Strong Charge-Photon Coupling in Planar Germanium Enabled by Granular Aluminium Superinductors
arxiv(2024)
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 have integrated a granular
aluminium resonator, having a characteristic impedance exceeding the resistance
quantum, with a germanium double quantum dot and demonstrate strong
charge-photon coupling with a rate of g_c/2π= (566 ± 2) MHz. This
was achieved due to the realisation of 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. This broadly applicable method opens the path for novel qubits
and high-fidelity, long-distance two-qubit gates.
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