstrating the
implementation of a controllable phase flip channel. This scheme can be applied
in a straightforward manner to characterize the resulting polarization states
of different types of phase or amplitude modulators and motivates the combined
use of polarization and spatial degrees of freedom in innovative applications.
