Quantum algorithm for weakly correlated molecular electronic ground states with sub-quintic scaling in the system size

Abstract

Weakly correlated electronic structures are predominant among molecules, and are routinely solved with heuristic classical methods such as the ``gold standard'' CCSD(T) coupled cluster model, incurring a formal time complexity $O(n^7)$ in the number of molecular orbitals and with no guarantee of energy error $\varepsilon$ at or below chemical accuracy. In this work, a quantum algorithm is presented for ground state energy estimation of weakly correlated molecules using Trotterized time evolution, with an end-to-end quantum operations cost that scales as $\tilde{O}(n^{4.8}/\varepsilon^2)$ in the limit of high-order product formulae when run as a low-depth, shot-noise limited algorithm, or $\tilde{O}(n^{4.8} + n^{3.8}/\varepsilon)$ when scaling the simulation time $T\propto 1/\varepsilon$. The algorithm combines adiabatic state preparation using a smoothed Gevrey schedule with eigenstate filtering via quantum prolate diagonalization (QPD). Both steps of the algorithm rely on a size-independent spectral gap assumption, which is validated by numerical studies on alanine chains with up to 64 monomers. We benchmark the performance of our eigenstate filtering algorithm on a set of 28 molecules to compare our approach against prior phase estimation algorithms, finding that QPD achieves state-of-the-art cost tradeoffs for $\varepsilon$-accurate results. Physical quantum resource estimates are obtained for running Trotter step circuits with up to 40 spin-orbitals, providing a clear path for scaling up quantum simulation of chemistry on early fault-tolerant hardware.