Experimental Ground-State Energy of a 125-Site Flat Kagome Antiferromagnet via Hamiltonian Engineering on Quantum Computer (2507.06361v1)

Abstract: We present an instance of utility-grade quantum computation by calculating the ground-state energy of a 125-site flat Kagome lattice under the antiferromagnetic Heisenberg model (KAFH), using IBM's Falcon and Hummingbird quantum processors. For spin-1/2 KAFH, our best per-site ground-state energy estimate reaches -0.417J, and after applying open-boundary corrections, it closely approaches the established thermodynamic value of -0.438J. To achieve this, we propose a hybrid approach that splits the variational quantum eigensolver (VQE) into local (classical) and global (quantum) components for efficient hardware utilization. We further introduce a Hamiltonian engineering strategy that increases coupling on defect triangles to mimic loop-flip dynamics, allowing us to simplify the ansatz while retaining physical accuracy. Using a single-repetition, hardware-efficient ansatz, we entangle up to 103 qubits with high fidelity to determine the Hamiltonian's lowest eigenvalue. This work demonstrates the scalability of VQE for frustrated 2D systems and lays the foundation for future studies using deeper ansatz circuits and larger lattices.