Uncertainty relations revisited

Abstract

Introductory courses on quantum mechanics usually include lectures on uncertainty relations, typically the inequality derived by Robertson and, perhaps, other statements. For the benefit of the lecturers, we present a unified approach -- well suited for undergraduate teaching -- for deriving all standard uncertainty relations: those for products of variances by Kennard, Robertson, and Schr\"odinger, as well as those for sums of variances by Maccone and Pati. We also give a brief review of the early history of this topic and try to answer why the use of variances for quantifying uncertainty is so widespread, while alternatives are available that can be more natural and more fitting. It is common to regard the states that saturate the Robertson inequality as "minimum uncertainty states" although they do not minimize the variance of one observable, given the variance of another, incompatible observable. The states that achieve this objective are different and can be found systematically.