Keeping the entropy of measurement: Szilard revisited

Szilard Revisited

Michael Devereux

Harvey S. Leff, Andrew F. Rex

It is widely believed that measurement is accompanied by irreversible entropy increase. This conventional wisdom is based in part on Szilard’s 1929 study of entropy decrease in a thermodynamic system by intelligent intervention (i.e., a Maxwell’s demon) and Brillouin’s association of entropy with information. Bennett subsequently argued that information acquisition is not necessarily irreversible, but information erasure must be dissipative (Landauer’s principle). Inspired by the ensuing debate, we revisit the membrane model introduced by Szilard and find that it can illustrate and clarify (1) reversible measurement, (2) information storage, (3) decoupling of the memory from the system being measured, and (4) entropy increase associated with memory erasure and resetting.

Heinz–Jürgen Schmidt, Thomas Bröcker

Abstract In order to reconcile the entropy reduction of a system through external interventions that are linked to a measurement with the second law of thermodynamics, there are two main proposals: (i) The entropy reduction is compensated by the entropy increase as a result of the measurement on the system (“Szilard principle"). (ii) The entropy reduction is compensated by the entropy increase as a result of the erasure of the measurement results (“Landauer/Bennett principle"). It seems that the LB principle is widely accepted in the scientific debate. In contrast, in this paper we argue for a modified S principle and criticize the LB principle with regard to various points. Our approach is based on the concept of “conditional action", which is developed in detail. To illustrate our theses, we consider the entropy balance of a variant of the well-known Szilard engine, understood as a classical mechanical system.

Göran Lindblad

Michael Devereux

Using an isolated measurement process, we calculate the effect measurement has on entropy for the multi‐cylinder Szilard engine. We find that the system of cylinders possesses an entropy associated with cylinder total energy states, and that it records information transferred at measurement. Contrary to other’s results, we find that the apparatus loses entropy due to measurement. The Second Law of Thermodynamics may be preserved if Maxwell’s demon gains entropy moving the engine partition.