Solution of SAT Problems with the Adaptive-Bias Quantum Approximate Optimization Algorithm

Abstract

The quantum approximate optimization algorithm (QAOA) is a promising method for solving certain classical combinatorial optimization problems on near-term quantum devices. When employing the QAOA to 3-SAT and Max-3-SAT problems, the quantum cost exhibits an easy-hard-easy or easy-hard pattern respectively as the clause density is changed, just as classical solvers do. We show by numerical simulations and analytical arguments that the adaptive-bias QAOA (ab-QAOA) does not show the easy-hard or easy-hard-easy transition, at least up to 10 variables. For this problem size, the ab-QAOA can solve the hard-region 3-SAT and Max-3-SAT problems at level 4 and level 8 respectively with near perfect fidelity. For the QAOA, level 24 and level 64 do not suffice for a solution. We argue that the improvement comes from a more targeted and more limited generation of entanglement during the time evolution. In the ab-QAOA local fields are used to guide the evolution, which suggests that classical optimization is not necessary. We demonstrate that this is the case: an optimization-free ab-QAOA can solve the hard-region problems effectively, though not as effectively as the ab-QAOA itself. Our work paves the way for realizing quantum advantages for optimization problems on NISQ devices.