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Quantum Heat Transformers

We propose a quantum heat transformer (QHT), a quantum thermodynamic device that modulates temperature gradients between two thermal junctions in quantum systems. Functionally, the QHT is analogous to classical absorption heat transformers in its ability to redistribute thermal energy without external work input. Moreover, we show that its performance ratio mirrors that of classical voltage transformers, where the intrinsic parameters of the system play a role similar to the coil turn ratios. We initially design the device for a three-qubit system, representing the smallest possible self-contained heat transformer model. Subsequently we extend to four-qubit systems, with a specific emphasis on exploring the step-down mode as the primary focus. We showcase the versatility and adaptability of the models by illustrating that a variety of self-contained setups can be constructed, each corresponding to different configurations of the interaction Hamiltonian and their associated self-contained conditions. An important effect in this study is the proof of existence of a necessarily transient step-down quantum heat transformer, that has a dual-mode characteristic, wherein the desired step-