  We put forward a new, versatile and highly-scalable experimental setup for
the realization of discrete two-dimensional quantum random walks with a
single-qubit coin and tunable degree of decoherence. The proposed scheme makes
use of a small number of simple optical components arranged in a multi-path
Mach-Zehnder-like configuration, where a weak coherent state with average
photon number of 1 is injected. Environmental effects (decoherence) are
generated by a spatial light modulator, which introduces pure dephasing in the
transverse spatial plane, perpendicular to the direction of propagation of the
light beam. By controlling the characteristics of this dephasing, one can
explore a great variety of scenarios of quantum random walks: pure quantum
evolution (ballistic spread), fast fluctuating environment leading to a
diffusive classical random walk, and static disorder resulting in the
observation of Anderson localization.
