The path most travelled

摘要

Trajectories of various kinds, such as those of aeroplanes and migrating birds, are a familiar part of our everyday experience. A trajectory is given, classically, by the position coordinates of an object as a function of time. In the realm of quantum mechanics, the state of an object is given not in terms of position, but rather by a more abstract mathematical construct - a vector in the Hilbert space. Nevertheless, quantum states can be parameterized by a set of coordinates whose evolution in time defines quantum trajectories. For a quantum system in isolation, quantum trajectories are deterministic, bearing a deceptive similarity to classical trajectories. However, the act of observation makes quantum trajectories random, revealing their fragile character. On page 570 of this issue, Weber et al. analyse these random trajectories and find that the most likely ones still provide insightful information about a quantum system.