  A scheme is proposed to entangle two systems that have not interacted by
using an ancillary particle in a Mach-Zehnder interferometer, by making a
suitable post--selection of the particle followed by a conditional feedback on
one of the subsystems to be entangled. For a strong interaction, the process
works deterministically. For a weaker interaction only the probability of
success is reduced, but the output continues to be a maximally entangled state.
