Proc. Natl Acad. Sci. USA 114, 7561–7564 (2017)

Can quantum technology help peek into somebody's mind? This may sound like the stuff of science fiction, but something of the sort is possible when the subject of the exercise is Maxwell's demon, as Nathanaël Cotteta and colleagues demonstrate.

Since James Clerk Maxwell formulated his famed thought experiment challenging the second law of thermodynamics 150 years ago — in a letter to Peter Tait dated 11 December 1867 — several experimental implementations have been realized. That of Cotteta et al., however, stands out in that they can control and measure at the quantum level all relevant thermodynamic quantities involved and directly track the work extracted by the 'demon'.

In the experiment, a microwave cavity takes the role of the demon. That cavity controls the quantum state of a superconducting qubit coupled to it, from which it can extract work. The precise amount depends on the information stored in the cavity, or on the state of the demon's memory. Cotteta and co-workers determined the full density matrix of that memory, providing direct insight into its role in the demonic process.