Thermal engines have been one of the principal topics of thermodynamics ever since its beginning. Today, in the era of the second quantum revolution, although the thermal processes in constant temperatures are relatively well understood in the language of resource theories, a framework to describe thermal engines is still in its infancy. In this work we formalize the resource theory of engines, initially put forward in [Quantum 8, 1222 (2024)], and define such quantities as engine efficiency. Then we turn to a detailed study of thermal engines based on free operations arising from the resource theory of athermality under different restrictions: thermal operation, semilocal thermal operations, and local thermal operation with classical communication. In order to provide analytic lower bounds for thermal engine operation we construct tree states—free states which can be obtained from Gibbs state in simple protocol consisting solely of two-level operations. Furthermore, we derive a full description of the engine based on semilocal thermal operations by describing free states, faithful monotones, and catalytic advantages.

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• Quantum algorithms
• Quantum entanglement
• Quantum heat engines & refrigerators