Interacting two-level defects as sources of fluctuating high-frequency noise in superconducting circuits

TL;DR

This paper investigates how interactions among two-level defects in superconducting circuits cause high-frequency noise fluctuations, impacting qubit coherence, and introduces a model supported by experimental evidence.

Contribution

It presents a qualitative model linking defect interactions to noise fluctuations in superconducting qubits, supported by experimental TLS energy fluctuation observations.

Findings

Fluctuations in qubit relaxation times are linked to defect interactions.

High-frequency TLS energy fluctuations are driven by interactions with thermally fluctuating TLS.

Experimental evidence shows energy fluctuations in a single TLS within a phase qubit.

Abstract

Since the very first experiments, superconducting circuits have suffered from strong coupling to environmental noise, destroying quantum coherence and degrading performance. In state-of-the-art experiments, it is found that the relaxation time of superconducting qubits fluctuates as a function of time. We present measurements of such fluctuations in a 3D-Transmon circuit and develop a qualitative model based on interactions within a bath of background two-level systems (TLS) which emerge from defects in the device material. In our model, the time-dependent noise density acting on the qubit emerges from its near-resonant coupling to high-frequency TLS which experience energy fluctuations due to their interaction with thermally fluctuating TLS at low frequencies. We support the model by providing experimental evidence of such energy fluctuations observed in a single TLS in a phase qubit circuit.