Beyond Weird: Why Everything You Thought You Knew about Quantum Physics Is Different

by Philip Ball

specifically, decoherence-inducing interactions with the environment simply transform a pointer state into an identical-looking state. Recall that the coherence of quantum states is a question of whether the phases of their wavefunctions – the positions of the peaks and troughs, you might say – are aligned. But pointer states are special states for which shifts of phases caused by interaction and entanglement with the environment make no difference. The state still looks the same after the shift. Crudely you can think of it rather like the difference between a circle and a square. You can rotate the circle by any angle you wish and it looks the same, but not so for the square. This implies that the environment doesn’t just squash quantumness indiscriminately: it selects particular states and trashes others, a process Zurek calls environment-induced superselection or ‘einselection’. The survivors are the pointer states, which are detectable. Superpositions of pointer states do not have this stability, and so they are not ‘einselected’.

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