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Maxwell’s demon is one of the most famous thought experiments in physics. In its traditional formulation, a demon sits next to a small hatch that separates two chambers. It observes the velocity of any gas molecules heading toward the hatch from one room and only opens the hatch when the velocity exceeds a certain value. Over time, the demon will raise the temperature of one room while cooling the second—something we know is thermodynamically impossible.

Over time, the demon’s domain has been expanded, as researchers realized the same issue applied to a variety of other problems. One reformulation came from physicist Leo Szilard, who noted you can have a demon-based engine. Now, 90 years later, researchers have built a Szilard engine that operates using a single electron. In the process, the researchers confirm that setting the digital bit of information describing the engine’s state has an energetic cost.

In its original formulation, the Szilard engine was a chamber with pistons at either end and a single gas molecule in the middle. Slide a divider down in the middle, and the gas molecule will wind up on one side or the other. This will push one of the pistons out, providing the potential for doing some work for « free » without the input of energy. (This being a thought experiment, the pistons are assumed to move without friction.) You can then remove the divider, let the chamber re-equilibrate, and do it all over again.

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