title: Collective state measurement of mesoscopic ensembles with single-atom resolution.
abstract: 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.
authors.affiliation: Department of Physics, MIT-Harvard Center for Ultracold Atoms, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Hao
authors.initials: H
authors.lastname: Zhang
authors.forename: Robert
authors.initials: R
authors.lastname: McConnell
authors.forename: Senka
authors.initials: S
authors.lastname: Cuk
authors.forename: Qian
authors.initials: Q
authors.lastname: Lin
authors.forename: Monika H
authors.initials: MH
authors.lastname: Schleier-Smith
authors.forename: Ian D
authors.initials: ID
authors.lastname: Leroux
authors.forename: Vladan
authors.initials: V
authors.lastname: Vuletić