Squeezing of quantum fluctuations by means of entanglement is a well recognized goal in the field of quantum information science and precision measurements. In particular, squeezing the fluctuations via entanglement between two-level atoms can improve the precision of sensing, clocks, metrology, and spectroscopy. Here, we demonstrate 3.4 dB of metrologically relevant squeezing and entanglement for ~ 10^5 cold cesium atoms via a quantum nondemolition (QND) measurement on the atom clock levels. We show that there is an optimal degree of decoherence induced by the quantum measurement which maximizes the generated entanglement. A two-color QND scheme used in this paper is shown to have a number of advantages for entanglement generation as compared to a single color QND measurement.
@article{arxiv.0810.3545,
title = {Mesoscopic atomic entanglement for precision measurements beyond the standard quantum limit},
author = {J. Appel and P. J. Windpassinger and D. Oblak and U. B. Hoff and N. Kjaergaard and E. S. Polzik},
journal= {arXiv preprint arXiv:0810.3545},
year = {2009}
}