= History-subspace propagation Hamiltonian
{title2=$E=\tfrac12\sum_t(|e_t\rangle-|e_{t-1}\rangle)(\langle e_t|-\langle e_{t-1}|)$}
In the orthonormal history basis $|e_t\rangle=|\psi_t\rangle|c_t\rangle$, circuit propagation restricts to $E=\frac12\sum_{t=1}^T(|e_t\rangle-|e_{t-1}\rangle)(\langle e_t|-\langle e_{t-1}|)$. This is a <graph Laplacian> on a path. Its uniform zero-energy vector is the <computational history state>, while initialization restricts to $D=I-|e_0\rangle\langle e_0|$.
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