Limitations on the creation of maximal entanglement

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Abstract
We study a limited set of optical circuits for creating near maximal polarization entanglement without the usual large vacuum contribution. The optical circuits we consider involve passive interferometers, feed-forward detection, down converters, and squeezers. For input vacuum fields we find that the creation of maximal entanglement using such circuits is impossible when conditioned on two detected auxiliary photons. So far, there have been no experiments with more auxiliary photons. Thus, based on the minimum complexity of the circuits required, if near maximal polarization entanglement is possible it seems unlikely that it will be implemented experimentally with the current resources.