Quantum Theory of the Classical

I will describe three insights into the transition from quantum to classical. After a brief discussion of decoherence I will give (i) a minimalist (and decoherence-free) derivation of preferred states. Such pointer states define events (e.g., measurement outcomes) without appealing to Born's rule. Probabilities and (ii) Born’s rule then follows from the symmetries of entangled quantum states. With probabilities at hand one can analyze information flows from the system to the environment in course of decoherence. They explain how, as a result of selective proliferation of information, robust classical reality arises from the quantum substrate by accounting for all the symptoms of objective existence of preferred pointer states of quantum systems through the redundancy of their records in the environment. Taken together, these three advances (i)-(iii) elucidate quantum origins of the classical.

W. H. Zurek, Quantum Darwinism, Classical Reality, and the Randomness of Quantum Jumps, Physics Today 10, 44-50 (2014), arXiv:1412.5206