  Quantum-mechanically correlated (entangled) states of many particles are of
interest in quantum information, quantum computing and quantum metrology.
Metrologically useful entangled states of large atomic ensembles have been
experimentally realized, but these states display Gaussian spin distribution
functions with a non-negative Wigner function. Non-Gaussian entangled states
have been produced in small ensembles of ions, and very recently in large
atomic ensembles. Here, we generate entanglement in a large atomic ensemble via
the interaction with a very weak laser pulse; remarkably, the detection of a
single photon prepares several thousand atoms in an entangled state. We
reconstruct a negative-valued Wigner function, an important hallmark of
nonclassicality, and verify an entanglement depth (minimum number of mutually
entangled atoms) of 2910(190) out of 3100 atoms. This is the first time a
negative Wigner function or the mutual entanglement of virtually all atoms have
been attained in an ensemble containing more than a few particles. While the
achieved purity of the state is slightly below the threshold for
entanglement-induced metrological gain, further technical improvement should
allow the generation of states that surpass this threshold, and of more complex
Schrodinger cat states for quantum metrology and information processing. More
generally, our results demonstrate the power of heralded methods for
entanglement generation, and illustrate how the information contained in a
single photon can drastically alter the quantum state of a large system.
