Dielectric losses in multi-layer Josephson junction qubits

TL;DR

This study investigates dielectric losses in multi-layer Josephson junction qubits, showing that removing certain dielectrics significantly improves qubit lifetimes and identifying the dielectric materials responsible for losses.

Contribution

It provides a detailed analysis of dielectric loss contributions in multi-layer Josephson qubits and demonstrates how material and design changes can enhance qubit coherence times.

Findings

Removing SiNx dielectric quadruples relaxation time.

Dielectric losses from AlOx, SiNx, and Si/SiO2 limit qubit lifetimes.

Material and design modifications can potentially improve lifetimes fourfold.

Abstract

We have measured the excited state lifetimes in Josephson junction phase and transmon qubits, all of which were fabricated with the same scalable multi-layer process. We have compared the lifetimes of phase qubits before and after removal of the isolating dielectric, SiNx, and find a four-fold improvement of the relaxation time after the removal. Together with the results from the transmon qubit and measurements on coplanar waveguide resonators, these measurements indicate that the lifetimes are limited by losses from the dielectric constituents of the qubits. We have extracted the individual loss contributions from the dielectrics in the tunnel junction barrier, AlOx, the isolating dielectric, SiNx, and the substrate, Si/SiO2, by weighing the total loss with the parts of electric field over the different dielectric materials. Our results agree well and complement the findings from other studies, demonstrating that superconducting qubits can be used as a reliable tool for high-frequency characterization of dielectric materials. We conclude with a discussion of how changes in design and material choice could improve qubit lifetimes up to a factor of four.