Modulated time evolution for efficient variational ground-state preparation

Abstract: Adiabatic state preparation aims to prepare the ground state of a target Hamiltonian starting from the easily prepared ground state of an initial Hamiltonian. While effective for time-dependent Hamiltonians with a significant energy gap to the first coupled excited state, this process becomes exceedingly slow as the gap becomes small. To accelerate it, we allow controlled diabatic excitations during the evolution and optimize the path to remove those excitations at the end of the evolution. This is done via a modulated time evolution (dynamically scaling the Hamiltonian) in addition to a field similar to the one used in local adiabatic time evolution, with the target to optimize the final energy of the unscaled Hamiltonian. This method is easy to implement and requires no complex counter-diabatic Hamiltonians and no prior knowledge of the system's energy gap. When the time evolution is further Trotterized, separating the spin-spin coupling terms and the magnetic-field terms into distinct factors, it becomes identical in structure to the quantum approximate optimization algorithm (QAOA). Compared to QAOA, the modulated time evolution often achieves the same level of performance with fewer layers (time steps).