Physicists read Maxwell’s Demon’s mind

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 thought experiment 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 energy 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 information 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.