Classical heat engines are bounded by the Carnot limit, which depends only on the temperatures of the hot and cold reservoirs. In the quantum regime, it has already been shown that engineered non-equilibrium reservoirs, such as squeezed thermal states, allow one to surpass this limit without violating the second law. In parallel, a line of research on relativistic quantum heat engines is developing, where the working fluid consists of Unruh–DeWitt detectors interacting with quantum fields. Neighboring studies explore accelerated motion, curved spacetime, and relativistic energies, but the question of whether motion itself can serve as a resource for boosting efficiency remained open.
The authors consider a quantum SWAP engine with two qubits, each locally coupled to a thermal reservoir at its own temperature. The qubits move at constant relativistic speeds through thermal baths of a scalar field. The effective temperature perceived by a moving qubit is computed from the detailed balance of transitions and depends on the qubit frequency and its velocity. Work and heat are evaluated via the cumulant generating function using a two-point measurement scheme. Two regimes are analyzed: the high-temperature regime (βω ≪ 1) and the low-temperature regime (βω ≫ 1).
It is shown that a moving qubit can perceive an effective temperature higher or lower than the reservoir temperature depending on frequency and speed. In the high-temperature regime, the effective temperature is always below the bath temperature, while in the low-temperature regime it is above. This allows the engine to operate as a heat engine in regimes where its static counterpart would act as a refrigerator. The efficiency at maximum power can exceed not only the Curzon–Ahlborn efficiency but also the standard Carnot limit defined by rest-frame temperatures. For example, when qubit B moves through the cold reservoir at speed 0.8c and the temperature ratio is $β_A$/$β_B$ = 1/2, the efficiency reaches values above $η_C$. When the reservoir temperatures are equal ($T_A$ = $T_B$), the standard Carnot limit is zero, but the relativistic engine still produces positive work. Entropy production remains non-negative, confirming consistency with the second law.
The work shows that relativistic motion of the working fluid is an independent thermodynamic resource, similar to squeezed reservoirs. Unlike neighboring studies that use accelerated motion or curved spacetime, here inertial motion at constant speed suffices. This simplifies experimental implementation and opens a new class of quantum heat engines where speed control allows tuning efficiency without changing reservoir temperatures.
The topic could develop toward accounting for the energy cost of maintaining the qubits' motion and the back-reaction of the detectors on the field. An interesting generalization would consider more realistic trajectories, including acceleration, and interaction with the electromagnetic field instead of a scalar field. One could also investigate many-particle working fluids and entangled states to further enhance performance.
The results will impact quantum thermodynamics, relativistic quantum information, and the design of microscopic thermal devices.
Test the predictions in experiments with moving atoms or superconducting qubits that mimic relativistic motion through modulation of the reservoir coupling.
The work directly relates to the fundamental problem of defining temperature in relativistic thermodynamics and to the question of efficiency limits for quantum heat engines with non-equilibrium reservoirs.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This paper examines a quantum SWAP engine built from two qubits moving at constant relativistic speeds through thermal reservoirs. The authors calculate the effective temperature experienced by a moving qubit and show that the efficiency at maximum power can exceed the Carnot limit defined by the reservoir rest temperatures. Entropy production remains nonnegative, consistent with the second law of thermodynamics.
Where its strength is
The work identifies inertial motion at constant velocity as an independent thermodynamic resource, requiring neither acceleration nor spacetime curvature. This greatly simplifies potential experimental realization compared with earlier approaches.
Where it could go next
Worth trying
Why this may matter
If relativistic motion can indeed surpass the Carnot limit without violating the second law, it may open up a new class of quantum heat engines with improved performance. Practical implementation of such devices, even in a laboratory setting, could provide a tool for controlling energy flows at the quantum level.
Nearby works in our archive
The closest work in the archive sits at 0.66 by meaning — the usual level for our corpus.
At ultra-relativistic speeds (v → c), a moving qubit perceives the thermal field as a vacuum at zero temperature, an effect akin to the Unruh effect.
The idea of using relativistic motion to extract work echoes the concept of an 'Unruh-effect engine' from science fiction, where acceleration creates a thermal bath from the vacuum.