Contextuality versus Incompatibility

Abstract

Our aim is to compare the fundamental notions of quantum physics - contextuality vs. incompatibility. One has to distinguish two different notions of contextuality, {\it Bohr-contextuality} and {\it Bell-contextuality}. The latter is defined operationally via violation of noncontextuality (Bell type) inequalities. This sort of contextuality will be compared with incompatibility. It is easy to show that, for quantum observables, there is {\it no contextuality without incompatibility.} The natural question arises: What is contextuality without incompatibility? (What is "dry-residue"?) Generally this is the very complex question. We concentrated on contextuality for four quantum observables. We shown that in the CHSH-scenarios (for "natural quantum observables") {\it contextuality is reduced to incompatibility.} However, generally contextuality without incompatibility may have some physical content. We found a mathematical constraint extracting the contextuality component from incompatibility. However, the physical meaning of this constraint is not clear. In appendix 1, we briefly discuss another sort of contextuality based on the Bohr's complementarity principle which is treated as the {\it contextuality-incompatibility principle}. Bohr-contextuality plays the crucial role in quantum foundations. Incompatibility is, in fact, a consequence of Bohr-contextuality. Finally, we remark that outside of physics, e.g., in cognitive psychology and decision making Bell-contextuality cleaned of incompatibility can play the important role.