We use a quantum non-demolition measurement to generate a spin squeezed state and to create entanglement in a cloud of 10^5 cold cesium atoms, and for the first time operate an atomic clock improved by spin squeezing beyond the projection noise limit in a proof-of-principle experiment. For a clock-interrogation time of 10 \mu\s the experiments show an improvement of 1.1 dB in the signal-to-noise ratio, compared to the atomic projection noise limit.
@article{arxiv.0912.3895,
title = {Entanglement-assisted atomic clock beyond the projection noise limit},
author = {Anne Louchet-Chauvet and Jürgen Appel and Jelmer J. Renema and Daniel Oblak and Niels Kjærgaard and Eugene S. Polzik},
journal= {arXiv preprint arXiv:0912.3895},
year = {2010}
}