  Superconducting quantum information processing machines are predominantly
based on microwave circuits with relatively low characteristic impedance, of
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
$R_Q = 6.4$ k$\Omega$. 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. Here we present a fluxonium qubit design using a granular aluminum
(grAl) superinductor strip. Granular aluminum is a particularly attractive
material, as it self-assembles into an effective junction array with a
remarkably high kinetic inductance, and its fabrication can be in-situ
integrated with standard aluminum circuit processing. The measured qubit
coherence time $T_2^R$ up to 30 $\mu$s illustrates the potential of grAl for
applications ranging from protected qubit designs to quantum limited amplifiers
and detectors.
