title: States of an ensemble of two-level atoms with reduced quantum uncertainty.
abstract: We generate entangled states of an ensemble of 5x10{4} 87Rb atoms by optical quantum nondemolition measurement. The resonator-enhanced measurement leaves the atomic ensemble, prepared in a superposition of hyperfine clock levels, in a squeezed spin state. By comparing the resulting reduction of quantum projection noise [up to 8.8(8) dB] with the concomitant reduction of coherence, we demonstrate a clock input state with spectroscopic sensitivity 3.0(8) dB beyond the standard quantum limit.
authors.affiliation: Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
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