The $S_x$ operator is defined as: \begin{equation} S_x = \sum_{\tau} \frac{1}{2}\left(a^{\dagger}_{\tau\uparrow}a^{\phantom{\dagger}}_{\tau\downarrow} + a^{\dagger}_{\tau\downarrow}a^{\phantom{\dagger}}_{\tau\uparrow}\right). \end{equation} The equivalent operator in Quanty is created by:
OppSx = NewOperator("Sx", NF, IndexUp, IndexDn)