macroscopic superposition

A macroscopic superposition is a superposition stretched far beyond the realm of single atoms, up to objects large enough that we might hope to see the quantum strangeness with the unaided imagination. Schrödinger's dead-and-alive cat is the cartoon version; the real laboratory versions are more modest but still remarkable — molecules made of hundreds of atoms passed through an interference grating, supercurrents flowing two ways at once in a loop, tiny vibrating drums put into superpositions of two motions. These are sometimes called 'cat states' in honour of the original thought experiment.

Creating and keeping such states is extraordinarily hard precisely because of decoherence. The bigger and warmer the object, the more vigorously it leaks which-way information into its environment, collapsing the delicate superposition almost as fast as it is made. Every advance in this field is therefore a feat of isolation, cooling, and speed: the experimenters must complete their interference before the environment notices what is going on.

These experiments matter because they probe directly where, if anywhere, quantum mechanics might break down. So far the theory has passed every test, with superpositions confirmed for ever-larger systems and no sign of a fundamental size limit. Some proposed theories predict that genuine, irreducible collapse should set in at some scale; pushing macroscopic superpositions to larger masses is one of the few ways to test such ideas experimentally rather than merely argue about them.