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 [1][2][3][4][5][6][7][8][9][10], 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 [11,12], and very recently in large atomic ensembles [13][14][15]. 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 Schrödinger 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.Entanglement is now recognized as a resource for secure communication, quantum information processing, and precision measurements. An important goal is the creation of entangled states of many-particle systems while retaining the ability to characterize the quantum state and validate entanglement. Entanglement can be verified in a variety of ways, with one of the strictest criteria being a negative-valued Wigner function [16,17], that necessarily implies that the entangled state has a non-Gaussian wavefunction. To date, the metrologically useful spin-squeezed states[1-10] have been produced in large ensembles. These states have Gaussian spin distributions and therefore can largely be modeled as systems with a classical source of spin noise, where quantum mechanics enters only to set the amount of Gaussian noise. Non-Gaussian states with a negative Wigner function, however, are manifestly non-classical, since the Wigner function as a quasiprobability function must remain nonnegative in the classical realm. While prior to this work a negative Wigner function had not been attained for atomic ensembles, in the optical domain, a negativevalued Wigner function has very recently been measured for states with up to 110 microwave photons [18]. Another entanglement measure is the entanglement depth [19], i.e. the minimum number of atoms that are demonstrably, but possibly weakly, entangled with one another. This paramete...

We demonstrate single-atom resolution, as well as detection sensitivity more than 20 dB below the quantum projection noise limit, for hyperfine-state-selective measurements on mesoscopic ensembles containing 100 or more atoms. The measurement detects the atom-induced shift of the resonance frequency of an optical cavity containing the ensemble. While spatially-varying coupling of atoms to the cavity prevents the direct observation of a quantized signal, the demonstrated measurement resolution provides the readout capability necessary for atomic interferometry substantially below the standard quantum limit, and down to the Heisenberg limit. PACS numbers:The rapidly progressing field of quantum metrology takes advantage of entangled ensembles of particles to improve measurement sensitivity beyond the standard quantum limit (SQL) arising from quantum projection noise for measurements on uncorrelated particles. Spinsqueezed states [1,2] improve the measurement signalto-noise ratio by redistribution of quantum noise, while GHZ states [3][4][5] enhance the signal via faster-evolving collective phase. GHZ states enable measurement at the Heisenberg limit, where noise-to-signal ratio scales with atom number N as 1/N [5].In both cases, very-high-precision readout is necessary to realize metrological gain. The performance of an entangled interferometer is determined not by the intrinsic fluctuations of the quantum system after detection noise subtraction, but by the full observed noise including detection noise [6][7][8][9][10][11]. Thus, the best observed spin squeezing of 6 dB [7] in a spin-1 2 system, and 8 dB of spin-nematic squeezing in a spin-1 system [11], were both limited by detection. For GHZ states, read-out of the collective phase requires a measurement of the parity of the population difference between two atomic states [5]. A state-selective measurement of atom number with singleatom resolution, which can be used to implement parity detection, therefore represents an important enabling technique for metrology beyond the SQL.An optical cavity can be used both to collect photons in a single mode [12][13][14][15][16][17][18][19][20][21][22], and to generate entangled states via light-mediated atom-atom interactions [7,23,24]. With respect to atom detection, counting of up to 4 atoms [12][13][14][15][16][17][18] and high-fidelity readout of the hyperfine state of a single neutral atom [19][20][21] have been achieved using cavity transmission measurements. Larger ensembles containing up to N = 70 atoms have been measured with atom detection variance (∆N ) 2 = 6 [25]. Spinsqueezed states of atoms in a cavity have also been prepared [7,22,26], and have enabled an atomic clock operating with variance a factor of 3 below the standard quantum limit [27].Single-atom resolution has also been achieved via fluorescence detection in free space. In optical lattices, the parity of site occupation has been measured for up to 5 atoms per lattice site without internal-state discrimination [28][29][30]. For strongly trapped ions...