Memory tests uphold the second law of thermodynamics
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 Proceedings of the National Academy of Sciences, a team of researchers 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.
“The state of the memory is very important,” says physicist Juan Parrondo of Complutense University of Madrid, because it is what confirms that the second law still holds. “This is the first experiment which really addresses this question,” says Parrondo, who was not involved with the research.
In the experiment, performed by physicist Benjamin Huard and colleagues, the demon extracts energy from the system, a tiny circuit made of superconducting metal, which can carry electricity without resistance. Light tuned to a particular frequency causes the system to jump from a low-energy to high-energy state, or vice versa, absorbing or emitting a photon, or particle of light, in the process. The demon — a superconducting cavity within which microwaves bounce back and forth — manipulates the system to ensure that energy can be drained from the system, but not absorbed, allowing the demon to capture the energy released.
If the system is in the high-energy state, the demon allows the system to drop to lower energy, in the process spitting out a photon, which the demon can harvest for energy. But if the system is in the low-energy state, the demon prevents it from absorbing photons. The net result: Energy is sapped from the circuit. But, says Huard, of École Normale Supérieure de Lyon in France, “the information the demon learns about the system is encoded into its memory.”
The researchers probed this memory through a process called quantum tomography — meaning that they repeated the experiment many times and cataloged the state of the memory. The results revealed that, as expected, the demon retained the information about what energy state the system was in.
In addition to reading the demon’s mind, the researchers measured the work extracted from the system in a more direct manner than previous experiments. “This is one new feature of this implementation,” says Roberto Serra of Federal University of ABC in Santo André, Sao Paulo, Brazil. “This work is a very nice experiment.”
In Huard’s scheme — in contrast to some previous experiments — the demon and the system both operate on a quantum level. While the rules of thermodynamics were originally understood only for large systems like steam engines, scientists now hope understanding how the rules translate to small scales could one day lead to designs for more efficient quantum machines (SN: 3/19/16, p. 18).
N. Cottet et al. Observing a quantum Maxwell demon at work. Proceedings of the National Academy of Sciences. Published online July 3, 2017. doi: 10.1073/pnas.1704827114.
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A. Grant. Ultrasmall engines bend second law of thermodynamics. Science News. Vol. 189, March 19, 2016, p. 18.
A. Grant. Maxwell’s demon faces the heat. Science News Online, December 1, 2015.