Spin Squeezing Criterion with Local Unitary Invariance

The non-classical nature of quantum entanglement is a key ingredient in the rapidly developing science of quantum information. One of the important tasks involved is quantifying quantum entanglement, which is essential in assessing the performance of a quantum system in several applications such as quantum teleportation, quantum cryptography, quantum computation and quantum communication There has been an ongoing effort devoted to characterizing quantum entanglement, especially the entanglement of multi-particle states shared by several distant parties. In view of the recent interest in creating and manipulating correlated collective atomic states, an experimentally relevant characterization of entanglement in terms of spin squeezing has been highlighted. Most studies are concentrated on collective symmetric multi-particle states