Classical-First Pathways to Early Fault-Tolerant Quantum Chemistry
Matthew Otten
University of Wisconsin-Madison
Physics
Fault-tolerant quantum computation is becoming an engineering reality, but early fault-tolerant machines will be resource-limited. For quantum chemistry, the near-term question is how to reach utility in this early-FTQC regime, where every qubit and gate matters. Guided by end-to-end resource estimation across the FTQC stack, I will describe a “classical-first” pathway that bridges NISQ-style workflows and early FTQC: state-of-the-art classical solvers provide (i) uncertainty-quantified baselines for target selection and (ii) compact wavefunction representations that lower quantum state-preparation cost.
I will highlight compact determinant expansions and ongoing benchmark studies that produce reproducible classical references for challenging catalytic and transition-metal systems. Building on these references, I will outline in-progress hybrid directions: loading compact expansions on a fault-tolerant device and refining them via quantum projection methods (imaginary-time evolution, phase-estimation variants), as well as non-orthogonal eigensolvers that combine many classical states with a small number of quantum-generated states. The goal is a transparent workflow for selecting early-FTQC targets and making competitiveness/advantage claims testable.