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Quantum bits can be described more precisely with the help of newly discovered harmonics, 30 researchers report in Nature Physics.

Physicists at Forschungszentrum Jülich and the Karlsruhe Institute of Technology have revealed that Josephson tunnel junctions – the basic building blocks of superconducting quantum computers – are more complex than previously thought. Just like overtones in a musical instrument, harmonics are superimposed on the fundamental mode. As a result, the corrections can lead to quantum bits that are 2 to 7 times more stable. The researchers support their findings with experimental evidence from several laboratories around the world, including the University of Cologne, Ecole Normale Supérieure in Paris, and IBM Quantum in New York.

It all started in 2019, when Dennis Willsch and Dennis Rieger — two PhD students at FZJ and KIT at the time and co-first authors of this paper — ran their experiments using the standard model of Josephson tunnel junctions to make sense of their difficulties. It was happening. This model won Brian Josephson the Nobel Prize in Physics in 1973. Excited to get to the bottom of it, the team led by Ioan Pop examined more data from the Ecole Normale Supérieure in Paris and a 27-qubit device at IBM Quantum. York, as well as data from previously published experiments. Independently, researchers at the University of Cologne were observing similar deviations of their data from the standard model.

“Fortunately, Gianluigi Catelani, who was involved in both projects and noticed the overlap, brought the research teams together!” Dennis Willsch recalls from FZ Jülich. “The timing was perfect,” says Chris Dickel from the University of Cologne, “because at the time, we were looking for completely different results for the same basic problem.”

Josephson tunnel junctions consist of two superconductors with a thin insulating barrier in between and, for decades, these circuit elements have been described with a simple sinusoidal model.

However, as the researchers show, this “standard model” fails to fully describe the Josephson junctions that are used to create quantum bits. Instead, an extended model including higher harmonics is needed to describe the tunneling current between two superconductors. The principle can also be found in the field of music. When the strings of an instrument are plucked, the fundamental frequency is covered by several harmonic overtones.

“It’s exciting that the measurements in the community have reached a level of accuracy at which we can fix these small improvements in a model that was considered adequate for more than 15 years,” remarked Dennis Rieger. Is.”

When the four coordinating professors — Ioan Pop and Gianluigi Catelani from KIT, Crystal Michielsen and David DiVincenzo from FZJ — realized the implications of these results, they enlisted the support of experimentalists, theorists, and material scientists in their efforts. Collected to add. In presenting a compelling case for the Josephson harmonics model. In a Nature Physics publication, researchers explore the origins and consequences of Josephson harmonics. “As an immediate consequence, we believe that Josephson harmonics will help engineer better and more reliable quantum bits by reducing errors to dimensions, allowing us to fully realize the dream of a universal superconducting quantum computer. The side brings one step closer.” The two first authors concluded.

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