Efficient Learning of Structured Fermionic States under General Quadratic Evolution

Abstract: We prove that unknown pure fermionic states prepared from disjoint-branch fixed-particle-number blocks can be reconstructed after general quadratic evolution, including pair creation and annihilation. For a fixed particle cap per block, adaptive or nonadaptive single-copy Gaussian measurements yield classical input and circuit descriptions with prescribed fidelity and confidence, using polynomial resources in system size and inverse infidelity. The adaptive procedure has the sharper copy guarantee. Connected quartic correlations and a covariance-compatible commutant identify the hidden blocks; local states are recovered from fresh measurements or a predetermined bounded-degree moment table. Stable branch recovery gives valid preparation descriptions. Neither particle-number conservation nor block homogeneity is required; no lower bound on branch weights or covariance gaps is assumed. A certified variant returns a passive preparation whenever one exists within the promised family, without advance membership information. On the full Gaussian orbit of paired magic states, quartic measurements alone suffice, with an explicit bound converting observable error into state error.