Superconducting Microstrip Losses at Microwave and Sub-millimeter wavelengths

TL;DR

This study measures microwave and sub-millimeter wave losses in superconducting NbTiN microstrips with amorphous silicon dielectrics, revealing very low dielectric losses but unexpectedly high sub-mm wave losses that challenge existing models.

Contribution

It introduces a lab-on-chip method for precise loss measurement at microwave and sub-mm frequencies using integrated resonators and reveals discrepancies with standard loss models.

Findings

Amorphous silicon exhibits very low dielectric loss tangent at microwave frequencies.

Sub-mm wave losses are significantly higher than microwave losses.

Standard two-level system models cannot fully explain the observed sub-mm wave losses.

Abstract

We present a lab-on-chip experiment to accurately measure losses of superconducting microstrip lines at microwave and sub-mm wavelengths. The microstrips are fabricated from NbTiN, which is deposited using reactive magnetron sputtering, and amorphous silicon which is deposited using plasma-enhanced chemical vapor deposition (PECVD). Sub-mm wave losses are measured using on-chip Fabry-P{\'e}rot resonators (FPR) operating around $350\ $GHz. Microwave losses are measured using shunted half-wave resonators with an identical geometry and fabricated on the same chip. We measure a loss tangent of the amorphous silicon at single-photon energies of $\tan\delta =3.7\pm0.5\times10^{-5}$ at $6\ $GHz and $\tan\delta = 2.1\pm 0.1\times10^{-4}$ at $350\ $GHz. These results represent very low losses for deposited dielectrics, but the sub-mm wave losses are significantly higher than the microwave losses, which cannot be understood using the standard two-level system loss model.