Emergent classicality in general multipartite states and channels

Understand

In a quantum measurement process, classical information about the measured system spreads throughout the environment.

• Meanwhile, quantum information about the system becomes inaccessible to local observers.
• Here we prove a result about quantum channels indicating that an aspect of this phenomenon is completely general.
• We show that for any evolution of the system and environment, for everywhere in the environment excluding an $O(1)$-sized region we call the "quantum Markov blanket," any locally accessible information about the system must be approximately classical, i.e.