Design and fabrication of a granular aluminum fluxonium qubit*

Superconducting materials with low microwave losses and high kinetic inductance are a valuable resource in quantum circuit design, enabling the design of so-called superinductors, which can provide electromagnetic environments with characteristic impedance larger than the resistance quantum R_Q = 6.5 kΩ. To implement superinductors, a promising alternative to the predominantly used mesoscopic Josephson junction arrays is granular aluminum (grAl) [1]. Its microstructure consists of pure aluminum grains embedded in an AlOx matrix, effectively forming a compact self-assembled Josephson junction network [2]. We present a superconducting fluxonium qubit employing a superinductor with impedance Z > R_Q, fabricated from a grAl thin film, in-situ integrated with a conventional Al/AlOx/Al Josephson junction. The measured qubit spectrum is in good agreement with the fluxonium Hamiltonian [3].

[1] L. Grünhaupt et al., Phys. Rev. Lett. 121, 117001 (2018)
[2] N. Maleeva et al., Nat. Commun. 9, 3889 (2018)
[3] L. Grünhaupt and M. Spiecker et al., arXiv:1809.10646 (2018)