Driven Spin Systems as Quantum Thermodynamic Machines: Fundamental Limits

We show that coupled two level systems like qubits studied in quantum information can be used as a thermodynamic machine. At least three qubits or spins are necessary and 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 heat pump or engine depending on the temperature difference of the baths $\mathrm{\Delta }T$ and the energy differences in the spin system $\mathrm{\Delta }E$. It can be shown that the efficiency is a function of $\mathrm{\Delta }T$ and $\mathrm{\Delta }E$.