Model Predictive Path Integral Control as a Quantum Query Problem
Model predictive path integral control computes its update from cost-weighted trajectory samples and may require many classical rollouts in rare-event or high-accuracy regimes. We reformulate each component of the finite-ensemble MPPI update as a ratio of bounded path expectations and construct reversible rollout oracles encoding them as success probabilities, making the update directly estimable by quantum amplitude estimation. This gives a quadratic improvement in the query dependence on accuracy and rare-event desirability over classical Monte Carlo sampling, matching known lower bounds for the underlying scalar problem below the exhaustive-evaluation threshold, while our coordinatewise construction incurs a linear dependence on the number of control inputs. For a fixed ensemble, the low-temperature weights concentrate on the minimum-cost trajectories, connecting the limiting control to quantum minimum finding when the minimizer is unique. A fully enumerable guidance example validates the predicted estimator scalings, and an illustrative operation-count model with a crossover condition separates query advantage from modeled implementation advantage.