Touch the back of a laptop, and the warmth you feel is energy that has already been paid for, processed by chips, and then dissipated as heat. The work by Toshimasa Fujisawa and colleagues, reported in Communications Physics as “Efficient heat-energy conversion from a non-thermal Tomonaga-Luttinger liquid” and summarized in the TechXplore piece “A new approach to energy harvesting opened up by the quantum world,” asks a simple but radical question: what if that waste heat could be turned back into useful electrical power more efficiently by exploiting the quirks of quantum physics rather than fighting them?[1]
To see why this matters, it helps to start with an understanding of what energy harvesting is in everyday terms. Imagine every hot surface in your life as a leaky battery. A car exhaust pipe, a factory chimney, the back of a server rack, or your laptop emits signals indicating that energy has been used and then allowed to drift away as heat. Energy harvesting technologies aim to capture some of that drifting energy and convert it back into electricity, much like placing small water wheels in warm rivers of air or electrons so that they spin and generate power again. In principle, this can reduce waste, cut cooling needs, and power small devices directly from their surroundings.
Classical energy harvesters, such as thermoelectric generators, operate according to classical thermodynamics. In that framework, a heat source is usually treated as being in thermal equilibrium, meaning that temperature is uniform and the microscopic motion of particles has settled into a smooth, featureless distribution. This is like a bathtub of water that has been left long enough for every part to reach the same lukewarm temperature. The problem is that the closer a system gets to this calm, uniform state, the less “usable” energy remains. The temperature difference that drives a heat engine shrinks, and so does the maximum possible efficiency, which is bounded by the Carnot limit. (The Carnot limit is the simple idea that no heat engine can turn all its heat into useful work because some of that heat must always be lost as the engine operates, providing a built-in ceiling on efficiency set by the temperatures of the hot and cold sides.)
This is where non-thermal states enter the story. A non-thermal state is one that has not relaxed into that smooth, bathtub-like equilibrium. Instead, it has structure. Some particles are much hotter than others, or there are distinct groups of energies that have not blended. A helpful analogy is a crowd at a concert. In thermal equilibrium, everyone would be milling around randomly with similar levels of excitement. In a non-thermal state, you might have a cluster of people still jumping and shouting near the stage, while others are already sitting quietly at the back. That unevenness can be exploited. If you can couple your “energy wheel” mostly to the excited group, you can extract more useful work than if everyone had calmed down.
Physicists have learned to create such non-thermal states in many ways, for example, by using lasers to cool or excite atoms, or by preparing coherent ensembles in which many atoms move in lockstep, like a choreographed dance. These methods are powerful but often require exquisite control and bulky apparatus, which makes them difficult to deploy in practical energy-recovery systems. What is needed is a system that naturally resists thermalization, that is, one that tends to stay non-thermal even when left to itself.
Tomonaga-Luttinger (TL) liquids are a leading candidate. A TL liquid is not a liquid in the everyday sense, but a special state of electrons confined so that they move in a very narrow, effectively one-dimensional channel. In such a channel, electrons cannot easily pass each other, so their motion becomes highly correlated. Instead of behaving like independent billiard balls, they move more like a tightly packed queue where a push at one end sends a collective ripple through the line. In the quantum Hall regime, these channels appear as edge states that carry current along the boundary of a two-dimensional electron gas in a strong magnetic field. Theory and prior experiments have shown that energy injected into such a TL liquid does not quickly smear out into a simple thermal distribution. Instead, it can settle into long-lived non-thermal configurations that are remarkably robust.
The Fujisawa team developed a compact device that leverages this property as the core of an energy harvester. On a tiny chip, they patterned a quantum point contact (QPC), which acts like a controllable constriction for electrons, and a quantum dot (QD), which acts as a very sharp energy filter. The QPC plays the role of an “active device” that generates waste heat when a voltage is applied across it. That heat is carried away along quantum Hall edge channels that form a TL liquid. Some distance downstream, the QD couples to one of these channels and selectively allows electrons of specific energies to tunnel through, converting their energy difference into an electrical voltage and current, analogous to a miniature heat engine.
Although the TechXplore article refers to a “setup for energy harvesting,” the structure functions much like a carefully engineered surface for electrons, sculpting where they can go and at what energies. The gates on the chip define the paths, the QPC injects hot carriers, and the QD acts as a tunable energy window. Together, they form a metastructure for electronic heat flow, even if it is not a meta-surface in the optical sense.
The key experimental trick was to prepare two distinct states in the TL liquid using the same total amount of heat. By tuning the QPC transmission, the researchers could create a strongly non-thermal state (NT) with a highly non-uniform energy distribution, or a quasi-thermal state (QT) that more closely resembled an ordinary hot Fermi gas. They then measured the voltage and power that the QD heat engine could generate in each case, and the efficiency with which it converted heat into electrical work.
The results were significant. For the same injected heat, the non-thermal TL state produced an electromotive force, essentially the maximum usable voltage, that was about two to three times larger than that of the quasi-thermal state. When they analyzed the data using an idealized single-level engine, they found that the effective conversion efficiency in the NT case exceeded that in the QT case, even surpassing the usual thermodynamic benchmarks for thermal reservoirs in the classical picture. In plain terms, the messy, structured energy distribution of the non-thermal state gave the engine more “high ground” to tap, because a significant population of high-energy electrons remained available instead of being washed out into a uniform temperature.
To understand this, the team modeled the NT state with a “binary” Fermi distribution. Rather than a single smooth curve of occupation versus energy, the distribution looked like a weighted mixture of a hot component and a cold component that coexist while preserving the overall entropy. Returning to the concert analogy, it is as if the crowd splits into a highly excited group and a calmer group, yet the overall disorder in the room remains the same. The presence of that hot subpopulation makes it easier for an energy filter like the QD to capture energetic electrons and convert their excess energy into voltage.
The researchers recommend two broad directions. First, refine the design of the energy filter, for example, by tailoring quantum dots or similar structures so that they more selectively harvest the high-energy side of the non-thermal distribution. Second, extend the concept to other systems that naturally resist thermalization, such as different TL platforms or other integrable quantum systems, and eventually to material platforms that could operate at higher temperatures and in more conventional environments.
If these ideas can be translated from cryogenic quantum Hall devices to more practical materials, the implications for super-efficient computation and materials are significant. Modern data centers and high-performance chips already struggle with heat management. Embedding quantum-inspired energy harvesters that operate on non-thermal electronic states could, in principle, enable the recycling of heat on chips, reducing cooling loads and improving overall energy budgets. At larger scales, industrial processes that generate strongly non-equilibrium electronic or vibrational excitations might be paired with tailored “quantum heat engines” that exploit those non-thermal features rather than waiting for them to decay into thermal waste.
In the near term, the real-world impact will be conceptual as much as technological. This work provides the first clear experimental evidence that naturally emerging non-thermal states in a TL liquid can outperform near-equilibrium states for thermoelectric conversion, validating a long-standing intuition in quantum thermodynamics and giving device designers a concrete target to aim for. The next steps will involve pushing these ideas to higher powers, higher temperatures, and more scalable platforms, while engineering better energy filters that can fully exploit the structured energy landscape of non-thermal carriers.
The broader message is simple and quietly provocative. Instead of treating disorderly, out-of-equilibrium quantum states as a nuisance on the way to thermal calm, it may be wiser to see them as a resource. If that perspective holds, the warm back of a laptop is not just a symptom of inefficiency, but a hint that there is still useful structure in the flow of energy, waiting for the right kind of engine to tap it.
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[1] https://six3ro.substack.com/p/turning-wasted-laptop-heat-into-quantum
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