Correlating Superconducting Qubit Performance Losses to Sidewall Near-Field Scattering via Terahertz Nanophotonics

Elucidating dielectric losses, structural heterogeneity, and interface imperfections is critical for improving coherence in superconducting qubits. However, most diagnostics rely on destructive electron microscopy or low-throughput millikelvin quantum measurements. Here, we demonstrate noninvasive terahertz (THz) nano-imaging/-spectroscopy of encapsulated niobium transmon qubits, revealing sidewall near-field scattering that correlates with qubit coherence. We further employ a THz hyperspectral line scan to probe dielectric responses and field participation at Al junction interfaces. These findings highlight the promise of THz near-field methods as a high-throughput proxy characterization tool for guiding material selection and optimizing processing protocols to improve qubit and quantum circuit performance.

💡 Research Summary

The paper introduces a non‑destructive terahertz scattering‑type scanning near‑field optical microscopy (THz‑sSNOM) platform for probing loss mechanisms in superconducting transmon qubits. By coupling broadband THz pulses (0–2 THz) to an atomic‑force‑microscope tip (≈20 nm radius) and demodulating the scattered signal at the tip‑tapping harmonics (n = 1–4), the authors achieve nanometer‑scale spatial resolution and sub‑nanosecond temporal resolution while operating at room temperature. They apply this technique to Nb‑based transmon qubits encapsulated with a thin AuPd capping layer, focusing on the exposed sidewalls where native Nb oxides form.

Four encapsulated qubits and four uncapped reference devices are examined. Line scans across sidewalls at four locations per qubit yield near‑field amplitudes s₁–s₄. The maximum scattered amplitude (s_peak) is normalized to the substrate background (s_sub) as (s_peak – s_sub)/s_sub. This normalized quantity correlates strongly with the average T₁ relaxation times measured for each qubit; higher normalized THz signals correspond to longer T₁. When the second‑order harmonic (s₂) is used, the reciprocal of the normalized signal, s_sub/(s_peak – s_sub), shows a linear relationship with 1/Q (where Q = 2πf T₁). The correlation disappears for uncapped devices, indicating that the sidewall condition is the dominant factor in the encapsulated samples.

Transmission electron microscopy of the sidewalls reveals two structural trends that explain the THz‑sSNOM results. First, the AuPd cap does not fully cover the Nb top surface, leaving a ≈100 nm exposed oxide region; larger exposed oxide correlates with lower Q. Second, trench depth (the recess of the sapphire substrate beneath the Nb) varies among devices; deeper trenches reduce electric‑field concentration at the sidewall, decreasing the energy participation ratio and thereby improving Q. Other parameters such as oxide thickness or sidewall curvature show negligible impact.

The technique also identifies a localized defect in an Al/AlOₓ/Al Josephson junction. A dark spot in the THz‑sSNOM image coincides with a ≈5 nm topographic depression and a 30–40 % drop in s₂. Frequency‑resolved THz time‑domain spectroscopy across the defect yields complex dielectric constants, revealing an anomalous suppression of free‑carrier response not observable with conventional microscopy.

Overall, the study demonstrates that (i) THz‑sSNOM can capture GHz‑scale electric‑field enhancements caused by nanometer‑scale geometric imperfections, (ii) sidewall geometry and oxidation are critical loss channels in Nb‑capped transmons, and (iii) the method provides a rapid, room‑temperature, high‑throughput proxy for qubit coherence that can guide material selection and process optimization. Limitations include the small sample set and the need for quantitative models linking THz near‑field responses to GHz loss mechanisms; future work should expand statistical sampling and integrate multi‑frequency electromagnetic simulations.