Do laboratory quantum switches realize genuinely indefinite causal order?
In plain words
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 statement
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.
What would settle it
A theorem or consensus definition linking process-matrix nonseparability to localized events, applied to the optical experiments.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
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.
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