Physics and Metaphysics of Wigner's Friends: Even performed pre-measurements have no results

Abstract

"The unambiguous account of proper quantum phenomena must, in principle, include a description of all relevant features of experimental arrangement" (Bohr). The measurement process is composed of pre-measurement (quantum correlation of the system with the pointer variable), and an irreversible decoherence via interaction with an environment. The system ends up in a probabilistic mixture of the eigenstates of the measured observable. For the pre-measurement stage, any attempt to introduce an 'outcome' leads, as we show, to a logical contradiction, $1=i$. This nullifies claims that a modified concept of Wigner's Friend, who just pre-measures, can lead to valid results concerning quantum theory.