Optimal control theory for maximum power of Brownian heat engines.

The pursuit of achieving the maximum output power in microscopic heat engines has gained increasing attention in the field of stochastic thermodynamics. We employ the optimal control theory to study Brownian heat engines and determine the optimal heat-engine cycles in a generic damped situation, which were previously known only in the overdamped and the underdamped limits. These optimal cycles include two isothermal processes, two adiabatic processes, and an extra isochoric relaxation process at the high stiffness constraint. Our results determine the maximum output power under realistic control constraints, and also bridge the gap of the optimal cycles between the overdamped and the underdamped limits. Hence, we solve an outstanding problem in the studies of heat engines by employing the optimal control theory to stochastic thermodynamics. These findings bring valuable insights for the design of high-performance Brownian heat engines in experimental setups.