Observation of classical-quantum crossover of 1/f flux noise and its   paramagnetic temperature dependence
  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. 
C. M. Quintana, Yu Chen, D. Sank, A. G. Petukhov, T. C. White, Dvir   Kafri, B. Chiaro, A. Megrant, R. Barends, B. Campbell,...
quant-ph cond-mat.mes-hall