Spin Squeezing Criterion with Local Unitary Invariance
A. R. Usha Devi,
and B. C. Sanders
Received May 27, 2003; accepted August 23, 2003
We propose a 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 unity.
KEY WORDS: Spin squeezing; entanglement; local unitary invariance.
PACS: 03.67.-a; 03.65.Ud.
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,
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
Australian Centre for Quantum Computer Technology, Department of Physics, Macquarie
University, Sydney, New South Wales 2109, Australia.
Department of Physics, Bangalore University, Jnanabharathi Campus, Bangalore 560 056,
Quantum Information Science Group, Department of Physics and Astronomy, University of
Calgary, Alberta T2N 1N4.
To whom correspondence should be addressed. E-mail: email@example.com
Quantum Information Processing, Vol. 2, No. 3, June 2003 (# 2003)
1570-0755/03/0600–0207/0 # 2003 Plenum Publishing Corporation