For over a century, the Maxwell’s Demon paradox has haunted physics. This thought experiment suggests that a tiny, energy-free “demon” could defy the second law of thermodynamics. Researchers put this to the test in a quantum setting, revealing that quantum mechanics allows surprising loopholes—though ultimately, thermodynamic balance always wins.
Researchers from Nagoya University in Japan and the Slovak Academy of Sciences have made a significant discovery about the relationship between quantum theory and thermodynamics. Their study, published in npj Quantum Information, reveals that while quantum theory does not strictly forbid violations of the second law of thermodynamics, quantum processes can still operate without actually breaking the law. This finding demonstrates that quantum mechanics and thermodynamics, despite being logically independent, can coexist in a complementary way.
Their research provides new insights into the thermodynamic limits of quantum technologies, including quantum computing and ” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>nanoscale engines. It also contributes to the ongoing study of the second law of thermodynamics—one of the most fundamental and intriguing principles in physics. This law states that entropy, a measure of disorder in a system, never decreases on its own. It also establishes that a cyclic engine cannot generate mechanical work solely by extracting heat from a single thermal reservoir and reinforces the idea of time’s one-way direction.
Maxwell’s Demon: A Thought Experiment That Won’t Die
Despite its foundational role, the second law remains one of the most debated and misunderstood principles in science. Central to this debate is the paradox of “Maxwell’s Demon,” a thought experiment proposed by physicist James Clerk Maxwell in 1867.
Maxwell envisioned a hypothetical being—the demon—capable of sorting fast and slow molecules within a gas at thermal equilibrium without expending energy. By separating these molecules into distinct regions, the demon could create a temperature difference. As the system returns to equilibrium, mechanical work is extracted, seemingly defying the second law of thermodynamics.
The paradox has intrigued physicists for over a century, raising questions about the law’s universality and whether it depends on the observer’s knowledge and capabilities. Solutions to the paradox have largely centered on treating the demon as a physical system subject to thermodynamic laws. A proposed solution is erasing the demon’s memory, which would require an expenditure of mechanical work, effectively offsetting the violation of the second law.
The Demonic Engine: Testing the Limits of Thermodynamics
To explore this phenomenon further, the researchers developed a mathematical model for a “demonic engine,” a system powered by Maxwell’s demon. Their approach is rooted in the theory of quantum instruments, a framework introduced in the 1970s and 1980s to describe the most general forms of quantum measurement.
The model involves three steps: the demon measures a target system, then extracts work from it by coupling it to a thermal environment, and finally erases its memory by interacting with the same environment.
Unexpected Results: Can the Second Law Be Broken?
Using this framework, the team derived precise equations for the work expended by the demon and the work it extracts, expressed in terms of quantum information measures such as von Neumann entropy and Groenewold-Ozawa information gain. When comparing these equations, they got a surprising result.
“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,” explained Shintaro Minagawa, a lead researcher on the project. “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.”
Quantum Theory’s Hidden Loopholes—But No Real Violations
Despite these loopholes, the researchers emphasize that they don’t pose a threat to the second law. “Our work demonstrates that, despite these theoretical vulnerabilities, it is possible to design any quantum process so that it complies with the second law,” said Hamed Mohammady. “In other words, quantum theory could potentially break the second law of thermodynamics, but it doesn’t actually have to. This establishes a remarkable harmony between quantum mechanics and thermodynamics: they remain independent but never fundamentally at odds.”
This discovery also suggests that the second law does not impose strict limitations on quantum measurements. Any process permitted by quantum theory can be implemented without violating thermodynamic principles. By refining our understanding of this interplay, the researchers aim to unlock new possibilities for quantum technologies while upholding the timeless principles of thermodynamics.
Quantum Technologies and the Future of Thermodynamics
“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,” Francesco Buscemi explained. “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.”
he implications of this study extend beyond theoretical physics. Illuminating the thermodynamic limits of quantum systems provides a foundation for innovations in quantum computing and nanoscale engines. As we explore the quantum realm, this research serves as a reminder of the delicate balance between the fundamental laws of nature and the potential for groundbreaking technological advancements.
Reference: “Universal validity of the second law of information thermodynamics” by Shintaro Minagawa, M. Hamed Mohammady, Kenta Sakai, Kohtaro Kato and Francesco Buscemi, 7 February 2025, npj Quantum Information.
DOI: 10.1038/s41534-024-00922-w
