Exchangeable qubit states
Remarks on the multiplicativity of the two-norm
Instituut voor Theoretische Fysica, Katholieke Universiteit Leuven, Leuven, Belgium
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.
© EDP Sciences, Springer-Verlag, 2008