Quantum Natural Gradient on Quotient Spaces

Abstract

Parametrized quantum circuits often contain state-preserving redundancies that make the quantum Fisher information matrix (QFIM) singular even when the physical state manifold is regular. We formulate quantum natural gradient (QNG) on the resulting parameter quotient and prove that, when the prescribed redundancy exhausts the Fisher kernel, the Moore--Penrose update is the minimum-norm horizontal lift of the quotient Riemannian gradient. A circuit-to-orbit transfer principle separates intrinsic state distinguishability from circuit-coordinate distortion and gives the exact condition under which a circuit realizes an intrinsic orbit-QNG direction. Representation theory then yields root-resolved Fisher scales on highest-weight flag orbits and isotropic intrinsic metrics for Slater and fermionic-Gaussian manifolds. On cominuscule embeddings, intrinsic fidelity QNG conserves principal-defect ratios and reduces to one scalar equation; Lie-retracted steps are locally cubic at $\eta=2$, with stability boundary $\eta=4$. For finite-shot implementations, we separate exact gauge removal from regularization of physical soft modes, obtain zero cumulative gauge drift under structural projection, and derive confidence-controlled soft-mode rules. Under depolarization, inverse-Fisher scaling restores deterministic scale only by amplifying fluctuations and therefore cannot recover a lost update signal-to-noise ratio. A redundant Slater/Givens circuit verifies the transfer law and the predicted finite-shot tradeoffs.