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RealQuadraticSum

Base class: RealBaseSum

The QuadraticSum operator is parametrized by a sparse matrix S(i,j)S(i,j) of size 2L×2L2L \times 2L . The operator represents the sum ∑i,jSi,j+,−fi†fj+Si,j−,+fifj†+Si,j+,+fi†fj†+Si,j−,−fifj\sum_{i,j}S_{i,j}^{+,-}f_i^{\dagger}f_j + S_{i,j}^{-,+}f_if_j^{\dagger} + S_{i,j}^{+,+}f_i^{\dagger}f_j^{\dagger} + S_{i,j}^{-,-}f_if_j with S=(S+,−,S+,+S−,−,S−,+)S =\begin{pmatrix}S^{+,-}, S^{+,+} \\ S^{-,-}, S^{-,+} \end{pmatrix}