  Cold atomic ensembles can mediate the generation of entanglement between
pairs of photons. Photons with specific directions of propagation are detected,
and the entanglement can reside in any of the degrees of freedom that describe
the whole quantum state of the photons: polarization, spatial shape or
frequency. We show that the direction of propagation of the generated photons
determines the spatial quantum state of the photons and therefore, the amount
of entanglement generated. When photons generated in different directions are
combined, this spatial distinguishing information can degrade the quantum
purity of the polarization or frequency entanglement.
