Physicists have now probed the memory of Maxwell’s demon, a devious, hypothetical beast. By peeking at information retained by a laboratory version of the creature, scientists confirmed the role of information in saving the second law of thermodynamics from the onslaught of a tiny, superpowerful being intent on wreaking havoc.
In work reported online July 3 in the created a quantum version of Maxwell’s demon in the lab and measured both the information stored in its memory and the energy it extracted from a system. The results directly illustrate that information plays a key role in the demon’s attempts to distill energy.
Since 1867, when the demon was proposed by physicist James Clerk Maxwell, scientists wondered whether such a creature could violate the second law, a sacred tenet of physics. It declares that the entropy, or disorder, of a closed system cannot decrease over time.
Maxwell suggested that a nefarious tiny being could shuttle around molecules to decrease entropy — for example, by putting all the fast-moving molecules on one side of a box containing a gas and the slower ones on the other side. Such an improbable reconfiguration would break the second law, allowing the demon to illegally siphon off energy.
A century later, a solution to this dilemma was found: The demon must record information about the molecules in order to manipulate them, and that information has physical relevance. Storing that information in its “brain” increases the entropy of the demon, compensating for the entropy decrease the demon produces. As the demon extracts energy, it must delete the contents of its memory in order to store new information and manipulate other molecules. That deletion, physicist Rolf Landauer determined in 1961, costs energy and releases entropy, with the result that the demon’s energy harvest is negated.
To show that the demon indeed remembered the properties of the system, the researchers probed the quantum state of the demon’s memory.
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