Article,
States of an Ensemble of Two-Level Atoms with Reduced Quantum Uncertainty
Physical Review Letters, 104 (7): 073604+ (February 2010)
DOI: 10.1103/physrevlett.104.073604
Abstract
We generate entangled states of an ensemble of 5×104 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.
