The Complexity Horizon of Quantum Branch-Access: Information-Theoretic Limits on Cross-Branch Retrieval in Everettian Quantum Mechanics

Abstract

We formalize the concept of quantum branch-access (QBA)-the retrieval of information from branches of the Everettian universal wave function other than the one an observer occupies-and characterize the computational complexity constraints that govern it. Building on Deutsch's argument that quantum computational speedup implies the physical reality of parallel branches, we address the principal objections (Steane, Aaronson) by reframing the question: rather than asking whether branches are real, we ask what the information-theoretic structure of branch-access looks like if they are.