Stoquastic simulations of non-stoquastic superconducting flux circuits

Abstract There is a tremendous interest in fabricating superconducting flux circuits that are nonstoquastic—i.e., have positive off-diagonal matrix elements—in their qubit representation, as these circuits are thought to evade sign-problem-free Quantum Monte Carlo approaches and could play a key role in the demonstration of speedups in quantum annealing protocols. We show, however, that for a broad class of such flux circuits, the sign problem can be eliminated by directly simulating the underlying continuous-variable circuit Hamiltonian. Our approach not only obviates the reduction of flux circuits to their qubit representation, but also produces results that are more directly tied to the experimental degrees of freedom. We discuss the implications of our work, arguing that apparent nonstoquasticity in the effective qubit description does not necessarily imply nonstoquasticity, or a QMC sign problem, at the circuit level. These findings call for a careful reassessment of attempts to engineer nonstoquasticity in superconducting flux-based quantum annealers in pursuit of universality.