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Cthugha
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DParlevliet said:According Wikipedia lasers can have coherence lengths of more the 100 m, so that cannot be a principle problem.
What do you mean by that? The coherence length of the initial laser is simply not important for the spectral width of the PDC. It just determines the width of the sum energy of the two photons. It is not really related to how asymmetric the distribution is, which gives you the spectral width of the PDC.
DParlevliet said:If you would have an ideal PDC, what would be the minimal spectral width caused by the uncertainty relation? Is that Planck constant compared to the photons energy?
What do you mean by ideal? Usually you do not want too narrow spectral width as that kills entanglement somewhat. If you want long (first order) coherence times you can always filter your light spectrally. However, that of course lowers the information you get from coincidence counting. The relative time delay between the photons in the photon pair is just known to be smaller than the respective coherence times.