  The high kinetic inductance offered by granular aluminum (grAl) has recently
been employed for linear inductors in superconducting high-impedance qubits and
kinetic inductance detectors. Due to its large critical current density
compared to typical Josephson junctions, its resilience to external magnetic
fields, and its low dissipation, grAl may also provide a robust source of
non-linearity for strongly driven quantum circuits, topological
superconductivity, and hybrid systems. Having said that, can the grAl
non-linearity be sufficient to build a qubit? Here we show that a small grAl
volume ($10 \times 200 \times 500 \,\mathrm{nm^3}$) shunted by a thin film
aluminum capacitor results in a microwave oscillator with anharmonicity
$\alpha$ two orders of magnitude larger than its spectral linewidth
$\Gamma_{01}$, effectively forming a transmon qubit. With increasing drive
power, we observe several multi-photon transitions starting from the ground
state, from which we extract $\alpha = 2 \pi \times 4.48\,\mathrm{MHz}$.
Resonance fluorescence measurements of the $|0> \rightarrow |1>$ transition
yield an intrinsic qubit linewidth $\gamma = 2 \pi \times 10\,\mathrm{kHz}$,
corresponding to a lifetime of $16\,\mathrm{\mu s}$. This linewidth remains
below $2 \pi \times 150\,\mathrm{kHz}$ for in-plane magnetic fields up to
$\sim70\,\mathrm{mT}$.
