We present a new method to measure 1/f noise in Josephson quantum bits (qubits) that
yields low-frequency spectra below 1 Hz. A comparison of the noise taken at positive and
negative bias of a phase qubit shows the dominant noise source to be flux noise and not
junction critical-current noise, with a magnitude similar to that measured previously in other
systems. Theoretical calculations show that the level of flux noise is not compatible with the
standard model of noise from two-level state defects in the surface oxides of the films.

We present the results of a novel measurement in a Josephson phase qubit that uses the
resonant response of the qubit to directly measure the spectrum of low-frequency noise. This
general method can be applied to any qubit system. By alternating the sense of the qubit
bias, we show that the noise is predominantly flux-like, as opposed to a critical-current noise.
The magnitude of the noise is compatible with previous measurements of excess low-
frequency flux noise in SQUIDs cooled to millikelvin temperatures. We present the results of …