In the last decades thermodynamics have been extended to small (microscopic or nanoscopic) scales, where fluctuations play a major role pushing systems out of equilibrium, and where genuine quantum effects cannot be neglected anymore. Quantum thermodynamics is an interdisciplinary and growing field that places at the intersection of quantum information and non-equilibrium statistical physics. It aims to study quantum systems from a new perspective, emphasizing the energetic and entropic costs of quantum operations, and investigating possible enhancements of classical thermodynamic tasks by means of genuine quantum effects. In this mini-tutorial I will introduce some of the building block models of quantum thermodynamics, namely, quantum thermal machines. These are small devices composed by few qubits or harmonic oscillators that are able to perform useful thermodynamic tasks, such as work production or refrigeration. Despite their simplicity, these models are of great importance to understand basic thermodynamic notions at the quantum level, raising important questions such as how small can a thermal machine be, or if it is possible to benefit from quantum effects to improve their performance. Moreover, in the last years pioneering experiments have implemented and tested some of these models in the laboratory using platforms such as ion traps, nuclear spins or nitrogen-vacancy centers in diamond.
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