2024/04/04 by Leo H. Chiang, Chiang, Leo, Ku, Calvin +5
Computer Science · Engineering · #Control Systems and Identification #FOS: Physical sciences #Quantum Physics (quant-ph) #Sensor Technology and Measurement Systems
paper · pdf · doi:10.48550/arxiv.2404.03268
openalex publication_date 2024/04/04 · openalex created_date 2024/04/06 · openalex updated_date 2026/07/28
Simulation of molecular ground states on near-term quantum hardware is constrained by qubit availability and the cost of variational optimization. To address these challenges, the Subspace Restriction Scheme (SRS) is introduced as a mathematical framework that projects the molecular Hamiltonian onto a selected Fock subspace prior to qubit encoding. By enforcing molecular multiplicity and a generalized Hund's rule, the Multi-Hund Subspace (MHS) is constructed. This physically motivated restriction significantly reduces the effective Fock-space dimension, asymptotically saving N qubits for a Hamiltonian of M spatial orbitals and N electrons. As a result, we successfully overcome classical memory bottlenecks and enable simulations of large systems, such as the H22 chain, which requires 44 qubits under standard Jordan-Wigner (JW) encoding. While the strict pairing structure may limit accuracy in strongly correlated dissociation regimes, MHS effectively captures the essential low-energy physics of closed-shell molecules near equilibrium. In Variational Quantum Eigensolver (VQE) benchmarks, MHS enhances optimization behaviour and achieves high accuracy with a shallow ansatz. These findings demonstrate that physically motivated subspace restriction offers an effective approach to more resource-efficient quantum-chemistry simulations.