Quantum bits can be described more precisely using newly discovered harmonics, as a team of 30 researchers reported in the journal Nature Physics.
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, harmonics are superimposed in a basic way. As a result, corrections can lead to qubits 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 – then two PhD students from FZJ and KIT and co-authors of the paper – were trying to understand their experiments using the standard model for Josephson tunnel junctions. This model won Brian Josephson the Nobel Prize in Physics in 1973. Eager to get to the bottom of things, the team led by Ioan Popa examined other data from the Ecole Normale Supérieure in Paris and the 27-qubit device at IBM Quantum in New York. York. York, as well as previously published experimental data. Regardless, the University of Cologne researchers observed similar differences 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 Dennis Willsch from FZ Jülich. “The timing was perfect,” adds Chris Dickel of the University of Cologne, “because at the time we were investigating very different ramifications of the same fundamental problem.”
Josephson tunnel junctions consist of two superconductors separated by a thin insulating barrier, and for decades these circuit elements were described by a simple sinusoidal model.
However, as the researchers show, this “standard model” fails to fully describe the Josephson junctions used to construct quantum bits. Instead, an extended model that includes more harmonics is needed to describe the tunneling current between two superconductors. The principle is also found in the field of music. When an instrument string is struck, the fundamental frequency is covered by several harmonics.
“It’s exciting that community measurements have reached a level of precision where we can make these small corrections to a model that has been considered adequate 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 findings, they gathered a broad collaboration of experimentalists, theorists and materials scientists to unite their efforts. presenting convincing arguments in favor of Josephson’s harmonic model. In the publication Nature Physics, researchers explore the origins and consequences of Josephson harmonics. “As an immediate consequence, we believe Josephson harmonics will help design better and more reliable quantum bits by reducing errors by an order of magnitude, bringing us closer to the dream of a fully universal superconducting quantum computer,” the report said. conclude the two first authors.