title: Observation of Classical-Quantum Crossover of 1/f Flux Noise and Its Paramagnetic Temperature Dependence.
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_{B}T/h≈1  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 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.
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Quintana
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: Yu
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: D
authors.initials: D
authors.lastname: Sank
authors.affiliation: NASA Ames Research Center, Moffett Field, California 94035, USA.
authors.forename: A G
authors.initials: AG
authors.lastname: Petukhov
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: T C
authors.initials: TC
authors.lastname: White
authors.affiliation: Google Inc., Venice, California 90291, USA.
authors.forename: Dvir
authors.initials: D
authors.lastname: Kafri
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: B
authors.initials: B
authors.lastname: Chiaro
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: A
authors.initials: A
authors.lastname: Megrant
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: R
authors.initials: R
authors.lastname: Barends
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: B
authors.initials: B
authors.lastname: Campbell
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: Z
authors.initials: Z
authors.lastname: Chen
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: A
authors.initials: A
authors.lastname: Dunsworth
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: A G
authors.initials: AG
authors.lastname: Fowler
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: R
authors.initials: R
authors.lastname: Graff
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: E
authors.initials: E
authors.lastname: Jeffrey
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: J
authors.initials: J
authors.lastname: Kelly
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: E
authors.initials: E
authors.lastname: Lucero
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: J Y
authors.initials: JY
authors.lastname: Mutus
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: M
authors.initials: M
authors.lastname: Neeley
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: C
authors.initials: C
authors.lastname: Neill
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: P J J
authors.initials: PJ
authors.lastname: O'Malley
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: P
authors.initials: P
authors.lastname: Roushan
authors.affiliation: Google Inc., Venice, California 90291, USA.
authors.forename: A
authors.initials: A
authors.lastname: Shabani
authors.affiliation: Google Inc., Venice, California 90291, USA.
authors.forename: V N
authors.initials: VN
authors.lastname: Smelyanskiy
authors.affiliation: Google Inc., Santa Barbara, California 93117, USA.
authors.forename: A
authors.initials: A
authors.lastname: Vainsencher
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: J
authors.initials: J
authors.lastname: Wenner
authors.affiliation: Google Inc., Venice, California 90291, USA.
authors.forename: H
authors.initials: H
authors.lastname: Neven
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.|Google Inc., Santa Barbara, California 93117, USA.
authors.forename: John M
authors.initials: JM
authors.lastname: Martinis