We propose and analyze a probabilistic but heralded scheme to generate pure, entangled, non-Gaussian states of collective spin in large atomic ensembles by means of single-photon detection. One photon announces the preparation of a Dicke state, while two or more photons announce Schr\"odinger cat states. The method produces pure states even for finite photon detection efficiency and weak atom-photon coupling. The entanglement generation can be made quasi-deterministic by means of repeated trial and feedback, enabling metrology beyond the standard quantum limit.
@article{arxiv.1308.6174,
title = {Generating Entangled Spin States for Quantum Metrology by Single-Photon Detection},
author = {Robert McConnell and Hao Zhang and Senka Ćuk and Jiazhong Hu and Monika H. Schleier-Smith and Vladan Vuletić},
journal= {arXiv preprint arXiv:1308.6174},
year = {2015}
}