Junction Resonances in Josephson Phase Qubits

Careful spectroscopic measurements of Josephson phase qubits have revealed avoided crossings characteristic of the qubit interacting with a set of two-level resonators. As these extra splittings degrade the quality of the qubit, it is extremely important to characterize and understand their origin. We will present data from a variety of qubits that show these resonators have an amplitude probability distribution which scales inversely with splitting magnitude up to a certain splitting size. This data can be explained with a model based on two-level critical-current fluctuators in the junction. This model is also reasonably consistent with previous measurements of 1/f (low frequency) critical-current noise.