Revision 1

Accepted · first revision · record

Title: Do laboratory quantum switches realize genuinely indefinite causal order?
Plain: A quantum switch sends a photon through devices A and B in a superposition of the orders A-then-B and B-then-A. Critics say each device actually acts at two different times, so the order is definite in spacetime and only appears indefinite.
Precise: For photonic quantum-switch experiments (operations applied at two distinct times on a time-delocalized subsystem), determine whether the process is causally nonseparable relative to spacetime-localized events, or is a causally ordered circuit on time-delocalized subsystems (Oreshkov, Quantum 2019). An answer is an agreed operational definition of an event and a classification of existing experiments under it.
Settled by: A theorem or consensus definition linking process-matrix nonseparability to localized events, applied to the optical experiments.
Kind: phenomenon
Answer type: classification
References: (empty)
Literature status: contested
Status note: Vilasini and Renner (Phys. Rev. A 110, 022227, 2024) proved that any fixed-spacetime realization uses systems not localized in spacetime and has a fine-grained definite-order description; whether the coarse-grained indefinite order counts as genuine remains disputed.
Posed since: (empty)