Looping on the Bloch sphere: Oscillatory effects in dephasing of qubits subject to broad-spectrum noise

TL;DR

This paper introduces a new mathematical approach to analyze broad-spectrum noise effects on qubits, revealing a novel noise-induced looping phenomenon on the Bloch sphere and demonstrating its relevance to superconducting qubits.

Contribution

The paper develops a comprehensive method to extract noise parameters from qubit measurements and predicts a new physical effect of noise-induced looping on the Bloch sphere.

Findings

Superconducting qubits are close to the regime where looping occurs.

The new method can characterize broad-spectrum noise across different qubit working points.

Evidence suggests the potential observation of noise-induced looping in experiments.

Abstract

For many implementations of quantum computing, 1/f and other types of broad-spectrum noise are an important source of decoherence. An important step forward would be the ability to back out the characteristics of this noise from qubit measurements and to see if it leads to new physical effects. For certain types of qubits, the working point of the qubit can be varied. Using a new mathematical method that is suited to treat all working points, we present theoretical results that show how this degree of freedom can be used to extract noise parameters and to predict a new effect: noise-induced looping on the Bloch sphere. We analyze data on superconducting qubits to show that they are very near the parameter regime where this looping should be observed.