Heat engines of finite power always seek a balance between speed and efficiency. An ideal reversible cycle gives maximum efficiency but zero power, so real machines operate in a compromise regime. In recent years, quantum thermodynamics has actively developed methods to speed up cycles, from 'shortcuts to adiabaticity' to engineered reservoirs. However, most approaches require additional energy expenditure or complex control over the Hamiltonian. The work by Burghardt and colleagues follows the search for alternative levers: they control the system-reservoir coupling, promising acceleration without increased dissipation. Nearby in the archive, there are already studies on collision-based quantum motors, quantum batteries, and the Mpemba effect, but here it is experimentally demonstrated for the first time how the microscopic structure of scattering rates determines the macroscopic performance of a machine.
The experiment is built on a mixture of two ultracold atomic gases: up to 40 cesium-133 atoms serve as the working fluid, while a cloud of several thousand rubidium-87 atoms acts as the reservoir. Cesium and rubidium interact via s-wave collisions, with inelastic spin-exchange processes transferring cesium atoms among seven Zeeman sublevels, exchanging energy quanta. Heating and cooling are realized through exothermic and endothermic collisions, while adiabatic strokes are implemented by linear magnetic field ramps. The key technique is varying the kinetic temperature of the rubidium cloud from 679 to 1140 nanokelvins, which changes the distribution of collision energies and thus the transition rates. The population dynamics are tracked via fluorescence images with spin-state resolution.
The main result is the control of the heat exchange law through the reservoir temperature. The scattering cross sections for exothermic collisions decrease monotonically with energy, while those for endothermic collisions have a threshold character, so the transition rates depend differently on temperature: heating slows down, cooling speeds up. This allowed changing the relative durations of the strokes and reducing the total cycle time. At fixed magnetic fields (1 G and 35 mG), the efficiency remained constant at 0.489(2), while the maximum power increased with temperature, reaching an optimum at 1410(5) nK with a peak value of 28.0 nK/ms in units of kB. Modeling showed that the multi-exponential relaxation characteristic of the real system yields lower power than the simplified single-exponential model: the latter overestimates performance by a factor of 1.5–1.7.
This work stands out from neighboring studies by demonstrating, for the first time, control over the heat exchange law at the microscopic level without changing the system Hamiltonian. Instead of phenomenological relaxation rates, it uses real energy-dependent scattering cross sections, linking atomic collision physics with finite-time thermodynamics. This opens the way to designing quantum heat engines where the spectrum of relaxation times becomes a resource rather than an obstacle. For quantum thermodynamics, the result is important as experimental confirmation that controlling the coupling to the reservoir can be more effective than controlling the system itself.
Further steps could include using non-Markovian reservoirs, where memory of past interactions alters dynamics, and optimizing control protocols beyond the fixed cycle. It is also interesting to investigate how multi-exponential relaxation affects power fluctuations and operational stability over many cycles. Theoretically, one could search for optimal modulation shapes of the coupling, not just the reservoir temperature.
The results apply to any system with state-dependent relaxation rates: spin baths, quantum dots, impurities in solids. This provides a tool for controlling heat flows at the nanoscale and optimizing microscopic heat engines.
The immediate task is to check whether the power gain persists when accounting for the finite lifetime of the reservoir and density inhomogeneities. It is also important to measure work fluctuations over long sequences of cycles to assess the engine's practical viability.
In quantum thermodynamics, the fundamental limits of power at a given efficiency remain an open question. This work shows that accounting for the real relaxation spectrum, rather than a single effective rate, changes predictions of optimal regimes. This directly relates to the problem of describing non-Markovian and many-body reservoirs, where simple heat exchange laws break down.
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
The work demonstrates a quantum Otto engine using ultracold Cs and Rb atoms, where heat exchange occurs through inelastic collisions. By varying the temperature of the rubidium cloud, the authors controlled the heating and cooling rates, finding the optimal temperature for maximum power at a fixed efficiency of 0.489. It is shown that multi-exponential relaxation reduces power compared to a single-exponential model.
Where its strength is
The strength of the work lies in the experimental demonstration of controlling the heat exchange law through microscopic scattering properties, which paves the way for accelerating cycles without altering the working fluid's Hamiltonian.
Where it could go next
Worth trying
Why this may matter
Controlling the reservoir coupling at the microscopic level could become a universal tool for optimizing quantum heat engines, especially in systems where direct Hamiltonian control is challenging.
Nearby works in our archive
The closest work in the archive sits at 0.69 by meaning — the usual level for our corpus.
as written by the authors arXiv:2608.12055v1
The non-equilibrium properties of open quantum systems are determined by the microscopic laws governing energy exchange with their environment. In particular, an enhancement of the performance of quantum heat engines has been predicted by speeding up the dynamics through control of the system--bath interaction. However, direct microscopic control of heat transfer between the machine and the reservoir has remained elusive so far. Here, we experimentally demonstrate such control in a quantum Otto engine realized with ultracold Cs-133 atoms coupled to an atomic reservoir of ultracold Rb-87 atoms. Heat exchange between the two is mediated by inelastic s-wave collisions whose energy-dependent scattering cross sections lead to an asymmetric equilibration dynamics in the isochoric heating and cooling strokes. By tuning the kinetic temperature of the atomic reservoir, we modify the associated microscopic scattering rates, and thereby the heat transfer law, giving control over the time allocation within the engine cycle through control over the microscopic, multi-exponential relaxation dynamics. This enables power output optimization at fixed efficiency. Our results establish microscopic control of system-reservoir interactions as a tool for manipulating heat flow at the nanoscale and engineering the finite-time performance of quantum thermal machines.
At the optimal temperature of 1410 nanokelvins for power, the thermal rubidium cloud moves at an average speed of about 2 millimeters per second — a thousand times slower than a housefly at room temperature.
In Stanisław Lem's novel 'The Invincible', microscopic machines use the environment for self-replication but face the problem of heat dissipation. Controlling heat exchange through microscopic collisions, as in this work, is a step toward solving such problems at the nanoscale.