Superconducting quantum information processing machines are predominantly based on microwave circuits with relatively low characteristic impedance, about 100 Ohm, and small anharmonicity, which can limit their coherence and logic gate fidelity. A promising alternative are circuits based on so-called superinductors, with characteristic impedances exceeding the resistance quantum R_Q = 6.4 kOhm. However, previous implementations of superinductors, consisting of mesoscopic Josephson junction arrays, can introduce unintended nonlinearity or parasitic resonant modes in the qubit vicinity, degrading its coherence.
I will present a fluxonium qubit design based on a granular aluminum (grAl) superinductor strip [1]. I will argue that granular aluminium forms a compact effective junction array with high kinetic inductance and low nonlinearity [2], and it can be in-situ integrated with standard aluminum circuit processing. The measured qubit coherence time T_2^* = 30 µs illustrates the potential of grAl for applications ranging from protected qubit designs to quantum limited amplifiers and detectors, even though quasiparticle poisoning is a limiting factor [3] and should be addressed in future works.
[1] Grunhaupt, Spiecker et al. Nature Materials 18, 816-819 (2019)
[2] Maleeva et al. Nature Comm. 9, 3889 (2018)
[3] Grunhaupt et al., Phys. Rev. Lett. 121, 117001 (2018)
