What the research is about
Touch the back of a laptop and it often feels warm. This is because part of the energy used for computation and communication escapes to the outside as heat. Yet even this "waste heat" still contains a great deal of usable energy. Technologies that convert such waste heat into electricity for reuse are known as energy harvesting.
Conventional energy-harvesting technologies have been developed within the framework of classical thermodynamics. In classical thermodynamics, a heat source is typically assumed to be in thermal equilibrium-a stable state in which temperature becomes uniform and heat flow is minimal. However, as waste heat approaches thermal equilibrium, the amount of energy that can be reused decreases, and consequently the amount that can be extracted as electricity also diminishes.
For this reason, researchers have focused on non-thermal states, special quantum states that do not settle into thermal equilibrium. Non-thermal states have been realized in various ways-for example, in atoms whose temperature distribution is controlled by lasers, or in coherent atomic ensembles (special states in which many atoms behave in synchrony, following the same rhythm). In many cases, however, creating these non-thermal states requires highly precise control, making practical applications to energy recovery challenging.
In recent years, a promising candidate has attracted increasing attention: the Tomonaga-Luttinger (TL) liquid. A TL liquid refers to a special state in which electrons are confined to a narrow channel and move collectively while strongly influencing one another. Rather than behaving independently, the electrons flow in a coordinated manner reminiscent of a liquid-hence the name. In TL liquids, electronic energy does not readily relax into thermal equilibrium, and non-thermal states can be sustained naturally. This has led to expectations that TL liquids could be useful for energy harvesting, but it had remained unclear whether they are truly advantageous for thermoelectric conversion.
Why this matters
A research team led by Professor Toshimasa Fujisawa of Institute of Science Tokyo (Science Tokyo) has now provided the world's first clear experimental evidence addressing this question. The team fabricated a compact energy-harvesting device that utilizes a naturally occurring non-thermal state (the NT state) in a TL liquid, and compared its ability to convert heat into electricity with that of a state close to thermal equilibrium (the QT state).
The results showed that, when the same amount of heat was supplied, the voltage generated in the NT state was approximately two to three times higher than that in the QT state. The team also confirmed that the conversion efficiency from heat to electricity was consistently higher in the NT state.
The key to understanding why the NT state is advantageous lies in how electronic energy is distributed. Analysis revealed that, in the NT state, electrons exhibit a distribution in which high-energy and low-energy populations coexist while maintaining disorder (entropy). In other words, instead of relaxing uniformly as in thermal equilibrium, a prominent population of high-energy electrons remains-making it easier to extract electrical energy.
What's next
This achievement marks a major advance in technologies that convert waste heat into electricity. Potential applications include large-scale recovery of exhaust heat in factories and data centers, self-powered operation of compact electronic devices, energy-saving technologies in extremely low-temperature environments, and extensions to other quantum systems and integrable systems.
Output power may also be increased by refining the design of energy filters that selectively extract the "high-energy side" of the non-thermal state. More broadly, this concept is expected to be applicable to other quantum systems and to a range of material systems that do not readily relax into thermal equilibrium.
Comment from the researcher
We have conducted experiments based on the belief that naturally emerging 'non-thermal states' in the quantum world can achieve high thermal efficiency, but proving this was more difficult than we expected. We feel that the day is truly approaching when heat that is currently lost all around us will become useful again through the power of quantum effects.
(Toshimasa Fujisawa, Professor, Department of Physics, School of Science, Science Tokyo)
