Operational modes of a Raman-coupled two-qubit quantum thermal machine

Abstract

We investigate a quantum thermal machine composed of two qubits coupled through a Raman-induced exchange interaction and driven by inhomogeneous transition frequencies. The system is analyzed within Carnot, Otto, and Stirling thermodynamic cycles, including the Stirling cycle with and without regeneration. We identify the conditions under which the device operates as a heat engine, refrigerator, thermal accelerator, or heater. Efficiency maps and operational-mode diagrams reveal well-defined boundaries in parameter space, governed by the frequency ratio $r=\barω/ω$, the coupling strength $g$, and the thermal gradient between reservoirs. The Carnot cycle exhibits sharp transitions between engine and refrigerator regimes, while the Otto cycle displays a richer structure with the coexistence of all operational modes. The Stirling cycle shows enhanced versatility and performance, particularly when assisted by a regenerator, where near-ideal efficiencies are achieved. Overall, the Raman-type interaction introduces a controllable left-right asymmetry that enables nontrivial manipulation of thermodynamic behavior through frequency tuning.

Figures (10)

• Figure 1: Schematic representation of the Raman-coupled two-qubit system, where the right-qubit transition frequency is externally tuned while the left qubit remains fixed.
• Figure 2: Visual depiction of the two-qubit working substance, where work is produced by tuning the right-qubit transition frequency from $\omega_{0}$ to $\omega_{1}$.
• Figure 3: Schematic representation of Quantum Carnot machine cycle in the $\omega_{0}-\omega_{1}$ plane. a) For $T_{c}=1$, and $T_{h}=2$. b) For $T_{c}=1$, and $T_{h}=5$. The parameters are set to $r=1$, and $g=1$.
• Figure 4: Operational modes of the quantum Otto cycle in the $\omega_{0}-\omega_{1}$ plane for different parameter configurations. a) $r=1$, $T_{c}=1$, and $T_{h}=2$. b) $r=1$, $T_{c}=1$, and $T_{h}=5$. c) $r=3$, $T_{c}=1$, and $T_{h}=2$. d) $r=3$, $T_{c}=1$, and $T_{h}=5$. The coupling parameter was fixed at $g=1$.
• Figure 5: Operational modes of the quantum Stirling cycle in the $\omega_{0}-\omega_{1}$ plane for different parameter configurations. a) $r=1$, $T_{c}=1$, and $T_{h}=2$. b) $r=1$, $T_{c}=1$, and $T_{h}=3$. c) $r=2$, $T_{c}=1$, and $T_{h}=2$. d) $r=2$, $T_{c}=1$, and $T_{h}=3$. The coupling parameter was fixed at $g=1$.