Quantum bits may be described more precisely with the assistance of newly discovered harmonics as a team of 30 researchers reports in Nature Physics.
Physicists from Forschungszentrum Jülich and the Karlsruhe Institute of Technology have uncovered that Josephson tunnel junctions — the elemental constructing blocks of superconducting quantum computers — are more complex than previously thought. Similar to overtones in a musical instrument, harmonics are superimposed on the elemental mode. As a consequence, corrections may result in quantum bits which are 2 to 7 times more stable. The researchers support their findings with experimental evidence from multiple laboratories across the globe, including the University of Cologne, Ecole Normale Supérieure in Paris, and IBM Quantum in Latest York.
It began in 2019, when Dennis Willsch and Dennis Rieger — two PhD students from FZJ and KIT on the time and joint first authors of the paper — were having a tough time understanding their experiments using the usual model for Josephson tunnel junctions. This model had won Brian Josephson the Nobel Prize in Physics in 1973. Excited to resolve this, the team led by Ioan Pop scrutinized further data from the Ecole Normale Supérieure in Paris and a 27-qubit device at IBM Quantum in Latest York, in addition to data from previously published experiments. Independently, researchers from the University of Cologne were observing similar deviations of their data from the usual model.
“Fortunately, Gianluigi Catelani, who was involved in each projects and realized the overlap, brought the research teams together!,” recalls Dennis Willsch from FZ Jülich. “The timing was perfect,” adds Chris Dickel from the University of Cologne, “since, at the moment, we were exploring quite different consequences of the identical underlying problem.”
Josephson tunnel junctions consist of two superconductors with a skinny insulating barrier in-between and, for many years, these circuit elements have been described with a straightforward sinusoidal model.
Nevertheless, because the researchers show, this “standard model” fails to completely describe the Josephson junctions which are used to construct quantum bits. As an alternative, an prolonged model including higher harmonics is required to explain the tunneling current between the 2 superconductors. The principle can be present in the sphere of music. When the string of an instrument is struck, the elemental frequency is overlaid by several harmonic overtones.
“It’s exciting that the measurements in the neighborhood have reached the extent of accuracy at which we are able to resolve these small corrections to a model which has been considered sufficient for greater than 15 years,” Dennis Rieger remarks.
When the 4 coordinating professors — Ioan Pop from KIT and Gianluigi Catelani, Kristel Michielsen and David DiVincenzo from FZJ — realized the impact of the findings, they brought together the massive collaboration of experimentalists, theoreticians, and material scientists, to hitch their efforts in presenting a compelling case for the Josephson harmonics model. Within the Nature Physics publication, the researchers explore the origin and consequences of Josephson harmonics. “As a right away consequence, we imagine that Josephson harmonics will assist in engineering higher and more reliable quantum bits by reducing errors as much as an order of magnitude, which brings us one step closer towards the dream of a completely universal superconducting quantum computer,” the 2 first authors conclude.
