The superposition principle of quantum mechanics decrees that every combination of quantum states is a legal state. That is at odds with our experience; we never see anything like what is depicted in figure 1. So why can’t a chef prepare chicken à la Schrödinger?

The answer is that interactions with the environment help select preferred states of the system (see Physics Today, October 1991, page 36). As it is impossible to follow every variable of the composite system–environment whole, one relies on a reduced density matrix,1 a statistical description of the system alone. It is obtained by averaging out the environment; Born’s rule,2 which states that the probability of finding a system with wavefunction ∣ψ〉 in a specific state ∣k〉 is the absolute square of 〈kψ〉, justifies that averaging.

The interactions that determine preferred states favor...

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