Observing a Quantum Maxwell Demon at Work

Janet Anders
University of Exeter, U.K.

Co-authors: N. Cottet,  S. Jezouin,  L. Bretheau,  P. Campagne-Ibarcq, Q. Ficheux,  A. Auffeves,  R. Azouit,  P. Rouchon, and B. Huard 

In apparent contradiction to the laws of thermodynamics, Maxwell’s demon is able to cyclically extract work from a system in contact with a thermal bath exploiting the information about its microstate. The resolution of this paradox required the insight that an intimate relationship exists between information and thermodynamics. We report on a realisation of a Maxwell demon that tracks the state of each constituent both in the classical and quantum regimes [1]. The demon is a microwave cavity that encodes quantum information about a superconducting qubit and converts information into work by powering up a propagating microwave pulse with stimulated emission. Thanks to the high level of control of superconducting circuits, we directly measure the extracted work and quantify the entropy remaining in the demon’s memory. This experiment provides an enlightening illustration of the interplay of thermodynamics with quantum information. 

[1] N. Cottet et al., PNAS 114 7561 (2017).