Post-Optimization Subspace Corrections to Quantum Ansätze
Abstract
Circuit depth and measurement count are the main limitations for chemical applications of the leading variational quantum algorithms on quantum computers. We show that both resource requirements can be reduced by augmenting adaptive variational quantum algorithms with post-optimization corrective techniques. We focus on two such techniques---Hamiltonian-moment Krylov subspace diagonalization and the identity-augmented quantum self-consistent equation of motion (q-sc-EOM)---applied to optimized ADAPT-VQE states without further ansatz growth or nonlinear optimization. Across molecular systems studied with up to 20 qubits, first-order Krylov correction reduces the optimized iterations (and the corresponding number of adaptive layers) needed to reach the 1.6 mEh threshold by up to approximately sixfold, with a general trend toward larger reductions at higher determinant Rényi entropy. We also find measurement-cost reductions of up to approximately fortyfold using a suitable resource-cost model. Identity-augmented q-sc-EOM improves molecular potential-energy curves obtained from truncated ADAPT-VQE ansätze with limited expressivity and recovers accuracy from states produced by sampled operator ranking without additional gradient-based operator selection, providing an advantage when small pool gradients make selection measurement intensive. Together, these post-optimization corrections offer routes to improve energy accuracy and reduce adaptive circuit growth in quantum algorithms as quantum hardware advances.
Related
- Classical Active-Space Hybrid Quantum Subspace Expansion (CASH-QSE): Quantum Corrections without Remeasuring the Classically Calculable Energy
- Quantum Subspace Expansion for Molecular Excited States: Representability, Stability, and Practical Considerations
- Machine-Learned Compact Subspace Generation for Quantum Selected Configuration Interaction within Density Matrix Embedding Framework
- Quantifying Teleportation Overhead in Distributed Unitary Coupled-Cluster Ansätze
- Resolving molecular biexcited states using classical leading-order triples corrections to quantum equation-of-motion UCCSD
BibTeX
@misc{kwao2026postoptimization,
title = {{Post-Optimization Subspace Corrections to Quantum Ansätze}},
author = {Prince Frederick Kwao and Bharath Sambasivam and Jason Saroni and Srivathsan Poyyapakkam Sundar and Edwin Barnes and Ayush Asthana},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.19109},
eprint = {2610.19109},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.19109},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Post-Optimization Subspace Corrections to Quantum Ansätze
AU - Prince Frederick Kwao
AU - Bharath Sambasivam
AU - Jason Saroni
AU - Srivathsan Poyyapakkam Sundar
AU - Edwin Barnes
AU - Ayush Asthana
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.19109
UR - https://arxiv.org/abs/2610.19109
AB - Circuit depth and measurement count are the main limitations for chemical applications of the leading variational quantum algorithms on quantum computers. We show that both resource requirements can be reduced by augmenting adaptive variational quantum algorithms with post-optimization corrective techniques. We focus on two such techniques---Hamiltonian-moment Krylov subspace diagonalization and the identity-augmented quantum self-consistent equation of motion (q-sc-EOM)---applied to optimized ADAPT-VQE states without further ansatz growth or nonlinear optimization. Across molecular systems studied with up to 20 qubits, first-order Krylov correction reduces the optimized iterations (and the corresponding number of adaptive layers) needed to reach the 1.6 mE_h threshold by up to approximately sixfold, with a general trend toward larger reductions at higher determinant Rényi entropy. We also find measurement-cost reductions of up to approximately fortyfold using a suitable resource-cost model. Identity-augmented q-sc-EOM improves molecular potential-energy curves obtained from truncated ADAPT-VQE ansätze with limited expressivity and recovers accuracy from states produced by sampled operator ranking without additional gradient-based operator selection, providing an advantage when small pool gradients make selection measurement intensive. Together, these post-optimization corrections offer routes to improve energy accuracy and reduce adaptive circuit growth in quantum algorithms as quantum hardware advances.
ER -