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Spin Squeezing on an Atomic-Clock Transition

We generate input states with reduced quantum uncertainty (spin-squeezed states) for a hyperfine atomic clock by collectively coupling an ensemble of laser-cooled and trapped 87Rb atoms to an optical resonator. A quantum nondemolition measurement of the population difference between the two clock states with far-detuned light produces an entangled state whose projection noise is reduced by as much as 9.4(8) dB below the standard quantum limit (SQL) for uncorrelated atoms. When the observed decoherence is taken into account, we attain 4.2(8) dB of spin squeezing, confirming entanglement, and 3.2(8) dB of improvement in clock precision over the SQL. The method holds promise for improving the performance of optical-frequency clocks.