The second law of thermodynamics underpins all of classical reality. It is the reason why it’s easier to make things messy, why you can’t have perpetual motion, why you age, and maybe even why time only moves in one direction. There have been considerations that the quantum world might escape the constraints of this law. New work reveals that this is indeed possible, but also it doesn’t look like quantum mechanics has to break this crucial law to work.
The example taken by this international team of scientists is Maxwell’s demon, probably the most famous thought experiment on the second law of thermodynamics. Imagine two chambers filled with gas divided by a small door. The demon has keen senses and can detect the energy of the molecules. By opening and closing the door, it divides the cold and hot portions of the gas, creating a system with lower entropy – the disorder of the system – and that would break the second law.
In the classical world, a realistic demon would not break the second law – there’s the mechanical work of the door, the information in the demon’s mind, etc. It all adds up to the total entropy. However, it turns out that it is possible to break it in quantum mechanics. The team developed a “demonic engine” – a system that could measure a target system, extract work by coupling it to a thermal environment, and then erase its memory using that thermal environment.
The team expected that they would find that the engine always needs more energy coming in than what they can get out. This is the cornerstone of the second law. Instead, they were able to find theoretical exceptions.
“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,” lead author Shintaro Minagawa, from Nagoya University, said in a statement. “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.”
You might be wondering why their work and this research are not all about the exciting violation of the second law of thermodynamics or ushering in the age of quantum perpetual motion machines. Firstly, let’s remember that we are still talking about theoretical setups!
The team found that yes, there are options for quantum theories to break from thermodynamics, but also plenty of solutions that instead respect the second law. In particular, if the protocol of the system is compatible with thermodynamic behavior, then no matter the quantum nature, it will obey the law. It is possible to create demonic engines that don’t exactly look like the engines you might expect.
“Our work demonstrates that, despite these theoretical vulnerabilities, it is possible to design any quantum process so that it complies with the second law,” explained co-author 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.”
The work is truly intriguing. Thermodynamics and quantum theory might be independent but interlinked in explaining reality. It is a fascinating area of exploration that could have an impact on both our theoretical understanding of the nature of reality and technologies based on quantum theories.
“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,” added co-author Francesco Buscemi. “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.”
The study is published in the journal npj Quantum Information.
