Electronic Decoherence of Two-Level Systems in a Josephson Junction

TL;DR

This study demonstrates that quasiparticles in superconducting Josephson junctions cause decoherence of two-level systems, affecting qubit performance and offering insights into their microscopic origins.

Contribution

It provides experimental evidence linking quasiparticle density to two-level system decoherence and introduces a method to locate these systems within the tunnel barrier.

Findings

Decoherence rates increase linearly with quasiparticle density.

Quasiparticles leaking into the barrier cause decoherence.

The method allows mapping two-level systems' positions across the barrier.

Abstract

The sensitivity of superconducting qubits allows for spectroscopy and coherence measurements on individual two-level systems present in the disordered tunnel barrier of an $\mathrm{Al/AlO_x/Al}$ Josephson junction. We report experimental evidence for the decoherence of two-level systems by Bogoliubov quasiparticles leaking into the insulating $\mathrm{AlO_x}$ barrier. We control the density of quasiparticles in the junction electrodes either by the sample temperature or by injecting them using an on-chip dc-SQUID driven to its resistive state. The decoherence rates were measured by observing the two-level system's quantum state evolving under application of resonant microwave pulses and were found to increase linearly with quasiparticle density, in agreement with theory. This interaction with electronic states provides a noise and decoherence mechanism that is relevant for various microfabricated devices such as qubits, single-electron transistors, and field-effect transistors. The presented experiments also offer a possibility to determine the location of the probed two-level systems across the tunnel barrier, providing clues about the fabrication step in which they emerge.