Molecular information ratchet

This series of image illustrates how the ratchet mechanism in the molecule "rotaxane" is analogous to "Maxwell's demon", a thought experiment devised by one of the esteemed forefathers of statistical thermodynamics. In (a), energy from a photon (hν) excites the ring part of rotaxane (represented by a particle in the left chamber). The ring is in such a position to transmit its acquired energy to the gate (represented by both the demon and the gate) so that the gate opens to let the ring pass (b). Once the ring is on the other side of the rotaxane molecule (c), it is then too far away to be able to transmit energy back to the gate (d), so instead it dissipates the energy to the environment.

According to David Leigh from the University of Edinburgh, (d) is crucial to understanding why Maxwell's demon does not provide a tenable argument for a violation of the second law of thermodynamics, because it corresponds to the demon "erasing" information about the particle's position and hence making the process irreversible – a flaw that IBM researcher Charles Bennett spotted in 1982. (Credit: David Leigh.)

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