Proc. Natl Acad. Sci. USA (in the press); preprint at http://arxiv.org/abs/1206.5553 (2012)

When Lord Kelvin weighed in on James Clerk Maxwell's formulation of a demon capable of defying the second law of thermodynamics, he did so by conjuring up “an intelligent being endowed with free will”. Although somewhat whimsical, this interpretation remains popular today: Maxwell's demon is often cast as an imaginary creature dreamed up to facilitate a thought experiment. A more modern take on the idea involves a purely mechanical demon that can generate work by manipulating fluctuations without any external influence. Dibyendu Mandal and Christopher Jarzynski have devised such a machine — the first of its kind, designed to operate as a Maxwell demon by amassing information that can later be erased to settle the system's thermodynamic account.

The proposed device makes thermally activated transitions between three states, interacting with a memory register that enables directed rotation through these states and allows the system to convert heat into the work required to raise a mass. Mandal and Jarzynski have solved the steady-state behaviour of a minimal model for their device, constructing a phase diagram that suggests the system can behave as either engine or eraser. The study offers a clever blueprint for a Maxwell demon — and a tangible system on which to base future discussion.