Decoherence to quantum theory from a causally indefinite post-quantum theory
Phys. Rev. A 113, 042433 – Published 14 April, 2026
DOI: https://doi.org/10.1103/kmmy-3dy3
Abstract
We find a process satisfying the axioms of hyperdecoherence, which produces standard quantum theory from the theory of quantum boxes (higher-order quantum theory with the nonsignaling tensor product). This hyperdecoherence map evades the no-go theorem of Lee and Selby [Proc. R. Soc. A 474, 20170732 (2018)] by relaxing constraints on signaling to the past and the uniqueness of purifications. We discuss natural opposing conclusions, namely, that the existence of this map might be evidence of a genuine hyperdecoherence process producing causal quantum theory from its causally indefinite higher-order theory or that it serves as an indication that the axioms of hyperdecoherence might need careful reconsideration, especially regarding the subtle albeit central role that purity plays.
