Coherence of a granular aluminum fluxonium qubit*

A promising alternative for the implementation of superinductors, compared to the predominantly used mesoscopic Josephson junction arrays, is granular aluminum (grAl), with a microstructure consisting of pure aluminum grains embedded in an AlOx matrix, effectively forming a self-assembled Josephson junction network [1]. This material offers a large kinetic inductance, while its non-linearity is orders of magnitude smaller than that of Josephson junction arrays [2]. We present a fluxonium qubit employing a granular aluminum superinductor with coherence times T₁ up to 23 μs and T₂^R up to 30 μs at the flux bias sweet spot. The measured T₂^Echo approaches the limit 2 × T₁ [3]. These coherence times recommend granular aluminum for increasingly complex protected superconducting quantum circuits, while they also evidence the need to further investigate and mitigate loss mechanisms in high impedance qubits.

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