Observation of quantum oscillations in the extreme weak anharmonic limit

We investigate a granular aluminium quantum circuit with an anharmonicity of the order of its decoherence rate in a three-dimensional microwave cavity. We perform single qubitlike manipulations such as Rabi oscillations and Ramsey fringes. Our findings, supported by quantitative numerical modeling, show that a very weakly anharmonic oscillator can also display quantum oscillations outside the qubit regime. These oscillations are hard to disambiguate from qubit oscillations in time domain measurements for a single driving frequency. This sheds light on recent findings for material superconducting quantum bits. Our platform shows in addition large magnetic field resilience which could find applications for quantum enhanced dark matter search.