Decoherence spectroscopy with individual two-level tunneling defects

TL;DR

This paper investigates individual two-level tunneling defects in superconducting qubits, revealing their environmental interactions, relaxation mechanisms, and dephasing behavior, which are crucial for improving quantum device coherence.

Contribution

It provides detailed experimental insights into the properties and environmental couplings of individual TLSs, highlighting phononic interactions over mutual TLS interactions as the relaxation mechanism.

Findings

TLS relaxation rates depend on frequency and are explained by phononic coupling.

Most TLSs show no pure dephasing at degeneracy points.

Incoherent low-frequency TLSs cause pure dephasing in high-frequency TLSs.

Abstract

Recent progress with microfabricated quantum devices has revealed that an ubiquitous source of noise originates in tunneling material defects that give rise to a sparse bath of parasitic two-level systems (TLSs). For superconducting qubits, TLSs residing on electrode surfaces and in tunnel junctions account for a major part of decoherence and thus pose a serious roadblock to the realization of solid-state quantum processors. Here, we utilize a superconducting qubit to explore the quantum state evolution of coherently operated TLSs in order to shed new light on their individual properties and environmental interactions. We identify a frequency-dependence of TLS energy relaxation rates that can be explained by a coupling to phononic modes rather than by anticipated mutual TLS interactions. Most investigated TLSs are found to be free of pure dephasing at their energy degeneracy points, around which their Ramsey and spin-echo dephasing rates scale linearly and quadratically with asymmetry energy, respectively. We provide an explanation based on the standard tunneling model, and identify interaction with incoherent low-frequency (thermal) TLSs as the major mechanism of the pure dephasing in coherent high-frequency TLS.