December 03, 2003

Quantum Darwinism

...is normally the kind of phrase which makes me reach for my eraser. On the other hand, I doubt anyone understands quantum mechanics and the measurement problem better than W. H. Zurek, so it behooves me to take the following very seriously.

Effective classicality of a property of a quantum system can be defined using redundancy of its record in the environment. This allowes quantum physics to approximate the situation encountered in the classical world:The information about a classical system can exist independently from its state.In quantum theory this is no longer possible: In an isolated quantum system the state and the information about it are inextricably linked, and any measurement may -- and usually will -- reset that state. However, when the information about the state of a quantum system is spread throughout the environment, it can be treated (almost) as in classical physics -- as (in effect) independent from the state of the open quantum system of interest. This is a central idea that motivates the quantum Darwinism approach to the interpretation problem. Quantum Darwinism differs from the traditional approach suggested by the von Neumann model of quantum measurement and offers a new perspective on the emergence of the everyday classical reality that is complementary to the one suggested by decoherence: Selection of preferred states occurs as a result of the `selective advertising', a proliferation of the information about the stable pointer states throughout the Universe. This view of the emergence of the classical can be regarded as (a Darwinian) natural selection of the preferred states. Thus, (evolutionary) fitness of the state is defined both by its ability to survive intact in spite of the immersion in the environment (i.e., environment-induced superselection is still important) but also by its propensity to create offspring -- copies of the information describing the state of the system in that environment. I show that this ability to `survive and procreate' is central to effective classicality of quantum states. Environment retains its decohering role, but it also becomes a ``communication channel'' through which the state of the system is found out by the observers. In this sense, indirect acquisition of the information about the system from its environment allows quantum theory to come close to what happens in the classical physics: The information about a classical system can be ``dissociated'' from its state. (In the case of an isolated quantum system this is impossible -- what is known about it is inseparably tied to the state it is in.) I review key ideas of quantum Darwinism and investigate its connections with the environment -- assisted invariance or envariance, a recently identified symmetry exhibited by pairs of entangled quantum systems that is responsible for the emergence of probability (allowing, in particular, a completely quantum derivation of the Born's rule) within the wholly quantum Universe.

Complexity ; Physics

Posted by crshalizi at December 03, 2003 17:44 | permanent link

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