Qubit Noise Spectroscopy of Superconducting Dynamics in a Magnetic Field

TL;DR

This paper demonstrates how magnetic noise spectroscopy using a single spin qubit can non-invasively probe and distinguish various vortex and fluctuation dynamics in two-dimensional superconductors under an applied magnetic field.

Contribution

It introduces a novel application of noise spectroscopy to analyze vortex phases and critical fluctuations in 2D superconductors with an applied magnetic field, providing detailed dynamical insights.

Findings

Magnetic noise spectrum near Tc deviates from quasiparticle predictions due to critical fluctuations.

Noise spectroscopy can distinguish between different vortex phases.

Extracts physical quantities like vortex oscillation frequencies and diffusivity.

Abstract

An applied magnetic field affects a superconductor in two ways -- by promoting pairing fluctuations, and by inducing topological defects called vortices that carry quantized magnetic flux. A quantitative characterization of the resultant field-induced superconducting dynamics with spatio-temporal resolution remains challenging, particularly in two-dimensional materials. In this work, we analyze magnetic noise measured by the depolarization rate of a proximate single spin qubit as a non-invasive probe of such dynamical fluctuations. We demonstrate that the temperature dependence of the magnetic noise spectrum near $T_c$ deviates from predictions based on quasiparticle excitations due to critical superconducting fluctuations, which in turn are enhanced by a weak applied field. By analyzing the magnetic noise due to vortex dynamics, we further show that noise spectroscopy is not only able to distinguish between different vortex phases, but also extract key physical quantities of interest, such as oscillation frequencies of pinned vortices, phonon dispersion of vortex lattices and vortex diffusivity in a vortex liquid. Complementing recent work on noise magnetometry of quasiparticle excitations and Berezinskii-Kosterlitz-Thouless transitions in two-dimensional superconductors without an applied field, our work highlights the ability of noise spectroscopy to reveal a wealth of superconducting dynamical phenomena in an applied field.