# Characterization and Reduction of Capacitive Loss Induced by Sub-Micron Josephson Junction Fabrication in Superconducting Qubits

## TL;DR

This paper investigates how sub-micron Josephson junction electrodes contribute to dielectric loss in superconducting qubits and introduces fabrication techniques to reduce this loss, significantly improving qubit relaxation times.

## Contribution

It develops an experimental method to quantify electrode-induced loss and demonstrates fabrication improvements that enhance qubit coherence.

## Key findings

- Loss scales with the number of electrodes added to resonators.
- Fabrication techniques can reduce dielectric loss from electrodes.
- Qubit relaxation times improved by a factor of 5.

## Abstract

Josephson junctions form the essential non-linearity for almost all superconducting qubits. The junction is formed when two superconducting electrodes come within $\sim$1 nm of each other. Although the capacitance of these electrodes is a small fraction of the total qubit capacitance, the nearby electric fields are more concentrated in dielectric surfaces and can contribute substantially to the total dissipation. We have developed a technique to experimentally investigate the effect of these electrodes on the quality of superconducting devices. We use $\lambda$/4 coplanar waveguide resonators to emulate lumped qubit capacitors. We add a variable number of these electrodes to the capacitive end of these resonators and measure how the additional loss scales with number of electrodes. We then reduce this loss with fabrication techniques that limit the amount of lossy dielectrics. We then apply these techniques to the fabrication of Xmon qubits on a silicon substrate to improve their energy relaxation times by a factor of 5.