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QI: Fachverband Quanteninformation

QI 11: Implementations IV

QI 11.5: Vortrag

Mittwoch, 11. März 2026, 10:45–11:00, BEY/0245

Spin Environment of a Superconducting Qubit in High Magnetic Fields — •Simon Günzler¹, Janic Beck¹, Dennis Rieger¹, Nicolas Gosling¹, Nicolas Zapata¹, Mitchell Field¹, Simon Geisert¹, Andreas Bacher¹, Judith K. Hohmann¹, Martin Spiecker¹, Wolfgang Wernsdorfer¹, and Ioan M. Pop^{1, 2} — ¹Karlsruhe Institute of Technology, Karlsruhe, Germany — ²Stuttgart University, Stuttgart, Germany

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.

Keywords: Fluxonium; Granular aluminum (grAl); Magnetic field resilience; Electron spin resonance (ESR); Flux noise freezing