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Interview: Reaching the quantum speed limit

Tommaso Calarco is known as one of the world's leading quantum physicists. The Jülich researcher is one of the founding fathers of the European Quantum Manifesto, which led to the EU's billion-dollar Quantum Flagship Programme a few years ago. His focus lies on optimising quantum processes. Researchers at Harvard and Vienna, among others, use his codes to adjust their quantum experiments.

More than ten years ago, the Jülich physicist was already driven by the question of where the upper limit lies beyond which quantum transport processes cannot be accelerated any further. In experiments with scientists from the University of Bonn, he has now succeeded in precisely determining this speed limit for complex quantum operations.

Prof. Dr. Tommaso CalarcoProf. Dr. Tommaso Calarco, Director at the Peter Grünberg Institute, Section "Quantum Control" (PGI-8), and Professor of Theoretical Physics at the University of Cologne
Copyright: Forschungszentrum Jülich / Sascha Kreklau

Professor Calarco, what is the significance of this experiment at the University of Bonn?

This was really a textbook experiment. There were already indications beforehand that there exists such a quantum speed limit. But there has not yet been a systematic experimental investigation, specifically for quantum transport. The result is of course highly interesting from a scientific point of view, and it is also technologically important.

How relevant are the results for future applications?

The problem is: with a quantum computer, you can't avoid creating so-called decoherence through interactions with the environment. After a certain time, every quantum system loses its quantum properties. Therefore, one must perform all operations within this coherence time.

The experiment was about the transport of atoms. Similar operations also take place in a quantum computer. When quantum bits are realised by atoms, they have to be moved from one region in the processor to another. This is exactly the process that has to be done very quickly, otherwise you lose your coherence. Thanks to the quantum speed limit, you can now predict exactly what speed is theoretically possible.

What does the limit imply for the computing speed of quantum computers - will they perhaps not be as fast as thought?

No, these are two completely different things. The fact that a quantum computer can calculate so fast has primarily not to do with the duration per se, but rather with the number of operations. A quantum computer needs far fewer operations than a classical computer to master a certain task. Calculating with a quantum computer is like finding the exit from a labyrinth without having to check all possible paths sequentially. That is also where the speed-up lies: I only have to send the quantum computer through the labyrinth once, whereas with a classical computer I have to try out a very large number of paths one after the other.

In this sense, there are therefore no consequences for the computing power of a quantum computer. But the quantum speed limit is interesting for another reason. And that is the question of how many quantum operations I can perform before decoherence occurs. The limit we have found shows that there are considerably more operations possible than we observe today.

You like to compare your quantum control methods with the work of an experienced waiter. Where is the connection there?

When I want to transport an atom from one place to another, my atom doesn't behave like a point, but like a wave, like a liquid sloshing in a glass, and I have to prevent it from spilling out. So we are dealing with a task similar to that of a waiter who wants to bring a tray of glasses to the guests at the table without spilling anything.

In the lab, we use laser fields that prevent the atom from getting lost. And that is difficult. I could move the atom very slowly, of course. But that's not very efficient. The waiter could also walk very slowly. But then it might take too long when he has to move around a lot, and the champagne gets warm. An experienced waiter will therefore tilt the tray to speed up. Then he turns it again to slow down and thus speed up the whole process.

You can really only iteratively learn how it works best. Our algorithm does it automatically, with atoms. The fastest way is not always directly from A to B. You often have to move the atoms back and forth, in a kind of wave motion. On the website, anyone can replay this task and see how it works.

Release of 22 February 2021: A speed limit also applies in the quantum world