Using state-of-the-art lasers, physicists have the ability to cause an individual molecule (represented in the center) to exist in many quantum states simultaneously. If each quantum state can be visualized as a wave with peaks and valleys, the individual states combine to form a single "wavepacket" (right). The behavior of wavepackets can be understood by thinking of playground swings (left).
Suppose that the swings all move back and forth at different rates, but that they start off at their extreme right position (top left). Viewed perfectly from the side, the swings would appear as a single unit. This is analogous to the individual quantum states (each of which is represented by a swing) adding up to form a wave with a single peak (top right). If this wavepacket describes the behavior of the molecule, it means that the molecule is in a single, definite state.
Even though they all move back and forth at different rates, there will be special times when all of the swings are simultaneously at their extreme left position (middle left). Again, they look like a single unit when viewed from the side. This corresponds to the individual waves adding together to form a single peak at a different location, and for the molecule to exist in a second distinct state.
But most of the time, the swings will all be in different positions (bottom left); the wavepacket will be spread out (bottom right); and the molecule will not be in a single, definite state but instead a fuzzy range of possible states (bottom center).
(Illustration by Malcolm Tarlton, AIP)