  Critical current fluctuations with a 1/ f spectral density (f is frequency)
are potentially a limiting source of intrinsic decoherence in superconducting
quantum bits (qubits) based on Josephson tunnel junctions. Prior measurements
of this noise were made at nonzero voltages whereas qubits are operated in the
zero voltage state. We report measurements of 1/ f noise in a dc
superconducting quantum interference device first, coupled to a resonant tank
circuit and operated in a dispersive mode at zero voltage, and, second,
operated conventionally with a current bias in the voltage regime. Both
measurements yield essentially the same magnitude of critical current 1/ f
noise.
