An ordinary magnet is like a compass: its needle points north. But a quantum compass can look in both directions at once. If a magnet floating in a vacuum is gently spun, its electron 'tops' synchronize so the whole piece of metal starts living by the rules of the microworld — freezes in a superposition, like a split pointer.
This gives unimaginable precision: the bigger the magnet, the more sensitive it is to weak fields, like the magnetic field of a single living cell. But the fragile split is easily shattered, decoherence from collisions with gas molecules, so you need a vacuum a billion times harder than outer space. The idea was proposed by Michael Horne, Anton Zeilinger, and Daniel Greenberger. The experiment will also test hypotheses beyond standard physics: perhaps quantum weirdness vanishes because of spacetime curvature, as Roger Penrose suggested.
To preserve the quantum superposition of a real magnet, a vacuum a billion times lower than atmospheric pressure is required — harder than that in interstellar space.
Like Schrödinger's cat in the form of a magnet — a fantastical symbol of quantum duality, ready to step from thought into reality.