Driven spin systems as quantum thermodynamic machines: Fundamental limits

We show that coupled two-level systems like qubits studied in quantum-information processing can be used as a thermodynamic machine. At least three qubits or spins are necessary and they must be arranged in a chain. The system is interfaced between two split baths and the working spin in the middle is externally driven. The machine performs Carnot-type cycles and is able to work as a heat pump or engine depending on the temperature difference of the baths, Delta T, and the energy difference in the spin system, Delta E. It can be shown that the efficiency is a function of Delta T and Delta E.