Spatially Delocalized Operators and Faster-than-light Signaling

by Carl Erik Hugo Fernström (KTH)

Thursday 28 Aug 2025, 13:00 → 14:00 Europe/Stockholm

Albano 3: 5230 - Xenon (12 seats) (Albano Building 3)

The talk is a presentation of the degree project in theoretical physics
(SI201X) by the presenter (KTH 2025).

Examinator: Professor Mats Wallin

The thesis explores the relationship between locality and causality for delocalized quantum operations within the framework of non-relativistic quantum mechanics. By defining and contrasting classes of local, shared-randomness localizable, shared-entanglement localizable, and causal bipartite superoperators, we establish a strict hierarchy of possible operations. Within the class of complete orthogonal measurement superoperators (COMS), we derive precise causality conditions and employ communication complexity arguments to determine nonlocalizability. Extending the analysis to Haar-random COMS, we compute the expected
signaling in finite dimensions and show numerically that it vanishes in the large-dimension limit, offering a quantitative interpretation of macroscopic causality as an emergent phenomenon. These results clarify the distinctions between mathematical implementability and physical admissibility, and provide a framework for further exploration of nonlocal and causal structures in quantum theory.