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Spin squeezing criterion with local unitary invariance

A. R. Usha Devi, Xiaoguang Wang, Barry C Sanders

quant-pharXiv:quant-ph/0304051

Abstract

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.

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