The work, which appeared as Volume 55, article number 86 of the journal in November 2025, situates itself within a tradition that stretches back to Ronald Fisher’s foundational 1922 paper on the mathematical foundations of theoretical statistics. Fisher information, a quantity that measures how much information a set of measurements carries about an underlying parameter, has long been known to play a dual role in physics. In thermodynamic fluctuation theory, as developed by George Ruppeiner in his influential 1995 review in Reviews of Modern Physics, the Fisher metric acts as a Riemannian metric on the space of equilibrium states: the squared distance between two nearby states is proportional to the probability of the fluctuations connecting them. What the new paper adds is the recognition that this fluctuation geometry, when applied carefully to a gas undergoing adiabatic change, contains the classical adiabatic law in its structure.

The central technical claim is a reinterpretation of the adiabatic exponent gamma, the familiar quantity that equals the ratio of the heat capacity at constant pressure to the heat capacity at constant volume, and takes the value 5/3 for a monatomic ideal gas. In the conventional textbook treatment, gamma is introduced through the thermodynamic identities governing reversible processes. Plastino and Pennini show instead that gamma admits a new interpretation as quantifying the relative stiffness of energy and volume fluctuations. Stiffness here has a precise statistical meaning. Around any equilibrium state, the energy of a system fluctuates with a variance controlled by the heat capacity, and the volume of a gas confined by a piston fluctuates with a variance controlled by its compressibility. The exponent gamma, in the Fisher-information picture, measures the ratio of these two fluctuation scales, expressing how reluctantly the system exchanges energy with its microscopic degrees of freedom compared with how reluctantly it changes its geometry.

This reading transforms the status of the adiabatic law. If gamma measures a ratio of fluctuation stiffnesses, then the condition PV^gamma equals a constant is no longer merely a rule about heat flow; it becomes a geodesic constraint in thermodynamic state space. In the language of differential geometry, a geodesic is the shortest, straightest path between two points on a curved surface, the path a freely moving particle would trace. Plastino and Pennini demonstrate that the adiabatic trajectories of an ideal gas are precisely the geodesics of the Fisher-geometric metric that describes thermodynamic fluctuations. The law that students memorize as an algebraic identity is, on this view, the signature of a straight line in the information geometry of equilibrium fluctuations. A quasi-static adiabatic process is the process that moves through thermodynamic state space most economically, without any excess fluctuation cost.

The connection between Fisher information and fluctuations is not new in itself, and the authors are careful to build on established results rather than claim wholesale novelty. Ruppeiner’s thermodynamic fluctuation theory, the geometrical treatment of statistical mechanics developed by David Brody and Nicolas Rivier, and the general framework of information geometry codified by Shun-ichi Amari and Hiroshi Nagaoka all provide the backdrop. Plastino’s group has contributed extensively to this literature over the past decade, including work on the Hellmann-Feynman connection for relative Fisher information, the Fisher thermodynamics of quasi-probabilities, and the scaling symmetries of Fisher information with S. P. Flego and A. R. Plastino. What distinguishes the new paper is the specific bridge it constructs between this fluctuation geometry and one of the oldest dynamical invariants in classical thermodynamics.

The authors also draw on a smaller but persistent line of inquiry concerning quantum origins of the adiabatic law. Work by T. Yarman and collaborators, published in the International Journal of Physical Sciences and later in Results in Physics, argued that the constancy expressed in the adiabatic gas law is ingrained within quantum mechanics, and that the second law of thermodynamics itself can be seen as a consequence of quantum mechanical structure. The new Fisher-information analysis provides a complementary and, the authors suggest, more general route to a similar conclusion. Rather than deriving the adiabatic constancy from the quantum mechanics of a specific gas model, the information-geometric approach shows how the law emerges from the statistical structure of fluctuations themselves, whatever their ultimate microscopic origin.

The practical implications of this reframing extend into several active research areas. In finite-time thermodynamics, researchers study how much extra work must be dissipated when a thermodynamic process is carried out in finite time rather than quasistatically. Seminal contributions by Peter Salamon and R. Stephen Berry in 1983, and later by T. Schmiedl and U. Seifert and by D. A. Sivak and G. E. Crooks, established that the dissipated availability along a finite-time protocol is proportional to a thermodynamic length computed from a fluctuation metric, and that optimal protocols are geodesics of that metric. The new result places the adiabatic law squarely within this framework, suggesting that the same geometry that governs optimal finite-time control also governs the idealized adiabatic paths of classical theory. This opens a pathway toward designing protocols for cold-atom platforms and other highly controllable quantum systems where the stiffness of fluctuations can be measured and manipulated.

Quantum metrology provides another natural arena of application. Fisher information is the central quantity of estimation theory: the Cramér-Rao bound, formalized by C. R. Rao, states that the variance of any unbiased estimator of a parameter is bounded below by the inverse of the Fisher information. In quantum systems, the quantum Fisher information sets the ultimate precision of measurements of temperature, phase, or field strength, and recent work on critical quantum metrology and coherence-enhanced thermometry has exploited near-critical fluctuations to sharpen estimates. The identification of the adiabatic exponent as a Fisher-geometric quantity suggests that thermodynamic processes themselves could be characterized, and optimized, by the estimation-theoretic properties of the fluctuations they carry. A quantum heat engine driven along its adiabatic geodesic would, in this picture, be an engine whose strokes minimize the information cost of moving between thermodynamic states.

The work is also part of a broader intellectual program that the authors describe as a step toward unifying epistemic and ontic perspectives on thermodynamic order. The epistemic view treats thermodynamic quantities as statements about knowledge and information, encoded in probability distributions over microstates. The ontic view treats them as objective features of the physical world, independent of any observer. Fisher information sits naturally at the boundary: it is defined epistemically, through the sensitivity of a probability distribution to parameter changes, yet the new result shows it fixing the form of an objective dynamical invariant, the adiabatic law, that governs the behavior of real gases. The suggestion that classical thermodynamic laws can be rederived as emergent signatures of Fisher-geometric structure points toward a deeper claim, namely that the laws of thermodynamics may be information-theoretic principles all the way down.

It should be emphasized that the analysis is theoretical and, as the authors state in their data availability section, no datasets were generated or analyzed in the study. The derivation concerns the equilibrium fluctuation geometry of gases, and extending the geodesic interpretation to non-ideal systems, to genuinely irreversible processes, and to strongly quantum regimes remains a task for future work. Nevertheless, the conceptual payoff is substantial. A law discovered in the nineteenth century, taught to every physics student as a piece of algebra, turns out to encode a statement about the geometry of fluctuations, a straight path through a curved space whose curvature is set by Fisher information. The research was partially supported by FONDECYT through grant 1251928, and the authors declare no competing interests. If the Fisher-geometric program continues to bear fruit, the adiabatic law may come to be seen not as an isolated rule but as the first recognizable landmark in an information-theoretic map of thermodynamic reality.

Thermodynamics was originally created to explain large machines such as steam engines, where enormous numbers of particles behave collectively and microscopic fluctuations can usually be ignored. Quantum physics, by contrast, describes atoms, photons and other particles whose energies and properties can occupy discrete states and fluctuate strongly. In modern laboratories, however, these two descriptions increasingly overlap. Quantum devices can absorb energy, transform it and release it, just as conventional machines do, but their working parts may consist of only one atom and a confined field of light. Understanding how such miniature engines operate could be essential for designing quantum sensors, optical technologies and new methods of controlling energy at microscopic scales.

The model examined by the Basel researchers places an atom inside a cavity formed by two mirrors. The atom can absorb photons and later emit them, while the mirrors confine the light long enough for repeated interactions to occur. A laser continuously injects additional photons into the cavity, replenishing the system’s energy. At the same time, the mirrors are only partially reflective, allowing some of the light to escape into the surrounding environment. This combination of constant driving and continuous loss makes the setup a driven-dissipative quantum system. It never reaches a simple equilibrium: energy is always entering, being converted through interactions between the atom and the electromagnetic field, and leaving again as emitted light.

In this picture, the atom functions much like the working substance of a tiny heat engine. Its discrete energy levels determine which photons it can absorb or emit, while the cavity field provides a controllable channel through which energy flows. Yet the escaping photons create a conceptual problem. In conventional thermodynamic analyses, energy leaving a system is often treated as waste heat. At the quantum scale, that assumption can be too crude. The outgoing light may retain structure, correlations and directed energy that could be transferred to another system to perform useful work. Treating every escaping photon as disordered heat risks erasing precisely the information that distinguishes usable energy from energy irreversibly lost to the environment.

The new approach builds on this distinction by dividing the emitted light into thermodynamic contributions rather than automatically classifying all of it as heat. Some of the energy carried away can be regarded as work-like energy, because it may be harnessed to influence another quantum device. The remaining portion represents heat or irreversible dissipation. This separation is not merely a matter of terminology. In a quantum system, the definition of work determines how researchers calculate efficiency, entropy production and fluctuations. A thermodynamic framework that assigns these quantities incorrectly can produce results that fail when the system is compared with its semiclassical counterpart.

The semiclassical limit provides a demanding test of the theory. In that limit, the atom remains a quantum object with discrete energy levels, but the light field is treated as a classical electromagnetic wave. This approximation is widely used because it simplifies calculations and accurately describes situations in which quantum fluctuations of the field are negligible. A consistent quantum theory should naturally approach this description when the relevant quantum effects are reduced. According to the researchers, their method passes this test because its treatment of the emitted light remains well behaved as the field becomes increasingly classical. The conventional approach, which counts all escaping energy as heat, fails to make the same transition consistently.

The calculations also reveal an important consequence of the atom–light interaction: quantum effects can reduce fluctuations in the escaping light. Normally, heat is associated with random disturbances that make precise control difficult. In this system, however, the atom can modify the statistical behavior of the photons leaving the cavity. Instead of producing light with completely ordinary fluctuations, the interaction can create a more ordered output in which variations are suppressed. This effect is significant because fluctuations are often the limiting factor in precision measurements. Light with reduced noise can improve the sensitivity of quantum sensors, help identify weak signals and support measurements of physical quantities that would otherwise be hidden by random variation.

The result suggests that heat and dissipation need not always be viewed solely as obstacles to quantum technology. Under carefully controlled conditions, fluctuations associated with an open system can become a resource. By engineering the interaction between a quantum emitter and a cavity field, researchers may be able to produce specially tailored states of light for quantum metrology, the science of making exceptionally precise measurements. The broader importance of the work lies in its attempt to place quantum and classical thermodynamics within one coherent framework. A single atom between two mirrors may be far removed from a steam engine, but both systems transform and release energy according to underlying thermodynamic principles. By clarifying what counts as heat, what counts as useful work and how quantum fluctuations evolve toward classical behavior, the Basel study provides a theoretical foundation for the next generation of microscopic machines.

Subject of Research: Quantum thermodynamics of a driven-dissipative atom–cavity system

Article Title: Bridging Quantum and Semiclassical Thermodynamics in Cavity QED

Web References: https://doi.org/10.1103/y6h7-sx93

References: Physical Review Letters; “Bridging Quantum and Semiclassical Thermodynamics in Cavity QED”; DOI: 10.1103/y6h7-sx93

Image Credits: Enrique Sahagún, Scixel / University of Basel, Department of Physics

Keywords

Quantum thermodynamics, cavity quantum electrodynamics, quantum heat engine, driven-dissipative systems, photons, atoms, semiclassical physics, quantum fluctuations, quantum metrology, thermodynamics

At the heart of this quantum engine lies a transmon qubit, a central component in many quantum computing architectures. Unlike conventional heat engines that require distinct hot and cold sources, this quantum heat engine employs a single, quantum-circuit refrigerator capable of acting as both heat source and sink. Tunable on demand, this element controls heat flow at an unprecedented quantum scale, allowing the engine to cyclically produce measurable positive work from minuscule ultracold energy fluctuations.

This experimental setup is a significant leap forward, proving that quantum thermodynamics can be engineered with precision in superconducting systems. The researchers designed their engine to operate near absolute zero inside a cryostat, carefully orchestrating microwave pulses that manage the cyclic Otto process. By monitoring the quantum state of the transmon, they showed direct evidence of work extraction—a feat that had long eluded quantum engineers.

The implications of this breakthrough extend beyond the laboratory. Autonomous quantum heat engines could help overcome significant challenges faced by the burgeoning quantum computing industry. Presently, high-qubit quantum computers rely on millions of costly microwave cables for qubit control and readout, cables that also introduce noise and complexity. Integrating heat engines capable of operating independently on-chip promises to drastically simplify these systems, reducing costs and improving stability.

Finland’s ambitious Quantum Technology Strategy foresees quantum computers with thousands of logical qubits on the horizon, demanding vast physical qubit arrays. Innovations like this superconducting quantum engine pave the way to meet such demands by minimizing external control infrastructure. The technology could enable quantum processors to perform essential functions such as qubit readout autonomously at cryogenic temperatures, bypassing the noisy transition to room temperature electronics.

This pioneering research was led by Academy Professor Mikko Möttönen and first author Tuomas Uusnäkki. Their paper detailing the construction and operation of the cyclic quantum heat engine was published in Nature Communications on July 13, 2026. Utilizing the state-of-the-art facilities at OtaNano, Finland’s national infrastructure for nano and quantum tech, the team demonstrated a clear proof of concept for a new class of quantum devices melding thermodynamics with quantum information science.

As quantum technologies continue to evolve, this fusion of quantum physics and thermodynamics may unlock novel functionalities and efficiencies. This development stands as a milestone indicating not only the feasibility of quantum heat engines but also their potential to reduce technological barriers in scaling future quantum computing systems.

Subject of Research: Superconducting quantum heat engine and quantum thermodynamics
Article Title: World’s first superconducting quantum heat engine offers path to larger quantum computers
News Publication Date: 13-Jul-2026
Web References: http://dx.doi.org/10.1038/s41467-026-72651-x
Image Credits: Heikka Valja / Aalto University

Keywords

Quantum heat engine, superconducting circuits, transmon qubit, quantum thermodynamics, Otto cycle, quantum refrigerator, quantum computing, cryogenic technology, autonomous quantum devices