Physicists develop a series of quality control tests for quantum computers

Quantum technology, especially quantum computers, has the potential to influence the future of technology. Scientists believe that quantum computers will help them solve problems that even the fastest supercomputers cannot. Large international IT companies, as well as countries such as the United States and China, have already invested heavily in the development of this technology. But because quantum computers are based on different laws of physics than traditional computers, laptops and smartphones, they are more prone to failure.

Now, an interdisciplinary team of researchers led by physicist Prof. Jens Eisert of Freie Universität Berlin has found a way to test the quality of quantum computers. Their related research was recently published in the scientific journal Nature Communications. These scientific quality control tests combine methods from physics, computer science and mathematics.

"Shadow estimation of gate-set properties from random sequences."

Prof. Jens Eisert, a quantum physicist at Freie Universität Berlin and author of the study, explains the science behind the research.

"Quantum computers work according to the physical laws of quantum mechanics, in which individual atoms or ions are used as computational units or, to put it another way, as controlled, tiny physical systems. What's remarkable about these future computers is that, at this level, nature operates extremely differently and in a very different way than our everyday experience of the world and the way we recognize and perceive it."

However, quantum computers have one major weakness: "They are very sensitive to sources of interference. If quantum computers are not adequately shielded from their surroundings, then the character of their computational power disappears. What we call the 'quantum advantage' tends to disappear. In other words, it simply won't work."

This conundrum raises an important question for researchers: how can they know if a quantum circuit is working properly? Just like testing whether a vehicle is safe for the road, we need to test the quality of quantum circuits. eisert explains, "without these methods, while you can get results from quantum computing, you don't know exactly how reliable they are."

An interdisciplinary research team consisting of members of the Free University of Berlin, the Helmholtz Center Berlin, the Heinrich Hertz Institute, QuSoft Amsterdam, the Quantum Research Center Abu Dhabi, QMath Copenhagen, and the Technical University of Munich has now found a way to test the quality of quantum computers.Eisert explains, "This method is as simple as it is amazing: implement random circuits and then output the measurements as 'quantum bits'."

The gate-set shadowing estimation protocol is carried out in two stages. First, a total of S random sequences of quantum gates of length m were experimentally implemented for a fixed initial state ρ and different sequence lengths m. Measurements were taken after each sequence. The team refers to the observed measurements and tuples of gate sequences as "gate-set shadows".

Numerical simulations of two potential applications, single noise optimization and crosstalk tomography, were performed.

These data provide a wealth of diagnostic information that can be used, for example, to find out how a quantum gate works, whether there are any interfering factors, or whether certain parts are inadvertently interacting with each other.

"If we stick to the analogy of sending a car in for a maintenance check, this is like using a rag to make a few random quick wipes on the exterior of the car, while also finding out if the engine is working properly, if there is any washer fluid in the windshield wipers, and if the brakes are adjusted correctly." Eisert said, "All of these processes are integrated into the same measurement, which means that the all-encompassing diagnostic is practically a one-two punch."

It is expected that this research will form the basis for new validation methods that will enable future quantum computers for technical equipment to deliver real economic and scientific benefits.