Spin Environment of a Superconducting Qubit in High Magnetic Fields

We leverage the magnetic field resilience of a granular aluminum nanojunction fluxonium qubit (Gralmonium) with a gradiometric design to uncover a paramagnetic spin-1/2 ensemble, which is the dominant Gralmonium loss mechanism when the electron spin resonance matches the qubit. We also report a suppression of fast flux noise measured in Spin-Echo experiments in magnetic fields exceeding 0.4 Tesla, which suggests the freezing of surface spins. In addition to these environments, by employing an active state stabilization sequence of the qubit, we hyperpolarize long-lived two-level systems (TLS), previously speculated to also be of magnetic origin. Surprisingly, the coupling to this TLS environment remains unaffected by magnetic fields, leaving the question of their origin open. These results demonstrate the gradiometric Gralmonium's potential for hybrid quantum architectures combining superconducting qubits with spins.