Zilong Chen, Justin G. Bohnet, Joshua M. Weiner, Kevin C. Cox, James K. Thompson
Probing the collective spin state of an ensemble of atoms may provide a means to reduce heating via the photon recoil associated with the measurement and provide a robust, scalable route for preparing highly entangled states with spectroscopic sensitivity below the standard quantum limit for coherent spin states. The collective probing relies on obtaining a very large optical depth that can be effectively increased by placing the ensemble within an optical cavity such that the probe light passes many times through the ensemble. Here we provide fundamental expressions for such cavity-aided non-demolition measurements. We also provide experimental examples related to previous work [Phys. Rev. Lett. 106, 133601 (2011)] using quantum non-demolition measurements to prepare conditionally spin-squeezed states in a large ensemble of nearly $10^6$ $^{87}$Rb atoms. Fundamental limits on spectroscopic enhancements in $^{87}$Rb are considered.
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http://arxiv.org/abs/1211.0723
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