The Guided Local Hamiltonian Problem for Stoquastic Hamiltonians

We show that the Guided Local Hamiltonian problem for stoquastic Hamiltonians is (promise) BPP-hard. The Guided Local Hamiltonian problem is a variant of the Local Hamiltonian problem that incorporates an additional input known as a guiding state, which is promised to overlap with the ground state. For a range of local Hamiltonian families, this problem is (promise) BQP-hard, though for stoquastic Hamiltonians, the complexity was previously unknown. Our results are achieved by first reducing from quantum-inspired BPP circuits to 6-local stoquastic Hamiltonians. We prove particular classes of quantum states, known as semi-classical encoded subset states, can guide the estimation of the ground state energy. Subsequent analysis shows the BPP-hardness is not dependent on the locality, i.e., the result holds for 2-local stoquastic Hamiltonians. Additional arguments further the BPP-hardness to Hamiltonians restricted to a square lattice. We also find for stoquastic Hamiltonians with a fixed local constraint on a subset of the system qubits, the Guided Local Hamiltonian problem is BQP-hard.