  We investigate the generation of entanglement between two non interacting
qubits coupled to a common reservoir. An experimental setup was conceived to
encode one qubit on the polarization of an optical beam and another qubit on
its transverse mode. The action of the reservoir is implemented as conditional
operations on these two qubits, controlled by the longitudinal path as an
ancillary degree of freedom. An entanglement witness and the two-qubit
concurrence are easily evaluated from direct intensity measurements showing an
excellent agreement with the theoretical prediction.
