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.
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