  We demonstrate the transition from local to global noise in a two-qubit
all-optical quantum simulator subject to classical random fluctuations. Qubits
are encoded in the polarization degree of freedom of two entangled photons
generated by parametric down-conversion (PDC) while the environment is
implemented using their spatial degrees of freedom. The ability to manipulate
with high accuracy the number of correlated pixels of a spatial-light-modulator
and the spectral PDC width, allows us to control the transition from a scenario
where the qubits are embedded in local environments to the situation where they
are subject to the same global noise. We witness the transition by monitoring
the decoherence of the two-qubit state.
