Quantum state readout, collapses, probes, and signals

Abstract

Theories involving localized collapse allow the possibility that classical information could be obtained about quantum states by mechanisms other than the standard quantum dynamical maps defined by positive operator valued measurements, but nonetheless without allowing superluminal signaling. We can model this by extending quantum theory to include hypothetical devices that read out information about the local quantum state at a given point, which is defined by considering unitary evolution modified only by the effects of collapses in its past light cone. Like Popescu-Rohrlich boxes, these hypothetical devices would have practical and scientific implications if realizable. These include signaling through opaque media, probing the physics of distant or opaque systems without needing a reflected signal and giving detailed information about collapse dynamics without requiring direct observation of the collapsing system. These potential applications add to the motivation for systematic searches for possible signatures of these nonstandard extensions of quantum theory, and in particular for relevant gravitational effects, such as the validity of semiclassical gravity on small scales.