Curator's Take
This article tackles two of ADAPT‑VQE’s biggest bottlenecks—slow convergence caused by redundant operators and the lack of dynamical correlation outside a chosen active space—by introducing a lightweight operator‑elimination scheme and coupling it with the one‑body downfolding framework (OBDF). The combined OBDF‑ADAPT‑VQE not only trims circuit depth and iteration counts dramatically but also delivers energies that approach full‑configuration interaction accuracy on benchmark systems such as H₆ chains and N₂, showing a clear path toward more practical quantum chemistry on noisy intermediate‑scale devices. While the results are still confined to simulated small molecules, the methodology offers a scalable strategy for extending near‑term algorithms to larger, chemically relevant problems.
— Mark Eatherly
Summary
Adaptive Derivative-Assembled Problem-Tailored variational quantum eigensolvers (ADAPT-VQE) represent one of the most promising approaches for quantum chemistry on near-term quantum devices. However, their optimization is slow and may stall due to vanishing parameters and redundant operators in the ansatz. In this work, we propose a simple strategy of operator elimination that removes non-contributing operators from the pool once they are detected, enabling the optimization to continue progressing toward convergence. We examine two variants, with and without pool restoration after elimination, and find that the former converges more smoothly and faster than the latter and the standard ADAPT-VQE. To capture dynamical correlations between the active space and its environment, we combine ADAPT-VQE with our recently developed downfolding approach, the one-body downfolding framework (OBDF). In OBDF, the bare molecular Hamiltonian in the active space is replaced by a correlated effective Hamiltonian that incorporates dynamical correlation effects outside the active space. We benchmark our implementation on a linear \ce{H_6} chain, an \ce{H_6} lattice, an \ce{H_6} ring, and the \ce{N_2} molecule using the OpenFermion simulator. Our results show that operator elimination significantly reduces circuit depth and iteration count, and that OBDF-ADAPT-VQE yields energies closer to the full configuration interaction (FCI) reference than the standard approach within the same active space.