Figure 21.2 — Experimental realisation of Maxwell’s demon Figure 21.2 — Experimental realisation of Maxwell’s demon. (a) A microscopic particle is held on a spiral-staircase-like potential with a step height comparable to the thermal energy kBT. Because of the presence of thermal fluctuations, the particle stochastically jumps between steps and, as the downward jumps along the gradient are more frequent than the upward ones, the particle on the average falls down the stairs. (b) When the feedback control is switched on, a block is placed behind the particle to prevent downward jumps whenever an upward jump has been observed. By repeating this cycle, the particle is expected to climb up the stairs without direct energy injection, using only the information gathered from the observation of its position.Reprinted by permission from Macmillan Publishers Ltd: Toyabe et al., Nature Phys. 6, 988–92, copyright 2010.