| Summary: | 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. Because of 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 nonlinearity for strongly driven quantum circuits, topological superconductivity, and hybrid systems. Having said that, can the grAl nonlinearity be sufficient to build a qubit? Here we show that a small grAl volume (10×200×500 nm^{3}) shunted by a thin film aluminum capacitor results in a microwave oscillator with anharmonicity α two orders of magnitude larger than its spectral linewidth Γ_{01}, effectively forming a transmon qubit. With increasing drive power, we observe several multiphoton transitions starting from the ground state, from which we extract α=2π×4.48 MHz. Resonance fluorescence measurements of the |0⟩→|1⟩ transition yield an intrinsic qubit linewidth γ=2π×10 kHz, corresponding to a lifetime of 16 μs, as confirmed by pulsed time-domain measurements. This linewidth remains below 2π×150 kHz for in-plane magnetic fields up to ∼70 mT.
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