Waiting for the quantum bus: The flow of negative probability

• Illustrates contrasting behaviour of classical and quantum probabilities.
• Describes the paradoxical quantum effect known as “probability backflow”.
• Summarizes known results concerning the size of the effect.
• Introduces negative probability to provide a striking interpretation.

It is 45 years since the discovery of the peculiar quantum effect known as ‘probability backflow’, and it is 20 years since the greatest possible size of the effect was characterized. Recently an experiment has been proposed to observe it directly, for the first time, by manipulating ultra-cold atoms. Here a non-technical description is given of the effect and its interpretation in terms of the flow of negative probability.