Rotated-Frame Determinant Discovery for Ground- and Excited-State Quantum Selected Configuration Interaction

Abstract

Quantum Selected Configuration Interaction (QSCI) constructs a variational determinant space from measurements of a prepared electronic state, but finite-shot sampling can repeatedly observe high-probability determinants, limiting efficient discovery of configurations important for an accurate selected-CI description. We introduce rotate--sample--unrotate (RSU) determinant discovery, in which the same state is sampled in complementary orbital frames and the observed rotated determinants generate a finite set of candidates in the canonical basis. The Hamiltonian is then diagonalized in an orthonormal selected-CI space without enumerating the full determinant space. State-targeted orbital frames are constructed from an initial selected-CI wavefunction and its external residual. For excited states, we combine RSU with the generative quantum eigensolver (GQE) to prepare target-specific sampling states. Candidate GQE circuits are evaluated by classical selected-CI diagonalization in finite sampled spaces, so the quantum procedure requires only state preparation and determinant sampling. For N$_2$, exact-state controls isolate the determinant-discovery effect: at the stretched geometry, residual-natural-orbital RSU reaches chemical accuracy with 5,000 shots, compared with 20,000 shots for conventional QSCI. The improvement persists with approximate GQE-prepared states and is also observed for target excited states. These results demonstrate improved finite-shot determinant discovery in QSCI for both ground and excited states.