Enhancing the power of a quantum heat engine via control of the system–reservoir coupling

How can the microscopic system-reservoir coupling be manipulated to enhance the finite-time performance of a quantum heat engine? While theoretical models often rely on phenomenological heat transfer laws (like Newton's law of cooling), this study seeks to experimentally control the underlying microscopic collision dynamics to optimize power output.

The researchers realized a quantum Otto engine using a working medium of ultracold Cesium-133 atoms immersed in a reservoir of Rubidium-87 atoms. The engine cycle consists of two isochoric strokes (heating and cooling) and two adiabatic work strokes. Heat exchange is mediated by inelastic s-wave spin-exchange collisions. By adjusting the kinetic temperature of the Rb reservoir, the team modified the energy-dependent scattering cross sections, which in turn altered the multi-exponential relaxation dynamics of the Cs atoms. This allowed for direct control over the time allocation of the engine cycle.

The study reveals that exothermal and endothermal spin-exchange collisions exhibit distinct energy scaling, leading to asymmetric equilibration dynamics. Because the transition rates respond differently to the reservoir's kinetic temperature, the researchers could tune the duration of the isochoric strokes. This tunability allows for the optimization of the engine's power output at a fixed efficiency. Furthermore, the team demonstrated that the heat transfer is governed by a multi-exponential process rather than the simple mono-exponential relaxation typically assumed in macroscopic thermodynamics, highlighting the importance of the system's spectral properties.

This work establishes microscopic control of system-reservoir interactions as a viable tool for engineering the performance of quantum thermal machines. By moving beyond phenomenological descriptions of heat flow, the findings provide a pathway for designing more efficient and powerful nanoscale devices, bridging the gap between fundamental quantum collision models and practical thermodynamic applications.