The physicist Erwin Schrödinger famously said that quantum theory would allow the existence of a cat that was simultaneously living and dead.

Now a team of physicists has published the recipe for making a large object - not cat-sized, but certainly bacterium-sized - in such a quantum quandary. A tiny mirror, they propose, can be in two places at once.

Scientists are resigned to atom-sized entities being capable of such feats. But they generally assume that at larger scales a phenomenon called decoherence intervenes, stamping out quantum weirdness and fixing everyday objects to a single, definite location.

William Marshall of the University of Oxford and his coworkers outline a scheme for evading decoherence to achieve a quantum superposition of states in an object with around a hundred trillion atoms. This is about a billion times larger than anything demonstrated previously...