Quantum stirling cycle in an anisotropic three-qubit Heisenberg chain

The thermodynamic performance of a quantum Stirling cycle is investigated using a three-qubit antiferromagnetic Heisenberg chain as the working substance. The analysis focuses on the roles of uniaxial magnetic anisotropy K, spanning both easy-axis K|0 and easy-plane K|0 regimes, and system topology (ring versus linear chain), with the external magnetic field employed as the control parameter. Key quantities, including work output, heat exchanges, and cycle efficiency, are evaluated under fixed temperature and coupling conditions. It is demonstrated that easy-axis anisotropy in the ring geometry enables the cycle to reach a local maximum efficiency of η|0.4 (which remains below the Carnot limit ηC|0.67 for the chosen temperatures Th=3TC) while delivering finite work W|0.2J at the quantum critical point. The true Carnot efficiency is not achieved in our parameter regime, but the results illustrate how anisotropy and topology can tune the cycle's performance close to criticality.This optimal performance is attributed to anisotropy-induced narrowing of the energy spectrum, which enhances the entropy exchange during the isothermal strokes. The results establish the quantum Stirling cycle as a highly versatile and efficient thermal machine, whose operational regime can be precisely tuned by anisotropy and topology for potential applications in quantum thermal management.