Exchangeable qubit states

Remarks on the multiplicativity of the two-norm
  • M. FannesEmail author
  • C. Vandenplas


We present a characterisation of the two-site marginals of exchangeable and Bose-exchangeable states and provide an elementary proof for the qubit case. A link between the ground state problem for ferromagnetic mean-field models and the additivity of the maximal two-norm for quantum channels is established. We then analyse the corrections on N-particle symmetric states with respect to the exchangeable ones. The finite-size effects on the average ground state energy of the BCS model are explicitly computed.


Density Matrix European Physical Journal Special Topic Ground State Energy Quantum Channel Qubit State 
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© EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2008

Authors and Affiliations

  1. 1.Instituut voor Theoretische Fysica, Katholieke Universiteit LeuvenLeuvenBelgium

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