Quantum Heat Engines with Spin-Chain-Star Systems

This study investigates a theoretical model of a Quantum Otto Cycle (QOC) that utilizes a working fluid spin-chain-star model. The system consists of a central atom interacting with multiple Heisenberg spin chains. Employing unitary transformations, the spin-chain-star system is transformed into a spin-star model. The work done and heat transferred for three distinct working fluid configurations: the $X$, $XX$, and $XYZ$ cases are discussed. The efficiency of the heat engine is examined, and a comparative study between the efficiencies of the three configurations is presented. The study assumes two interaction scenarios for the central atom: either with a single chain (resulting in a two-qubit system after transformation) or with three Heisenberg chains. The results demonstrate that increasing the ratio between the central atom's frequency in the hot bath and the cold bath leads to an enhancement in positive work performed for the $X$ and $XX$ cases. In the $XYZ$ case, the magnitude of this enhancement exhibits a dependence on the system's temperature. The QOC employing the $X$ configuration working fluid exhibits superior efficiency compared to the other two configurations. Moreover, increasing the central atom's relative frequency improves efficiency for all three cases.