N. Materise | Searcharxiv

arXiv2025

Superconducting qubit decoherence correlated with detected radiation events

Most quantum error correction (QEC) protocols for superconducting qubits assume spatially and temporally uncorrelated decoherence events; however, recent evidence suggests that cosmic radiation induces spatially correlated errors. We present a platform that sandwiches a superconducting transmon qubit between two microwave kinetic inductance detector (MKID) arrays, enabling real-time detection of radiation-induced phonon bursts. By synchronizing MKID event detection with single-shot measurements of qubit energy relaxation ($T_1$) and phase coherence ($T_2$), we observe statistically significant reductions in both $T_1$ and $T_2$-up to 30.5%-immediately following dual MKID events attributed to penetrating muons. Our findings directly link radiating events to correlated qubit decoherence. Furthermore, our experimental platform provides a foundation for systematic studies of radiation effects, the development of shielding and mitigation techniques, and the refinement of error-correction algorithms tailored to correlated noise sources.

quant-ph↗

arXiv2025

Non-equilibrium Dynamics of Two-level Systems directly after Cryogenic Alternating Bias

Two-level systems (TLSs) are tunneling states commonly found in amorphous materials that electrically couple to qubits, resonators, and vibrational modes in materials, leading to energy loss in those systems. Recent studies suggest that applying a large alternating electric field changes the oxide structure, potentially improving the performance of qubits and resonators. In this study, we probe the effect of alternating bias at cryogenic temperatures on TLS dynamics within amorphous oxide parallel-plate capacitors operating in the strongly coupled regime. We bias the TLSs in the capacitors using an electric field. This allows us to spectroscopically image TLSs and extract their densities and dipole moments. When an in-situ alternating bias is applied, the steady-state spectra from the standard TLS model disappear. Post-alternating bias TLS spectroscopy reveals transient behavior, in which the TLS frequency fluctuates on the order of minutes. Thermal cycling above 10 K reverses these effects, restoring the TLS spectrum to its original state, indicating a reversible mechanism. Importantly, the intrinsic loss tangent of the LC oscillator remains unchanged before and after the application of the alternating bias. We propose that the disappearance of the steady-state spectrum are caused by non-equilibrium energy build up from strain in the oxide film introduced by the pulsed voltage bias sequence. Understanding this non-equilibrium energy could inform future models of time-dependent TLS dynamics.

quant-ph↗