Einstein causality of quantum measurements in the Tomonaga-Schwinger picture

Samuel Fedida

Published 2025-06-17, updated 2025-06-21Version 2

When quantum measurements are conducted over spacelike-separated regions of spacetime, a natural and commonly assumed physical postulate, called Einstein causality, asserts that they should commute. In this paper, we provide a generalisation to L\"uders' rule \`a la Aharonov-Albert in those globally hyperbolic spacetimes which allow unitarily equivalent Hilbert spaces to be defined along Cauchy hypersurfaces, thus relying on the existence of an interaction picture \`a la Tomonaga-Schwinger. We show that under this rule, selective quantum measurements satisfy a state-independent anyonic commutation relation over spacelike-separated (pre)compact regions. We highlight that this propagates to positive operator-valued measures (POVMs), where the commutation is necessarily bosonic. In the simplistic scenario where the measurements are assumed to be instantaneous, this implies quantum no-signalling for non-selective measurements. We then examine Sorkin's impossible measurements and show that immediate contradictions can be averted as long as collapse-inducing measurements are irreversible. We finish by discussing the possibility of extending such results beyond the interaction picture.