  Parametric down-conversion can produce photons that are entangled both in
polarization and in space. Here we show how the spatial entanglement can be
used to purify the polarization entanglement using only linear optical
elements. Spatial entanglement as an additional resource leads to a substantial
improvement in entanglement output compared to a previous scheme.
Interestingly, in the present context the thermal character of down-conversion
sources can be turned into an advantage. Our scheme is realizable with current
technology.
