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

Yu Chen
Andre Petukhov
Ben Chiaro
Anthony Megrant
Rami Barends
Brooks Campbell
Zijun Chen
Andrew Dunsworth
Austin Fowler
Rob Graff
Josh Mutus
Charles Neill
Alireza Shabani
Vadim Smelyanskiy
Amit Vainsencher
Jim Wenner
John Martinis
Phys. Rev. Lett., vol. 118 (2017), pp. 057702


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 2kT/h ≈ 1GHz, 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 Hz. The antisymmetric component displays a 1/T dependence below 100 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.