Broken arrows: Hardy-Unruh chains and quantum contextuality

Abstract

Hardy (1993) and Unruh (2018) constructed a family of non-maximally entangled states of pairs of particles giving rise to correlations that cannot be accounted for with a local hidden-variable theory. Rather than pointing to violations of some Bell inequality, however, they pointed to clashes with the basic rules of logic. Specifically, they constructed these states and the associated measurement settings in such a way that the outcomes will satisfy a set of two or three conditionals, which we call Hardy-Unruh chains, but not a conditional entailed by this set. Quantum mechanics avoids such broken 'if ... then ...' arrows because it cannot simultaneously assign truth values to all conditionals involved. Measurements to determine the truth value of some preclude measurements to determine the truth value of others. Hardy-Unruh chains thus nicely illustrate quantum contextuality: which variables do and do not get definite values depends on what measurements we decide to perform. We use the framework inspired by Bub (2016) and Pitowsky (1989) and developed in Janas, Cuffaro and Janssen (2022} to construct and analyze Hardy-Unruh chains in terms of fictitious bananas mimicking the behavior of spin-1/2 particles.