Power maximization of two-stroke quantum thermal machines
Two different versions of a two-stroke quantum thermal machine are studied . In both versions, two collections of identical systems with evenly spaced nonvariable energy levels can be put in contact, respectively, with a cold and a hot thermal bath. In the first version, a system of a collection interacts with a system of the other one, and then they thermalize. In the second one, a mediator system interacts alternately with one or more systems of each collection. We show that the efficiencies of these machines depend only on the energy gaps of the systems and are equal to the efficiency of “equivalent” Otto cycles. Focusing on the cases of qubits or harmonic oscillators for both models, we maximize the engine power and analyze, in the model without the mediator, the role of the waiting time between subsequent interactions. We find that in both cycles, the power peaks of qubit systems can surpass the Curzon-Ahlborn efficiency. Finally, we compare our cycle without the mediator with previous schemes of the quantum Otto cycle showing that high coupling is not required to achieve the same maximum power.  N. Piccione, G. De Chiara, and B. Bellomo, Phys. Rev. A 103, 032211 (2021).