  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.
