Retrievability of information in quantum and realistic hidden variable theories

Abstract

We propose a generalisation of the Leggett-Garg conditions for macrorealistic behaviour. Our proposal relies on relaxing the postulate of non-invasive measurability with that of retrievability of information. This leads to a strictly broader class of hidden variable theories than those having a macrorealistic description. Crucially, whereas quantum mechanical tests of macrorealism require one to optimise over all possible state updates, for retrievability of information it suffices to use the basic L\"uders state update, which is present in every quantum measurement. We show that in qubit systems the optimal retrieving protocols further relate to the fundamental precision limit of quantum theory given by Busch-Lahti-Werner error-disturbance uncertainty relations. We implement an optimal protocol using a photonic setting, and report an experimental violation of the proposed generalisation of macrorealism.