In the 19th century, James Clerk Maxwell imagined a tiny demon. It knows the speed of every gas molecule and opens a door: fast ones to the left, slow ones to the right. This creates a temperature difference without expending energy, seemingly violating the laws of physics. But the puzzle's solution lies in information: to remember the speeds and decide, the demon needs memory. Erasing these records releases heat, and the overall disorder (entropy) still increases.
A modern experiment recreated the demon with light. Instead of gas — photons, flying at the speed of light and obeying the statistics of Bose–Einstein (part of the Standard Model). Detectors measured their number in different channels, then the device swapped the routes: wherever there were many particles, it sent even more. Quantum law: indistinguishable particles tend to bunch together. So the temperature difference between channels turned out more pronounced than for distinguishable photons. This "quantum herd instinct" acts like fuel for the demon.
These experiments link heat, information, and quantum physics. They're needed for testing future quantum circuits that use light. And the most surprising: the price of erasing memory, releasing heat, inevitably restores disorder to the world. Even a quantum demon can't cheat nature.
In the experiment, the temperature difference was only a few millionths of a degree, but it was detected by ultrasensitive electronics.
The idea of a demon controlling heat with information has inspired science fiction writers. For example, in 'A Fire Upon the Deep' by Vernor Vinge, superintelligent entities manipulate an invisible measure of chaos on a galactic scale.