https://doi.org/10.1140/epjd/e20020045
Pairwise entanglement in symmetric multi-qubit systems
Institute of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark
Corresponding author: a moelmer@ifa.au.dk
Received:
9
May
2001
Revised:
16
November
2001
Published online: 15 March 2002
For pairs of particles extracted from a symmetric state
of N two-level systems, the two-particle density matrix
is expressed in terms of expectation values of collective
spin operators 
for the large system. Results
are presented for experimentally relevant examples of pure states:
Dicke states 
, spin coherent, and spin squeezed states,
where only the symmetric subspace, S=N/2 is populated,
and for thermally entangled mixed states populating
also lower S values. The entanglement of the extracted
pair is then quantified by a calculation of the concurrence,
which provides directly the entanglement of formation of the pair.
PACS: 03.65.Ud – Entanglement and quantum nonlocality (e.g. EPR paradox, Bell's inequalities, GHZ states, etc.) / 03.67.Lx – Quantum computation / 75.10.Jm – Quantized spin models
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2002
