Quantum Algorithms for Molecular Response Functions: Resolvent Kernels, Time-Domain Sampling, Query Lower Bounds with Matched Single-Point Calibers, and a Six-Route Comparison

Abstract

Quantum algorithms for molecular response functions have mostly been studied one route at a time, most prominently the indicator-binning algorithm of Kharazi et al. We compare six routes within one accounting framework and state the conversions between their resource calibers. On the resolvent route the Lorentz-broadened spectrum is exactly a resolvent expectation; at finite gamma two boundary terms, a reversal term and a permanent-dipole pseudo-line, enter a disjoint error budget. Because the kernel subnormalization is exactly 1, the published binning accounting carries a removable Theta(alpha+1+omega) factor (49.69x and 7.75x at the definitional caliber), a correction that applies to both routes. Estimating the square root of the kernel amplitude lowers the readout cost from beta^2/mu_tilde_2 to beta/sqrt(mu_tilde_2). On the time-domain route we prove a single-point amplitude-estimation theorem and convert the generalized QPE of Loaiza et al. to the same accounting. The lower bounds are Theta_tilde(1/gamma) for parameter location, Theta(1/(gamma epsilon)) for windowed values, Omega(n/gamma) inside a restricted correlation-sampling class, and Omega(1/(epsilon gamma^2)) on the Lorentzian object; in the single-circuit-resolution caliber, gamma^(-n) is a grid-coverage artifact. Six routes, their calibers, an n=2 assembly with a component-grade coherence, and a phase diagram over the tested ensembles form one accounting map.