Physics & Astronomy

Spotlight:

Mapping the optimal route between two quantum states

Nature July 2014A representation of the individual quantum trajectories that connect two points in quantum state space. 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 the theory of University of Rochester theorists Areeya Chantasri, Justin Dressel, and Andrew N. Jordan. This is 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. (Cover: Kater Murch)

 

As a quantum state collapses from a quantum superposition to a classical state or a different superposition, it will follow a path known as a quantum trajectory. For each start and end state there is an optimal or “most likely” path, but it is not as easy to predict the path or track it experimentally as a straight-line between two points would be in our everyday, classical world.

Measurement data showing the comparison with the 'most likely' path (in red) between initial and final quantum states (black dots). The measurements are shown on a representation referred to as a Bloch sphere. Credit: Areeya Chantasri

Measurement data showing the comparison with the ‘most likely’ path (in red) between initial and final quantum states (black dots). The measurements are shown on a representation referred to as a Bloch sphere. Credit: Areeya Chantasri

In a new paper featured this week on the cover of Nature, scientists from the University of Rochester, University of California at Berkeley and Washington University in St. Louis have shown that it is possible to track these quantum trajectories and compare them to a recently developed theory for predicting the most likely path a system will take between two states.

Andrew N. Jordan, professor of physics at the University of Rochester and one of the authors of the paper, and his group had developed this new theory in an earlier paper. The results published this week show good agreement between theory and experiment.

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