  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.

Quantum Physics