For more than a century, thermodynamics has described how heat flows and engines run, while quantum mechanics has ruled the strange behavior of atoms and photons. Physicists have long suspected that these two pillars must ultimately fit together, but the rules of boiling kettles and steam turbines seemed impossible to reconcile with the fragile superpositions inside a quantum chip. Now a new generation of researchers is showing how the familiar laws of energy and entropy can be rewritten so they still hold in the quantum world, turning a conceptual clash into a roadmap for future technologies.
By treating information, randomness and measurement as physical resources, these teams are building a unified language that works from single particles up to power plants. Their work is not just philosophical housekeeping. It is beginning to define how efficient quantum machines can be, how much heat a qubit must generate, and how far clever feedback or even “demonic” engines can go without breaking the second law.
From steam engines to single quanta
Classical thermodynamics was born in the 19th century to explain why steam engines waste so much fuel and why some processes, like mixing cream into coffee, never run in reverse. In that framework, entropy measures the degree of disorder, and the second law says entropy in a closed system can only stay the same or increase, which is why In the classical picture the universe trends toward disorder. That story works beautifully for gases in a cylinder or water in a kettle, where you can ignore the microscopic details and focus on averages like temperature and pressure.
At quantum scales, however, those averages hide the very effects that matter most. Individual atoms can occupy superpositions, entangle with distant partners and fluctuate wildly, so the neat classical variables no longer capture what is going on. As researchers began probing physics at the smallest scales, they found that the old definitions of work, heat and entropy needed to be sharpened to cope with coherent quantum states and the information stored in them, which is why modern efforts in quantum thermodynamics explicitly revisit that 19th century legacy of entropy and randomness.
Why thermodynamics and quantum theory seemed incompatible
For decades, the tension between quantum mechanics and thermodynamics was framed as a battle between microscopic reversibility and macroscopic irreversibility. The Schrödinger equation is time symmetric, so if you reverse the sign of time, the evolution still makes sense, while the second law insists that entropy rises and processes like heat flowing from a colder body to a warmer one simply do not occur. Earlier work on the foundations of statistical mechanics tried to bridge this gap with probabilistic arguments, but as quantum technologies matured, that uneasy truce began to look inadequate and the Many competing proposals for rebuilding thermodynamics from quantum laws started to proliferate.
Some theorists suggested that the second law might be only approximate, or that new “resource theories” of coherence and entanglement were needed to define what counts as useful work in the quantum regime. Others pushed more radical ideas about emergent time arrows and observer dependent entropy. The result was a heated debate over whether the thermodynamic laws were fundamental or derivative, and whether they could survive intact once quantum effects were fully accounted for, a debate that set the stage for the more concrete frameworks now emerging.
Basel’s “Quantum Challenge” to classical physics
The latest breakthrough comes from Researchers at the University of Basel, who have framed a Quantum Challenge to Classical Physics by asking how thermodynamic quantities should be defined when the working medium is a fully quantum system. Led by Professor Patrick Potts, the team built a formalism that tracks not only energy exchanges but also the quantum information encoded in the system and its environment. In their analysis, work and heat are no longer just bulk flows, they are tied to changes in the underlying quantum state, including coherence between energy levels.
In a companion description of the same project, the group is presented as Researchers at the University of Basel, led by Professor Patrick Potts, with doctoral student Aaron Daniel explaining how their approach recovers the familiar thermodynamic laws in the right limit while still respecting quantum mechanics. By carefully defining work as the part of an energy change that can be extracted in a controlled way, and heat as the part tied to uncontrollable fluctuations and entanglement with the environment, they show that the second law survives even when the system is a single quantum oscillator or a few photons in a cavity.
Making entropy precise at the quantum scale
One of the thorniest issues in this merger has been entropy, which in classical thermodynamics is a coarse measure of disorder but in quantum theory is tied to the full density matrix of a system. Recent work on Quantum scale thermodynamics argues that by treating entropy as a property of quantum states, not just ensembles, one can give a tighter definition that works for small systems and single-shot processes. In that view, entropy is not just about counting microstates, it is about how much uncertainty and entanglement are present, and how much of that structure can be harnessed for useful tasks.
Physicists tracing the rise in entropy to quantum information have gone further, showing that the second law can be understood as a statement about the flow and degradation of information. Several independent groups, highlighted in a Quanta Science Podcast The feature, argue that entropy increase is tied to the loss of access to the quantum resource of information as systems interact and decohere. In this picture, thermodynamic irreversibility is not a brute fact but a reflection of how observers become entangled with what they measure and how much of the underlying quantum detail they can actually control.
Mixing thermodynamics and quantum physics in the lab
These ideas are not confined to blackboards. Experimentalists are now building tabletop setups where individual atoms, ions or superconducting circuits act as tiny engines and refrigerators, allowing them to test how heat and work behave one quantum at a time. One researcher profiled as a steampunk enthusiast describes Mixing Thermodynamics and Quantum Physics by contrasting the macroscopic laws that govern engines with the delicate quantum effects that are most apparent in individual particles, and by designing experiments where a single trapped ion plays the role of a working fluid.
Educational efforts are keeping pace, with explainers such as a video titled what is quantum thermodynamics walking viewers through how the principles of thermodynamics and quantum mechanics intersect. These resources emphasize that temperature, work and entropy must be reinterpreted when dealing with qubits and photons, but they also stress continuity with the classical laws, which still emerge when many quantum systems are averaged together. The result is a growing community of students and practitioners who see quantum thermodynamics not as an exotic niche but as a practical toolkit for the devices they are already trying to build.
Quantum machines, demons and the limits of efficiency
The push to reconcile thermodynamics with quantum theory is not just about consistency, it is about figuring out what kinds of machines are possible. As engineers design quantum heat engines and refrigerators, they must contend with the fact that Worse still, quantum states are even more delicate than a molecular motor, and a single gas molecule colliding with a quantum device can destroy its coherence and overheat it. That fragility makes thermal management a central challenge for quantum technologies, since every unwanted interaction with the environment both adds noise and pumps entropy into the system.
At the same time, theorists are revisiting classic thought experiments like Maxwell’s demon in fully quantum terms. Recent work on a “demonic” engine shows that Our results showed that under certain conditions permitted by quantum theory, even after accounting for all costs, the work extracted by the demon can be positive, as a researcher from Nagoya University said in a statement. Crucially, the bookkeeping still respects the second law once the information the demon gathers and erases is included, reinforcing the idea that information is a thermodynamic resource that must be paid for in energy and entropy.
Rewriting the laws for the quantum age
One of the clearest demonstrations that thermodynamics can be made to work in the quantum regime comes from optical cavity experiments. In a study described as By University of Basel November, researchers analyze what happens when laser light passes through a cavity filled with a quantum medium and show that familiar thermodynamic quantities like temperature and free energy can be defined in terms of the quantum state of the light field. When they drive the system out of equilibrium, they can still write down generalized second laws that constrain how much work can be extracted and how quickly the system can relax back to a steady state.
In a related theoretical development, another report on the same Basel work notes that In the future, we can use our formalism to consider more subtle problems in quantum thermodynamics, says Daniel Thi, highlighting how their framework can be extended to complex many body systems and information processing tasks. By grounding thermodynamic concepts directly in the structure of quantum states, they argue that quantum mechanics and thermodynamics can both be true at once, with no need to sacrifice one set of laws for the other.
Information, randomness and the emergence of reality
As the mathematical pieces fall into place, some physicists are using quantum thermodynamics to probe deeper questions about how classical reality emerges from the quantum substrate. One line of thought, explored in a feature on a bold new take on quantum theory, suggests that the act of measurement and the flow of information might be the key to understanding why we see definite outcomes instead of superpositions. When physicists considered this question, they found that the way quantum systems interact with their environment, and the thermodynamic cost of recording and erasing information, could help explain why macroscopic objects behave classically.
Others revisit the notion of randomness itself, arguing that what looks like disorder at large scales may hide structured correlations at small ones. A reflection on order in disorder notes that classical thermodynamics painted a picture of inevitable decay into randomness, but modern quantum physics at the smallest scales reveals intricate patterns of entanglement and interference. Quantum thermodynamics sits at this crossroads, translating between the microscopic rules and the macroscopic arrow of time by treating information and entropy as two sides of the same coin.
A revolution in how we think about heat
For working physicists and engineers, the most immediate impact of this merger is a conceptual one. As one analysis of what happens when you mix thermodynamics and the quantum world puts it, extending the 19th century laws to the quantum realm amounts to a Revolution in how we think about heat, work and information. Instead of treating heat as a vague flow of microscopic motion, researchers now track individual quanta of energy and the correlations they carry, asking not just how much energy moves but how much control and knowledge is required to move it.
This shift is also changing how the field is taught and organized. One overview of Quantum Thermodynamics emphasizes that theoretical advancements in this area pave the way for real world applications, from more efficient quantum heat engines to enhanced control over quantum entanglement and coherence. By framing thermodynamics as a resource theory that quantifies what transformations are possible under given constraints, the field gives students a unified language that applies equally to steam turbines and superconducting qubits.
From theory to technology and policy
Although much of quantum thermodynamics is still theoretical, it is already influencing how laboratories and companies think about future devices. Nicole Yunger Halpern, described as a quantum thermodynamicist, notes that A lot of theory work is still going on now, and quantum thermodynamics is still, she thinks, primarily theoretical, but concrete proposals for experiments and technologies have multiplied during the past decade. That trajectory mirrors the early days of quantum information science, when abstract discussions of qubits and entanglement gradually gave way to working prototypes of quantum computers and communication links.
Policymakers are starting to pay attention as well, since the thermodynamic efficiency of quantum technologies will shape their economic and strategic value. Celia Merzbacher In quantum as an emerging technology has argued that the United States needs a coordinated push to stay competitive in areas such as quantum sensing, secure communication and computing, sectors that span civilian and defense uses such as military, aerospace, space and so on, as highlighted in a call for a quantum leap. As governments invest in quantum infrastructure, the frameworks developed by thermodynamic theorists will help determine how powerful, reliable and energy efficient those systems can ultimately become.
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