We propose a new spin squeezing criterion for arbitrary multi-qubit states that is invariant under local unitary operations. We find that, for arbitrary pure two-qubit states, spin squeezing is equivalent to entanglement, and multi-qubit states are entangled if this new spin squeezing parameter is less than 1.
@article{arxiv.quant-ph/0304051,
title = {Spin squeezing criterion with local unitary invariance},
author = {A. R. Usha Devi and Xiaoguang Wang and Barry C Sanders},
journal= {arXiv preprint arXiv:quant-ph/0304051},
year = {2007}
}