Fundamental equation of superconducting qubits revised

Physicists from Forschungszentrum Jülich and the Karlsruhe Institute of Technology have discovered that Josephson tunnel junctions – the fundamental building blocks of superconducting quantum computers – are more complex than previously thought.

Just like the harmonics of a musical instrument, the harmonics are superimposed on the fundamental mode. As a result, corrections can lead to qubits that are two to seven times more stable. The researchers support their findings with experimental evidence from several laboratories around the world, including the University of Cologne, the Ecole Normale Supérieure in Paris and IBM Quantum in New York.

It all started in 2019, when two doctoral students, Dr. Dennis Willsch and Dennis Rieger, studied. students of FZJ and KIT at the time and co-authors of a new article published in Natural physics— had difficulty understanding their experiments using the standard Josephson tunnel junction model. This model earned Brian Josephson the Nobel Prize in Physics in 1973.

Eager to get to the bottom of things, the team led by Professor Ioan Pop examined further data from the École Normale Supérieure in Paris and a 27-qubit device from IBM Quantum in New York, as well as than previously published experimental data. Independently, researchers at the University of Cologne observed similar discrepancies between their data and the standard model.

“Fortunately, Gianluigi Catelani, who was involved in both projects and realized the overlap, brought the research teams together,” recalls Dr. Dennis Willsch from FZ Jülich. “The timing was perfect,” adds Dr. Chris Dickel of the University of Cologne, “since at that time we were exploring very different consequences of the same underlying problem.”

Josephson tunnel junctions consist of two superconductors separated by a thin insulating barrier, and for decades these circuit elements have been described with a simple sinusoidal model (see image below).

However, as the researchers demonstrate, this “standard model” fails to fully describe the Josephson junctions used to construct quantum bits. Instead, an extended model including higher harmonics is needed to describe the tunneling current between the two superconductors. The principle is also found in the field of music. When the string of an instrument is struck, the fundamental frequency is covered by several harmonics.

“It is exciting that the measurements made in the community have reached the level of precision at which we can make these small corrections to a model that has been considered sufficient for over 15 years,” notes Dennis Rieger.

When the four coordinating professors – Ioan Pop from KIT and Gianluigi Catelani, Kristel Michielsen and David DiVincenzo from FZJ – realized the impact of their results, they brought together a broad collaboration of experimentalists, theorists and materials scientists to unite their efforts to present a convincing argument for Josephson’s harmonic model.

In the Natural physics publication, researchers explore the origin and consequences of Josephson harmonics. “As an immediate consequence, we believe that Josephson harmonics will help design better and more reliable quantum bits by reducing errors by up to an order of magnitude, bringing us closer to the dream of a fully universal superconducting quantum computer,” he said. stated the report. conclude two first authors.