Average Fidelity in n-Qubit systems

This letter generalizes the expression for the average square fidelity of single qubits, as described by Bowdrey Bowdrey2002 et al., to the case of two or more qubits. We use symmetries evident in an algebraic approach utilizing basis elements for the density-matrix expansion expressed as Kronecker products of n Pauli spin matrices. The results have applications to measurements of quantum information, for example in ion-trap and NMR experiments