Classical systems are unmoved when a measurement is performed. Not so quantum systems, where continuous monitoring can direct the quantum state along a random path. Steve Weber et al. have tracked the quantum trajectories in a qubit, consisting of two aluminum paddles connected by a tunable Josephson junction deposited on silicon. The authors manage to determine which of the possible paths between an initial and a final quantum state is the most probable and show that these ‘optimal paths’ are in agreement with the route predicted by theory, a quantum relative of the principle of least action that defines the correct path linking two points in space and time in classical mechanics. As well as giving insights into the interplay between measurement dynamics and evolution of a system, this work opens up new possibilities for first-principles synthesis of control sequences for complex quantum systems and in information processing.
