Environment as a witness: Selective proliferation of information and emergence of objectivity in a quantum universe

We study the role of the information deposited in the environment of an open quantum system in the course of the decoherence process. Redundant spreading of information---the fact that some observables of the system can be independently read off from many distinct fragments of the environment---is investigated as the key to effective objectivity, the essential ingredient of classical reality. This focus on the environment as a communication channel through which observers learn about physical systems underscores the importance of quantum Darwinism---selective proliferation of information about ``the fittest states'' chosen by the dynamics of decoherence at the expense of their superpositions---as redundancy imposes the existence of preferred observables. We demonstrate that the only observables that can leave multiple imprints in the environment are the familiar pointer observables singled out by environment-induced superselection (einselection) for their predictability. Many independent observers monitoring the environment will therefore agree on properties of the system as they can only learn about preferred observables. In this operational sense, the selective spreading of information leads to appearance of an objective classical reality from within the quantum substrate.