Spin squeezing of collective atomic spins can be achieved conditionally via probing with light and subsequent homodyne detection, as is done in a Quantum Nondemolition measurement. Recently it has been shown that squeezing can also be created unconditionally by a properly designed dissipative dynamics. We compare the two approaches in a Gaussian description, and optimize over all Gaussian light-matter interactions. We find that in the optimal unconditional scheme based on dissipation the level of squeezing scales as d−1/2. In contrast, the optimal conditional scheme based on measurement of light -- which in fact is not a Quantum Nondemolition measurement -- can provide squeezing which scales as d−1 in the most relevant regime of moderate optical depths. Our results apply directly also to the creation of entanglement in the form of non-local spin squeezing of two atomic ensembles.
@article{arxiv.1303.5888,
title = {Dissipative versus Conditional Generation of Gaussian Entanglement and Spin Squeezing},
author = {Denis V. Vasilyev and Christine A. Muschik and Klemens Hammerer},
journal= {arXiv preprint arXiv:1303.5888},
year = {2013}
}