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Abstract
A growing number of experiments make use of multiple pairs of photons generated in the process of spontaneous parametric down-conversion. We show that entanglement in unwanted degrees of freedom can adversely affect the results of these experiments. We also discuss techniques to reduce or eliminate spectral and spatial entanglement, and we present results from two-photon polarization-entangled source with almost no entanglement in these degrees of freedom. Finally, we present two methods for the generation of four-photon polarization-entangled states. In one of these methods, four-photon can be generated without the need for intermediate two-photon entanglement.
Acknowledgements
This work was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the US Department of Energy. W.G. also acknowledges the Governments' support in the publication of this paper under AFRL Contract No. F4HBKC1013G001.