Observation of classical-quantum crossover of 1/f flux noise and its paramagnetic temperature dependence

TL;DR

This study investigates the classical-quantum crossover of flux noise in a flux qubit by analyzing its environmental noise spectrum, revealing a $1/f$ noise behavior and paramagnetic temperature dependence, with implications for qubit coherence.

Contribution

It provides the first detailed measurement of the two-sided flux noise spectrum across the classical-quantum crossover in a flux qubit, highlighting the paramagnetic environment's role.

Findings

Symmetric flux noise follows a $1/f$ power law near 1 Hz.

Antisymmetric noise component shows a 1/T dependence below 100 mK.

Spectrum extrapolation predicts qubit tunneling linewidth and reorganization energy.

Abstract

By analyzing the dissipative dynamics of a tunable gap flux qubit, we extract both sides of its two-sided environmental flux noise spectral density over a range of frequencies around $2k_BT/h \approx 1\,\rm{GHz}$, allowing for the observation of a classical-quantum crossover. Below the crossover point, the symmetric noise component follows a $1/f$ power law that matches the magnitude of the $1/f$ noise near $1\,{\rm{Hz}}$. The antisymmetric component displays a 1/T dependence below $100\,\rm{mK}$, providing dynamical evidence for a paramagnetic environment. Extrapolating the two-sided spectrum predicts the linewidth and reorganization energy of incoherent resonant tunneling between flux qubit wells.