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arXiv:2601.03139

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

Alonso Alcalá, Charlie Oncebay, Onofre Rojas, Moises Rojas

quant-phSubmitted Jan 6, 2026• Announced Jan 7, 2026• 0 citations
arXivHTMLPDF

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.

Comments
13 pages, 10 figures

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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=ωˉ/ωr=\barω/ωr=ωˉ/ω, the coupling strength ggg, 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.
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We investigate a quasi-static quantum Stirling cycle employing a spin-chain-star working fluid, comprising a central ancilla coupled to multiple Heisenberg chains via anisotropic exchange. A unitary transformation reduces the many-body thermodynamics to exact diagonalisation within a Hilbert space of dimension two to the power of the chain number plus one. We evaluate purely transverse, isotropic in-plane, and fully anisotropic configurations. The cycle exhibits tunable heat engine, refrigerator, and quantum accelerator regimes governed by ancilla frequency and coupling strength disparities. The transverse configuration demonstrates superior engine performance approaching the Carnot limit, corroborated by a closed-form partition function that permits the engine-refrigerator boundary to be evaluated exactly at arbitrary chain number. Conversely, the fully anisotropic architecture undergoes a genuine many-body crossover from engine to refrigerator with escalating chain multiplicity, driven by collective coupling amplification. Regenerator functionality proves non-ideal, rendering the cycle irreversible. Perfect regeneration emerges only along a precise quantum resonance locus where isochoric heat exchanges balance. Deviating from this condition exacerbates thermal imbalance with increasing system size, constituting the primary irreversibility source. Finally, kinematic coherence maximizes during cold-bath compression and amplifies with anisotropy and chain number. Originating from Hamiltonian basis rotation rather than non-equilibrium dynamics, this coherence remains dissociated from thermodynamic efficacy.
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