Researchers from the Nagoya University and Slovak Academy of Sciences have made a ground-breaking discovery that sheds new light on how quantum theory and thermodynamics interact. The group showed that although quantum theory can in principle go against the second law of thermodynamics, quantum processes can be used without breaking it. The study was published in npj Quantum Information.
Despite the two fields' logical independence, this discovery highlights their peaceful coexistence. Their research provides new insights into the thermodynamic limits of quantum technologies, including nanoscale engines and quantum computing.
This discovery advances the long-standing investigation of the second law of thermodynamics, which is frequently thought to be one of the most profound and mysterious physics concepts.
According to the second law, entropy, which is a measure of a system's disorder, never naturally falls. It also emphasizes the idea of a unidirectional flow of time and asserts that a cyclically operating engine cannot generate mechanical work by drawing heat from a single thermal environment.
The second law is still one of the most contested and misinterpreted scientific concepts, despite its fundamental role. The paradox of “Maxwell's Demon,” a thought experiment put forth by physicist James Clerk Maxwell in 1867, is at the heart of this discussion.
Maxwell imagined a fictitious entity, the demon, that could distinguish between fast and slow molecules in a gas at thermal equilibrium without consuming energy.
The demon might produce a temperature differential by dividing these molecules into different areas. In apparent violation of the second law of thermodynamics, mechanical work is extracted as the system reaches equilibrium.
For more than a century, physicists have been fascinated by the paradox, which asks whether the law is universal and whether it is dependent on the skills and knowledge of the observer. Treating the demon as a physical system governed by thermodynamic laws has been the main strategy used to resolve the paradox.
One proposed solution is to erase the demon's memory, but this would require mechanical work and effectively offset the violation of the second law.
The researchers created a mathematical model for a system driven by Maxwell's demon, dubbed a “demonic engine,” to investigate this phenomenon in greater detail. The theory of quantum instruments, which was developed in the 1970s and 1980s to characterize the broadest types of quantum measurement, serves as the foundation for their methodology.
Three steps make up the model: the demon measures a target system, then couples it to a thermal environment to extract work from it, and finally, it interacts with the same environment to erase its memory.
In terms of quantum information measures like von Neumann entropy and Groenewold-Ozawa information gain, the team used this framework to derive exact equations for the work that the demon expends and the work that it extracts. They discovered an unexpected outcome when they compared these equations.
Our results showed that under certain conditions permitted by quantum theory, even after accounting for all costs, the work extracted can exceed the work expended, seemingly violating the second law of thermodynamics. This revelation was as exciting as it was unexpected, challenging the assumption that quantum theory is inherently ‘demon-proof.’ There are hidden corners in the framework where Maxwell’s Demon could still work its magic.
Shintaro Minagawa, Lead Researcher, Nagoya University
Notwithstanding these shortcomings, the researchers stress that the second law is unaffected by them.
Our work demonstrates that, despite these theoretical vulnerabilities, it is possible to design any quantum process so that it complies with the second law. In other words, quantum theory could potentially break the second law of thermodynamics, but it does not actually have to. This establishes a remarkable harmony between quantum mechanics and thermodynamics: they remain independent but never fundamentally at odds.
Hamed Mohammady, Institute of Physics, Slovak Academy of Sciences
This finding also implies that quantum measurements are not strictly constrained by the second law. Any procedure that is allowed by quantum theory can be carried out without going against the laws of thermodynamics.
The researchers hope to advance the knowledge of this interaction and preserve the laws of thermodynamics while opening up new avenues for quantum technologies.
One thing we show in this paper is that quantum theory is really logically independent of the second law of thermodynamics. That is, it can violate the law simply because it does not ‘know’ about it at all. And yet and this is just as remarkable any quantum process can be realized without violating the second law of thermodynamics. This can be done by adding more systems until the thermodynamic balance is restored.
Francesco Buscemi, Graduate School of Informatics, Nagoya University
This study has ramifications that go beyond theoretical physics. Understanding the thermodynamic bounds of quantum systems lays the groundwork for advancements in nanoscale engines and quantum computing.
This research serves as a reminder of the delicate balance between the potential for revolutionary technological advancements and the fundamental laws of nature.
Journal Reference:
Minagawa, S., et al. (2025) Universal validity of the second law of information thermodynamics. Npj Quantum Information. doi.org/10.1038/s41534-024-00922-w.