Credit: (a) Shutterstock.com/Scott Maxwell/LuMaxArt; S. W. Kim et al., [2]; (b) S. W. Kim et al., [2]

Figure 1: (a) The Maxwell demon purportedly violates the second law of thermodynamics by directing hot molecules in a gas to perform work without a heat gradient. The classical Szilard engine, made up of a single molecule coupled to a thermal bath, illustrates how the second law survives because the demon must generate a compensating entropy. (b) Steps in the operation of a quantum Szilard engine. (I) Three molecules are prepared in a closed box with size $L$. (II) A wall, depicted by a vertical bar, is isothermally inserted at location $l$. (III) Information on the number of molecules $m$ on the left is acquired by quantum “measurement.” (IV) The wall shifts isothermally to reach its equilibrium location $lm$. The cycle is complete when the wall is removed isothermally to reach step (I).