Compatible and incompatible observables in the paradigmatic multislit experiments of quantum mechanics

The present investigation is about the quantum mechanics of multislit interference experiments. One of the cornerstones of our understanding of quantum mechanics is provided by the analysis of this type of experiments. Beginning with Bohr and Einstein, the discussion revolved mostly around single-slit diffraction and double-slit interference, probing the complementarity of path determination and the appearance of interference fringes, and contrasting the familiar with the quantum. Here, we are concerned with developing a systematic understanding and description of multislit interference experiments, i.e. setups in which a plurality of slits is illuminated. We provide a characterisation of a number of relevant observables, discussing those that are compatible and may be measured jointly, and also incompatible observables which cannot be measured jointly but instead display quantum uncertainty.
We begin with a discussion of a particular modification of the classic double- slit interference experiment which highlights the realisation of specific position and momentum observables which are jointly measurable. Although there are technical results regarding the coexistence of specific position and momentum observables, it may be surprising that ubiquitous experimental setups provide a preparation of such quantum states. We proceed with a discussion of the particular character of the incompatibility of certain measurements by building on an initial heuristic argument provided by Aharonov, Pendleton and Petersen. We prove, extend and discuss a formulation of un- certainty suitable for the context of multislit experiments. We conclude with a comparison of this formulation of uncertainty with an alternative uncertainty formulation developed by Uffink and Hilgevoord. Although these two uncertainty formulations are very different technically, we demonstrate that the same tradeoff is expressed independently.