Dear Editor,

in the enclosed manuscript we present an investigation on the quality of the
entanglement contained in the polarization degree of freedom of photon pairs
generated by four-wave mixing in a cold atomic vapour of Rb-87.

Despite the fact that we work with an atomic ensemble in a mixed state of
Zeeman states of a fixed hyperfine level, we observe polarization-entangled
photon pair states with a remarkably high purity, and reasonably close to a
maximally entangled state. 

Due to two very closely spaced intermediate levels in the cascade decay scheme
we use in Rb, we can observe an interference between two decay possibilities
in the parametric conversion process that are closely enough spaced to lead to
observable quantum beats between the two decay paths - and we can control both
phase and contrast of the quantum beat by appropriate post-selection of the
photon polarizations.

The observed interference pattern is very clear, and can be explained by a
remarkably simple model that we think this could be of interest to the wide
readership of the New Journal of Physics.

Looking forward for your reply,

with Best Regards on behalf of all authors,

Christian Kurtsiefer