The flow of electric current between two superconductors without any external voltage has revolutionized the field of superconductivity, paving the way for numerous technological advancements today. This was possible due to the path breaking discovery of Brian Josephson (later Prof and 1973 Nobel laureate), a 22-year-old PhD student at Cambridge University in 1962, who worked out a theory that was later named after him.
Developments in applications of superconductivity include superconducting quantum interference devices (SQUIDs) and, more recently, quantum computing platforms based on superconducting circuits. However, the journey towards achieving reliable quantum computing is fraught with challenges for scientists. Unlike classical computers, quantum computers use qubits, which can exist in superpositions of states, to store information. To attain functional qubits, tech majors IBM and Google, but also startups and research institutions, are conducting experiments with small superconducting circuits by combining elements such as capacitors and Josephson junctions. One of the main hurdles is the accurate prediction of the properties of superconducting quantum bits based on Josephson junctions. These junctions, comprising two aluminum films (blue and yellow in the picture) separated by a thin oxide layer (dark grey in panel B), are a critical component within a quantum processor.
Improving qubit modelling
An international research collaboration that includes Dr. Gianluigi Catelani, Lead Researcher at the Quantum Physics group, Quantum Research Center (QRC), Technology Innovation Institute (TII) in Abu Dhabi, and teams from Germany (Karlsruhe Institute of Technology, Juelich Research Center, University of Cologne), France (Ecole Normale Superieure), Romania (Center for Advanced Research and Technologies for Alternative Energy), and USA (IBM), has conducted in-depth investigations into Josephson tunnel junctions for quantum computing applications. The results of their work, coordinated by Profs Pop (KIT), DiVincenzo and Michielsen (FZJ) and Dr. Catelani, were published recently in the Nature Physics journal under the title, ‘Observation of Josephson Harmonics in Tunnel Junctions’.
Dr Catelani has highlighted the need for a more comprehensive model to capture the complexities of these circuits. “These circuits don't have just two states. Instead, they display multiple states at different energies. These energies do not follow the predictions from Josephson’s model of the junction. The oxide layer forming the junction is not consistently uniform throughout, and its irregularities could explain this difference,” he said.
A promising future
He goes on to say that understanding these intricacies is vital to making sound predictions about the properties of quantum circuits. It will also help scientists develop alternative circuit designs, thereby achieving greater accuracies in quantum computers without slowing them down. Moreover, this may also enable researchers to improve the design of other components, such as travelling wave parametric amplifiers (TWPAs), used to read the basic units of information in qubits. Dr Catelani also highlights that advancements in Josephson junction-based devices hold potential for applications in metrology, as the standard for voltage measurements is based on the Josephson effect, or in sensing magnetic fields using SQUIDs. To read the full research paper, please visit: Observation of Josephson harmonics in tunnel junctions | Nature Physics
Scanning (left) and transmission (right) electron microscope images of a typical aluminum Josephson junction. Adapted from D. Willsch et al., Nat. Phys. (2024), https://doi.org/10.1038/s41567-024-02400-8; used under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)