A speed limit also applies in quantum world

Jülich, 22 February 2021 – Even in the world of the smallest particles with their own special rules, things cannot proceed infinitely fast. Physicists has now shown which speed limit applies to complex quantum operations. The study involved researchers from the Universities of Bonn, Hamburg, Cologne and Padua as well as the US American MIT and Forschungszentrum Jülich. The results are important for the realization of quantum computers, among other things.

Suppose you observe a waiter (the lockdown is already history) who on New Year’s Eve has to serve an entire tray of champagne glasses just a few minutes before midnight. He rushes from guest to guest at top speed. Thanks to his technique, perfected over many years of work, he nevertheless manages not to spill even a single drop of the precious liquid.

Atoms are in some ways similar to champagne. They can be described as waves of matter, which behave not like a billiard ball but more like a liquid. Anyone who wants to transport atoms from one place to another as quickly as possible must therefore be as skillful as the waiter on New Year’s Eve.

Somewhere, however, there is a limit beyond which the process cannot be accelerated. This applies to atoms just as much as to the champagne glasses on the tray. The researchers have now experimentally investigated exactly where this limit lies. The results have been published in the renowned journal Physical Review X.

Cesium atom as a champagne substitute

They used a cesium atom as a champagne substitute and two laser beams perfectly superimposed but directed against each other as a tray. This superposition creates a standing wave of light: a sequence of mountains and valleys in which the cesium atom can be captured and transported.


Transport einer Materiewelle mittels stehender Lichtwellen
Transport of matter waves by standing light waves

Copyright: M. R. Lam et al., Demonstration of Quantum Brachistochrones between Distant States of an Atom, Phys. Rev. X, https://doi.org/10.1103/PhysRevX.11.011035 (CC BY 4.0)

The physicists’ findings are important not least for quantum computing. The computations that are possible with quantum computers are mostly based on the manipulation of multi-level systems. Quantum states are very fragile, though. They last only a short lapse of time, which physicists call coherence time. It is therefore important to pack as many computational operations as possible into this time.

Limit for complex operations

The fact that a fundamental speed limit also applies to quantum transport was already theoretically demonstrated by the two Soviet physicists Leonid Mandelstam and Igor Tamm as early as the middle of the last century. However, one can only reach it under certain circumstances, namely in systems with only two quantum states.

The situation is different, however, when the two-level system becomes a multi-level system. For example, because the distance grows to several dozens of matter wave widths as in the Bonn experiment and the particle changes its location in several intermediate steps. Also, computations that are possible with quantum computers are mostly based on the manipulation of multi-level systems. The study shows that a lower speed limit applies to such processes than that predicted by the two Soviet physicists.

Original publication:

Manolo R. Lam, Natalie Peter, Thorsten Groh, Wolfgang Alt, Carsten Robens, Dieter Meschede, Antonio Negretti, Simone Montangero, Tommaso Calarco und Andrea Alberti:

Demonstration of Quantum Brachistochrones between Distant States of an Atom

Physical Review X (published online 19 February 2021), DOI: 10.1103/PhysRevX.11.011035

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