Improving engine efficiency by extracting laser energy from hot exhaust gas

We show that it is possible to improve the efficiency of a classical Otto-cycle heat engine by adding a high-Q microwave cavity and a laser system that can extract coherent laser energy from thermally excited “exhaust” atoms. This improvement does not violate the second law of thermodynamics, i.e., we show that a combined high-Q microwave cavity and a laser system does not improve the efficiency of a classical Carnot-cycle heat engine.