Beyond Ground States: Physics-Inspired Optimization of Excited States of Classical Hamiltonians

Abstract: We introduce excited local quantum annealing (ExcLQA), a classical, physics-inspired algorithm that extends local quantum annealing (LQA) to identify excited states of classical Ising Hamiltonians. LQA simulates quantum annealing while constraining the quantum state to remain in a product state and uses a gradient-based approach to find approximate solutions to large-scale quadratic unconstrained binary optimization problems. ExcLQA extends this framework by adding a penalty term in the cost function to target excited states, with a single hyperparameter that can be tuned via binary search to set the desired penalization level. We benchmark ExcLQA on the shortest vector problem (SVP), a fundamental lattice problem underlying the security of many postquantum cryptographic schemes. Solving an SVP instance can be mapped to identifying the first excited state of a Hamiltonian, with approximate solutions located among nearby excited states. Our results show that ExcLQA manages to solve SVP instances up to rank 46, and outperforms the Metropolis-Hastings algorithm in solved ratio, number of shots, and approximation factor in the tested instances.