Quantum technologies, and especially quantum computers, are considered a promising technology for the future. They are expected to solve problems that even the fastest supercomputers currently struggle to handle.

An interdisciplinary research team from Freie Universität Berlin, Helmholtz-Zentrum Berlin, Heinrich Hertz Institute, QuSoft in Amsterdam, Quantum Research Center in Abu Dhabi, QMath in Copenhagen, and Technische Universität München has now found out how to test the quality of quantum computers. The study has been published in the journal Nature Communications. The scientific "Computer TÜV" incorporates methods from physics, computer science, and mathematics.

The renowned quantum physicist at Freie Universität Berlin and one of the authors of the study, Prof. Dr. Jens Eisert, explains: "Quantum computers work on the basis of quantum mechanical laws of physics, in which individual atoms or ions are used as computational units, i.e. controlled, tiny physical systems. What's extraordinary about these future computers is that at this level, nature functions extremely and radically differently from our everyday experience of the world we perceive and know."

However, quantum computers also have a weak point: "They are true mimosas when it comes to interactions with the environment. If the quantum computer is not adequately shielded from the environment, the properties responsible for its computational power disappear. The so-colled 'quantum advantages' disappear. In short, then it doesn't work," says the Professor of Theoretical Physics.

This raises a critical question for research: How can we know whether a quantum circuit has functioned correctly? Similar to the car's TÜV inspection, which tests whether a car is still safe for use on the road, are needed to determine the quality of a quantum circuit's implementation. "Without such methods, you get results in quantum computation, but you can't rely on them," says Jens Eisert.

In the publication in Nature Communications, the research team presented an approach for testing the quality of quantum computers. "The method is as simple as it is amazing: one performs certain completely random quantum circuits and then reads out the elementary parts of the quantum computer, called 'quantum bits,'" the professor further explains. The data obtained in this way allow for a whole range of diagnostic information.

From the same data, one can learn how well elementary gates, i.e., computational units, function, what interferences are present, or whether certain parts unintentionally interact with each other.

"To continue with the TÜV metaphor, it's as if you randomly go over the car with a washcloth several times with a few erratic movements to find out if the engine is working, the windshield wiper fluid is filled and the brakes are properly adjusted - all with the same random measurement. Such a measurement provides the entire diagnostic in one go," emphasizes Jens Eisert.

With the help of this method of verification, research promises to turn quantum computers into real technological devices in the future that can actually be used economically and scientifically.

Read publication