Power maximization of two-stroke quantum thermal machines

TL;DR

This paper investigates two-stroke quantum thermal machines using quantum systems, analyzing their efficiencies and power output, and compares models with and without mediators to optimize performance and surpass classical efficiency limits.

Contribution

It introduces a detailed analysis of two-stroke quantum cycles with and without mediators, highlighting conditions for power maximization and efficiency improvements over classical bounds.

Findings

Efficiencies depend only on energy gaps and match Otto cycle efficiencies.

Power peaks can surpass the Curzon-Ahlborn efficiency in qubit systems.

Mediator presence can enhance performance when interaction and waiting times are comparable.

Abstract

We present a detailed study of quantum thermal machines employing quantum systems as working substances. In particular, we study two different types of two-stroke cycles where two collections of identical quantum systems with evenly spaced energy levels are initially prepared at thermal equilibrium by putting them in contact with a cold and a hot thermal bath, respectively. The two cycles differ in the absence or the presence of a mediator system, while, in both cases, non-resonant exchange Hamiltonians are exploited as particle interactions. We show that the efficiencies of these machines depend only on the energy gaps of the systems composing the collections 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. It turns out that the case with the mediator can bring performance advantages when the interaction times are comparable with the waiting time of the correspondent cycle without the mediator. 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.