Quantum mechanics, unlike classical physics, allows objects to exist in more than one state at the same time. This idea is often illustrated by Schrödinger's cat, imagined as being both alive and dead until it is observed. In the laboratory, physicists can create less dramatic but very real versions of this effect by placing atoms, light or motion into two distinct quantum states at once. Creating and controlling these superpositions is essential for applications ranging from quantum computing to precision timekeeping.