Data processing makes POVMs coarser and observational entropies larger

Abstract

We find a criterion to compare POVM measurements and decide which ones can extract more information from physical systems, with coarser POVMs always extracting less information. This criteria generalizes the previous definition of coarser POVM, and is motivated by the idea that information cannot be gained by processing the measurement outcomes. The information that a measurement cannot extract is quantified by observational entropy or coarse-grained entropy. Adequately, coarser measurements have larger observational entropies. Moreover, the characterization and properties of coarser measurements that we provide allow to straightforwardly derive several previously known results about observational entropy.