New research led by Paris Diderot University offers a view into the inner workings of the ‘mind’ of Maxwell’s demon, a famous thought experiment in physics.
Sketch of the quantum Maxwell demon experiment: (A) after preparation (step 1) in a thermal or quantum state by a pulse at frequency fs, the system S (superconducting qubit) state is recorded (step 2) into the demon’s quantum memory D (microwave cavity); a pulse incoming toward port a at fD populates the cavity mode with a state ραin only if the qubit is in the ground state |g>s; this information is used to extract work W (step 3) which charges a battery B (a microwave pulse at frequency fs on port b) with one extra photon; importantly, the system emits this photon only when the demon’s cavity is empty; the work is determined by amplifying and measuring the average output power at fs on bout; the memory reset (step 4) is performed by cavity relaxation. (B) when the system starts in a quantum superposition of |g>s and |e>s, the demon and system are entangled after step 2. Image credit: Cottet et al, doi: 10.1073/pnas.1704827114.
The Maxwell’s demon 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.
The thought experiment first appeared in a letter British mathematical physicist James Clerk Maxwell wrote to Scottish mathematical physicist Peter Guthrie Tait on December 11, 1867.
Maxwell hypothesized 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.
“In the 1980s it was discovered that this is not the full story,” said co-author Dr. Janet Anders, a theoretical physicist at the University of Exeter, UK.
“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 physicists created a quantum Maxwell demon, manifested as a microwave cavity, that draws energy from a superconducting qubit.
They were able to fully map out the memory of the demon after its intervention, unveiling the stored information about the qubit state.
“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,” Dr. Anders said.
“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.”
The research is published in the Proceedings of the National Academy of Sciences (arXiv.org preprint).
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Nathanaël Cottet et al. Observing a quantum Maxwell demon at work. PNAS, published online July 3, 2017; doi: 10.1073/pnas.1704827114
