The (macro)-reality of superpositions

          @misc{ pirsa_PIRSA:15030085,
            doi = {10.48660/15030085},
            url = {https://pirsa.org/15030085},
            author = {Allen, John-Mark},
            keywords = {Quantum Foundations},
            language = {en},
            title = {The (macro)-reality of superpositions},
            publisher = {Perimeter Institute},
            year = {2015},
            month = {mar},
            note = {PIRSA:15030085 see, \url{https://pirsa.org}}
          }
          

Abstract

This talk touches on three questions regarding the ontological status of quantum states using the ontological models

framework: it is assumed that a physical system has some underlying ontic state and that quantum states correspond to probability distributions over these ontic states.

The first question is whether or not quantum states are necessarily real---that is, whether or not the distributions for different quantum states must be disjoint. The PBR theorem proves the reality of quantum states by making assumptions about the ontic structure of bipartite systems, assumptions that have been challenged. Recent work has therefore concentrated on single systems, producing theorems proving the existence of pairs of quantum states whose overlap region on the ontic state space is very small.

The second question is whether the ontology of a quantum system can be macro-realist---that is, can there be "macroscopic" quantities which always have determinate values? The Leggett-Garg inequalities claim to rule out this possibility, but this conclusion has been disputed.

The third question is less familiar: Must quantum superpositions be ontic? That is, for some superposition with respect to some orthonormal basis, must ontic states exist which can be obtained by preparing the superposition, but not by preparing any of the basis states? In other words, can Schrödinger's cat always be either alive xor dead?