Rotating-frame relaxation as a noise spectrum analyzer of a superconducting qubit undergoing driven evolution

TL;DR

This paper introduces a method to analyze environmental noise spectra affecting superconducting qubits during driven evolution, using spin-locking to measure relaxation and identify specific noise features.

Contribution

It demonstrates a novel noise spectroscopy technique during driven evolution, revealing spectral features and defects that impact quantum gate fidelity.

Findings

Resolved spectral features of flux noise during driven evolution

Identified a 1MHz defect signature in a time-domain experiment

Complemented free-evolution noise characterization methods

Abstract

Gate operations in a quantum information processor are generally realized by tailoring specific periods of free and driven evolution of a quantum system. Unwanted environmental noise, which may in principle be distinct during these two periods, acts to decohere the system and increase the gate error rate. While there has been significant progress characterizing noise processes during free evolution, the corresponding driven-evolution case is more challenging as the noise being probed is also extant during the characterization protocol. Here we demonstrate the noise spectroscopy (0.1 - 200 MHz) of a superconducting flux qubit during driven evolution by using a robust spin-locking pulse sequence to measure relaxation (T1rho) in the rotating frame. In the case of flux noise, we resolve spectral features due to coherent fluctuators, and further identify a signature of the 1MHz defect in a time-domain spin-echo experiment. The driven-evolution noise spectroscopy complements free-evolution methods, enabling the means to characterize and distinguish various noise processes relevant for universal quantum control.