Scalable Quantum-Classical Hybrid Algorithm for Excited States Based on Divide-and-Conquer Unitary Coupled-Cluster...

This work introduces a divide-and-conquer (DC) quantum linear-response framework for scalable excited-state simulations, in which excitation energies and oscillator strengths are extracted from the poles of the frequency-dependent dynamical polarizability. This feature naturally enables a fragmentation-based formulation, while retaining the ability to describe nonlocal excitations beyond predefined localization regions. The method, termed DC-qUCCSD-LR, builds upon the established self-consistent quantum linear-response (qLR) theory combined with the variational unitary coupled-cluster ansatz with single and double excitations. For linear hydrogen chains, nH₂, the DC-qUCCSD-LR method reproduces full configuration interaction (FCI) excitation energies while significantly reducing quantum resource requirements, achieving favorable scaling of O(n^1.5) for gate counts and O(n^2.2) for measurements with respect to the molecular size, n. These results demonstrate that the polarizability-based qUCC-LR framework and its DC extension provide an accurate and scalable foundation for quantum excited-state simulations.