States of an Ensemble of Two-Level Atoms with Reduced Quantum Uncertainty

Abstract

We generate entangled states of an ensemble of 5*10^4 rubidium-87 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.

Letter (4 pages, 3 figures) followed by Auxiliary Material (10 pages, 6 figures). Minor changes in presentation and analysis of data. Significant expansion of Auxiliary Material. Broken images fixed

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