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IT IS often said that the very small is governed by quantum physics, and the large by classical physics. There seems to be one set of rules for fundamental particles and another for us. But everything, including us, is made of particles. So why can鈥檛 we too be in superpositions or show wave-like interference when we pass through a doorway, as a photon or electron does when it passes through narrow slits? Ditto any large, inanimate object?

To cut to the chase: we don鈥檛 know the answer. One of the most intriguing ideas now being tested, however, is that classical reality might emerge through a process analogous to evolution by natural selection.

That notion has its origins in the 1970s, when physicists first came to realise that a particle鈥檚 quantum behaviours of superposition, entanglement and suchlike leak out into its environment, disappearing as a result of interactions with other particles 鈥� a process called decoherence. 鈥淭he coupling to the macroscopic environment spoils the quantum coherences so fast that they are unobservable,鈥� says in Paris, France. Experiments have demonstrated that decoherence is a real, physical process, albeit one that happens in the blink of an eye.

What it can鈥檛 tell us, however, is why various definite properties, such as position or velocity, emerge for us to observe. Why do these properties survive the transition from quantum to classical, while some other quantum features don鈥檛?

To at the Los Alamos National Laboratory in New Mexico, it looked a lot like there was some sort of going on. That filtering, he realised, is caused by decoherence itself: it turns out that it destroys some states, like superpositions, but leaves others unchanged.

Zurek also noticed that to measure those robust states, what we really do is look at the imprints they leave on the environment. For example, the position of an object is imprinted on the photons of light that bounce off it, so we can deduce the position by looking at the reflected light. Intriguingly, it turns out that those states selected by their robustness to decoherence are precisely the ones that are also good at making many imprints 鈥� copies, you might say 鈥� of themselves in the environment.

This survival of states by virtue of their ability to make copies reminded Zurek of evolution by natural selection, so he called the idea . 鈥淨uantum Darwinism says that the preferred [observable] states are those that disseminate copies of themselves in the environment so as to more easily allow a set of independent observers to reach a consensus about the result of the measurement,鈥� says Raimond.

In recent years, Zurek and others have begun to put the idea to the test. They realised that if there is some form of natural selection going on at the quantum-classical transition, you should see a clear signature of it as a quantum object interacts with its environment. Specifically, quantum Darwinism predicts that most of the information we can gather about the object will appear within the first few copies it imprints on the surroundings, with subsequent copies adding very little that is new. In other words, the information transferred from the object to its environment 鈥渟aturates鈥� rapidly.

With that in mind, looked at quantum systems that could be described precisely enough for this signature to be clearly observable. All of them have found exactly the kind of information saturation predicted. As Raimond points out, however, these experiments involved simplified systems. 鈥淚 do not think there is yet a general result that states that [this theory of] decoherence should work for all systems,鈥� he says.

And one question remains: why do we only see one of all the possible values a particular property could have when measured? A superposition of two positions for a particle can鈥檛 survive the quantum Darwinian filter, but both classical positions can 鈥� so what happens to the one not observed? 鈥淒ecoherence predicts that the measuring device is in a statistical mixture of all the possible states,鈥� says Raimond. 鈥淪o how is it that just a single result emerges? This problem is not at all addressed by the decoherence mechanism.鈥�

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