Physicists read Maxwell's Demon's mind

Credit: CC0 Public Domain

Pioneering research offers a fascinating view into the inner workings of the mind of 'Maxwell's Demon', a famous thought experiment in physics.

An international research team, including Dr Janet Anders from the University of Exeter, have used superconducting circuits to bring the 'demon' to life.

The demon, first proposed by James Clerk Maxwell in 1867, is a hypothetical being that can gain more useful energy from a thermodynamic system than one of the most fundamental laws of physics—the second law of thermodynamics—should allow.

Crucially, the team not only directly observed the gained energy for the first time, they also tracked how information gets stored in the demon's memory.

The research is published in the leading scientific journal Proceedings of the National Academy of Sciences (PNAS).

The original was first proposed by mathematical physicist James Clerk Maxwell—one of the most influential scientists in history—150 years ago.

He hypothesised that gas particles in two adjacent boxes could be filtered by a 'demon' operating a tiny door, that allowed only fast energy particles to pass in one direction and low energy particles the opposite way.

As a result, one box gains a higher average energy than the other, which creates a pressure difference. This non-equilibrium situation can be used to gain energy, not unlike the energy obtained when water stored behind a dam is released.

So although the gas was initially in equilibrium, the demon can create a non-equilibrium situation and extract energy, bypassing the second law of thermodynamics.

Dr Anders, a leading theoretical physicist from the University of Exeter's physics department adds: "In the 1980s it was discovered that this is not the full story. The information about the particles' properties remains stored in the memory of the demon. This information leads to an energetic cost which then reduces the demon's energy gain to null, resolving the paradox."

In this research, the team created a quantum Maxwell demon, manifested as a microwave cavity, that draws energy from a superconducting qubit. The team was able to fully map out the memory of the demon after its intervention, unveiling the stored information about the qubit state.

Dr Anders adds: "The fact that the system behaves quantum mechanically means that the particle can have a high and low at the same time, not only either of these choices as considered by Maxwell."

This ground-breaking experiment gives a fascinating peek into the interplay between quantum and thermodynamics, and is an important step in the current development of a theory for nanoscale thermodynamic processes.

'Observing a Quantum Maxwell demon at Work' is published in PNAS.

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More information: Nathanaël Cottet et al. Observing a quantum Maxwell demon at work, Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1704827114
Citation: Physicists read Maxwell's Demon's mind (2017, July 5) retrieved 18 June 2019 from
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Jul 05, 2017
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Jul 05, 2017
The idea that the demon must have a memory for events is flawed: there is absolutely no reason whatsoever that the demon must have a memory beyond a fixed memory of the rules required to evaluate energies and open the door.

Whenever there are particles approaching the door from different sides the demon must evaluate the energy and decide if the energy of the particle in the high energy side is lower than the energy of the particle approaching from the low energy side. If so, the door is opened and the particles are allowed to pass through. If not the demon resets and waits for the next suitable interaction. No storage of information beyond that one event is required. Each event entirely fills the demon's buffer and his only long term memory is the fixed rules (ROM memory) which never changes or updates.

Jul 07, 2017
I don't agree with the second law, an undisturbed system does not spontaneously go into any direction of stability. It is not a random event. The system of only charge, will seek a stable state depending upon the dynamics; or an unstable state with a different set of dynamics. This is a conjecture, not a law. Typically, with only stable charge pairs, the system will self assemble.

Well, with fast and slow, set the gate threshold!

Jul 07, 2017
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Jul 07, 2017
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Jul 08, 2017
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Jul 10, 2017
Anyway, how can we make law? It is what it is, a bunch of like charges will always fly apart, a group of like charges will tend to aggregate. Mix it up, with different speeds, etc. it will eventually reach an equilibrium. So WTF is the second law?

Jul 12, 2017
Maxwell would make a long sigh had he known that so many "research papers" have been published to "resolve the paradox".

Jul 12, 2017
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Jul 12, 2017
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