We address ultracold N-atom degenerate cascade systems and show that stationary subradiant states, already observed in the semiclassical regime, also exist in a fully quantum regime and for a small number of atoms. We explicitly evaluate the amount of stationary entanglement for the two-atom configuration and show full inseparability for the three-atom case. We also show that a continuous variable description of the systems is not suitable to detect entanglement due to the nonGaussianity of subradiant states.
@article{arxiv.1007.3825,
title = {Stationary entanglement in N-atom subradiant degenerate cascade systems},
author = {Massimo Borrelli and Nicola Piovella and Matteo G. A. Paris},
journal= {arXiv preprint arXiv:1007.3825},
year = {2011}
}