  We propose an all-optical setup, which couples different degrees of freedom
of a single photon, to investigate entanglement generation by a common
environment. The two qubits are represented by the photon polarization and
Hermite-Gauss transverse modes, while the environment corresponds to the photon
path. For an initially two-qubit separable state, the increase of entanglement
is analyzed, as the probability of an environment-induced transition ranges
from zero to one. An entanglement witness that is invariant throughout the
evolution of the system yields a direct measurement of the concurrence of the
two-qubit state.
