We derive spin squeezing inequalities that generalize the concept of the spin squeezing parameter and provide necessary and sufficient conditions for genuine 2-, or 3- qubit entanglement for symmetric states, and sufficient condition for general states of N qubits. Our inequalities have a clear physical interpretation as entanglement witnesses, can be relatively easy measured, and are given by complex, but {\it elementary} expressions.
@article{arxiv.quant-ph/0504005,
title = {Spin squeezing inequalities and entanglement of $N$ qubit states},
author = {J. Korbicz and J. I. Cirac and M. Lewenstein},
journal= {arXiv preprint arXiv:quant-ph/0504005},
year = {2009}
}
Comments
formula (24) corrected, minor changes, final version