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Return the s^{2} operator.

\[ \begin{align} S^{2} = S^{-} S^{+} + S^{z}( S^{z} + 1) \end{align} \]

n_spatial_orbitals number of spatial orbitals (n_qubits + 1 // 2).

operator (FermionOperator): corresponding to the s+ operator over n_spatial_orbitals.


The indexing convention used is that even indices correspond to spin-up (alpha) modes and odd indices correspond to spin-down (beta) modes.