a team of european researchers have created a quantum mechanical version of maxwell’s demon, a thought experiment devised by physicist james clerk maxwell in 1867 about the second law of thermodynamics, according to a report on science news.

researchers have created a quantum mechanical version of
maxwell's demon. (science news)

the demon, according to maxwell, could break the second law of thermodynamics, which states that the total entropy of an isolated system can only increase over time. in thermal terms, it means that it is impossible to convert heat energy to mechanical energy with 100 percent efficiency.

the article explained, “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.”

further research indicated another problem with the demon - in order to manipulate molecules, the demon must also record information about them and that has a physical component. storing the information increases the entropy of the demon, which counteracts the entropy decrease it caused.

“as the demon extracts energy, it must delete the contents of its memory in order to store new information and manipulate other molecules,” the article continued. “that deletion, physicist rolf landauer determined in 1961, costs energy and releases entropy, with the result that the demon’s energy harvest is negated.”

the researchers performed an experiment to probe the quantum state of the demon’s memory.

the article explained how the experiment was performed. it said:

“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.”

when the system is in a high-energy state, the demon allows it to drop to a lower-energy state, which releases a photon that the demon collects, but it does not allow the low-energy state to absorb photons, which takes energy from the circuit.

“the researchers probed this memory through a process called quantum tomography,” the article added, “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.”

by demonstrating how the rules of thermodynamics translate to the quantum scale, could lead to more efficient designs for quantum machines.

the research was published in the proceedings of the national academy of sciences. the abstract stated:

“in apparent contradiction to the laws of thermodynamics, maxwell’s demon is able to cyclically extract work from a system in contact with a thermal bath, exploiting the information about its microstate. the resolution of this paradox required the insight that an intimate relationship exists between information and thermodynamics.

“here, we realize a maxwell demon experiment that tracks the state of each constituent in both the classical and quantum regimes. the demon is a microwave cavity that encodes quantum information about a superconducting qubit and converts information into work by powering up a propagating microwave pulse by stimulated emission.

“thanks to the high level of control of superconducting circuits, we directly measure the extracted work and quantify the entropy remaining in the demon’s memory. this experiment provides an enlightening illustration of the interplay of thermodynamics with quantum information.”