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

We generate entangled states of an ensemble of 5x104 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.