Quantum Thermal Machines Improved by Internal Coupling: From Equilibrium to Non-equilibrium Limit Cycles

TL;DR

This paper explores how internal coupling enhances the performance and operational regimes of quantum Otto cycles, enabling them to function as engines or refrigerators even in regimes where uncoupled systems fail, and improves efficiency and power output.

Contribution

It demonstrates that internal coupling broadens operational regimes and enhances efficiency of quantum Otto cycles, including regimes where uncoupled systems cannot operate as thermal machines.

Findings

Internal coupling broadens operational regimes of quantum Otto cycles.

Coupled systems can function as engines or refrigerators where uncoupled systems cannot.

Efficiency and power output are improved by internal coupling and finite interaction times.

Abstract

We investigate how internal coupling influences the operation and performance of a quantum Otto cycle operating as the Gibbs-state limit cycle (GSLC), equilibrating limit cycle (ELC), and non-equilibrating limit cycle (NELC). We show that the internal coupling significantly broadens the operational regime of the cycle. In particular, in parameter regimes where the uncoupled Otto cycle fails to operate as any thermal machine, the coupled system can function as an engine or a refrigerator. For the GSLC, in which we assume that the system quickly equilibrates during the isochoric processes, the internal coupling not only shifts and enlarges the operational regime but also enhances the efficiency and the coefficient of performance (COP), allowing the performance to exceed the standard Otto bounds while remaining below the Carnot limit. For ELC and NELC, we validate the global approach of the Gorini--Kossakowski--Sudarshan--Lindblad (GKSL) master equation by comparison with the GSLC, and examine the NELC for finite interaction time and the ELC for infinite interaction time. Although the efficiency and COP of NELC are lower than those of ELC, shorter interaction times yield higher power output, consistent with the power--efficiency trade-off.