Eavesdropping on the Decohering Environment: Quantum Darwinism, Amplification, and the Origin of Objective Classical Reality

Abstract

"How much information about a system S can one extract from a fragment F of the environment E that decohered it?" is the central question of Quantum Darwinism. To date, most answers relied on the quantum mutual information of SF, or on the data extracted by measuring S directly. These are reasonable upper bounds on what is really needed but much harder to calculate -- the channel capacity of the fragment F for the information about S. We consider a model based on imperfect c-not gates where all the above can be computed, and discuss its implications for the emergence of objective classical reality. We find that all relevant quantities, such as the quantum mutual information as well as the channel capacity exhibit similar behavior. In the regime relevant for the emergence of objective classical reality this includes scaling independent from the quality of the imperfect c-not gates or the size of E, and even nearly independent of the initial state of S.