Strong charge-photon coupling in planar germanium enabled by granular aluminium superinductors

Marián Janík (), Kevin Roux, Carla Borja-Espinosa, Oliver Sagi, Abdulhamid Baghdadi, Thomas Adletzberger, Stefano Calcaterra, Marc Botifoll, Alba Garzón Manjón, Jordi Arbiol, Daniel Chrastina, Giovanni Isella, Ioan M. Pop and Georgios Katsaros
Additional contact information
Marián Janík: Institute of Science and Technology Austria
Kevin Roux: Institute of Science and Technology Austria
Carla Borja-Espinosa: Institute of Science and Technology Austria
Oliver Sagi: Institute of Science and Technology Austria
Abdulhamid Baghdadi: Institute of Science and Technology Austria
Thomas Adletzberger: Institute of Science and Technology Austria
Stefano Calcaterra: Physics Department, Politecnico di Milano
Marc Botifoll: CSIC and BIST
Alba Garzón Manjón: CSIC and BIST
Jordi Arbiol: CSIC and BIST
Daniel Chrastina: Physics Department, Politecnico di Milano
Giovanni Isella: Physics Department, Politecnico di Milano
Ioan M. Pop: Karlsruhe Institute of Technology
Georgios Katsaros: Institute of Science and Technology Austria

Nature Communications, 2025, vol. 16, issue 1, 1-9

Abstract: Abstract High kinetic inductance superconductors are gaining increasing interest for the realisation of qubits, amplifiers and detectors. Moreover, thanks to their high impedance, quantum buses made of such materials enable large zero-point fluctuations of the voltage, boosting the coupling rates to spin and charge qubits. However, fully exploiting the potential of disordered or granular superconductors is challenging, as their inductance and, therefore, impedance at high values are difficult to control. Here, we report a reproducible fabrication of granular aluminium resonators by developing a wireless ohmmeter, which allows in situ measurements during film deposition and, therefore, control of the kinetic inductance of granular aluminium films. Reproducible fabrication of circuits with impedances (inductances) exceeding 13 kΩ (1 nH per square) is now possible. By integrating a 7.9 kΩ resonator with a germanium double quantum dot, we demonstrate strong charge-photon coupling with a rate of gc/2π = 566 ± 2 MHz. This broadly applicable method opens the path for novel qubits and high-fidelity, long-distance two-qubit gates.

Date: 2025
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DOI: 10.1038/s41467-025-57252-4

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