Drew G. Wild | Searcharxiv

arXiv2026

Comparison of Two-Level System Microwave Losses in Pure Bulk Microcrystalline Nb2O5 and NbO2 Oxide Samples

Losses from two-level systems (TLS) associated with amorphous oxides remain one of the primary limitations to the performance of superconducting qubits and microwave cavities. Niobium resonators are widely used in quantum science experiments, yet niobium's natural oxide layer contains various types of oxides whose relative contributions to TLS loss have not been clearly distinguished. Here, we use a superconducting 3D microwave cavity to measure commercial 99.9\% trace metal pure, microcrystalline oxide powders \ch{Nb2O5} and \ch{NbO2} in bulk amounts. Using this approach, we directly compare the loss characteristics of \ch{Nb2O5} and \ch{NbO2}. Our measurements show that the nominal \ch{Nb2O5} bulk oxide powder samples exhibit losses which have the power and temperature behavior expected for TLS. Moreover, the measurements agree with existing theoretical models. Analogous measurements performed on \ch{NbO2} bulk powder samples do not show any detectable TLS loss signatures. Based on our results we propose that the TLS losses might be reduced if a high quality microcrystalline \ch{NbO2} oxide dominates the \ch{Nb2O5} oxide in practical Nb cavities. These results establish a materials based strategy for isolating oxide specific TLS losses and provide a reference measurement for niobium oxide phases relevant to superconducting quantum devices.

cond-mat.supr-con↗

arXiv2024

Signatures of Majorana Bound States in the Diffraction Patterns of Extended Superconductor-Topological Insulator-Superconductor Josephson Junctions

In an extended superconductor-topological insulator-superconductor (S-TI-S) Josephson junction in a magnetic field, localized Majorana bound states (MBS) are predicted to exist at the cores of Josephson vortices where the local phase difference across the junction is an odd-multiple of $π$. These states contribute a supercurrent with a $4π$-periodic current-phase relation (CPR) that adds to the conventional $2π$-periodic sinusoidal CPR. In this work, we present a comprehensive experimental study of the critical current vs. applied magnetic field diffraction patterns of lateral Nb-Bi$_2$Se$_3$-Nb Josephson junctions. We compare our observations to a model of the Josephson dynamics in the S-TI-S junction system to explore what feature of MBS are, or are not, exhibited in these junctions. Consistent with the model, we find several distinct deviations from a Fraunhofer diffraction pattern that is expected for a uniform sin$(ϕ)$ CPR. In particular, we observe abrupt changes in the diffraction pattern at applied magnetic fields in which the current-carrying localized MBS are expected to enter the junction, and a lifting of the odd-numbered nodes consistent with a $4π$-periodic sin$(ϕ/2)$-component in the CPR. We also see that although the even-numbered nodes often remain fully-formed, we sometimes see deviations that are consistent with quasiparticle-induced fluctuations in the parity of the MBS pairs that encodes quantum information.

cond-mat.supr-con↗